This is a chapter from an Exchange Server book which was never published. A few years ago several colleagues and I decided to write an Exchange Server 2016 book, but scheduling and bandwidth issues caused us to delay it until Exchange Server 2019. However, we soon realized we were simply too busy to commit the amount of time needed to properly see the book to its completion. As a result, I was left with a few chapters destined to die a slow death in a folder on my desktop. As an alternative to that grim scenario, I’ve decided to publish them here (with zero warranty or proper editing). Enjoy.

Also, if you enjoy the below topic on Exchange Backup, Restore, & DR, checkout the Exchange Server Troubleshooting Companion now available for free on TechNet

Having spent much of my career in a support role, working with the largest of the large to smallest of the small companies, one theme rang consistently true in all escalations involving disaster recovery; improper planning and testing. One tell-tale sign of an operationally mature IT department is their commitment to planning and testing for failures within their environment. What happens when the WAN link fails? Whom do we contact when our critical database goes down? Can we deliver adequate performance if multiple servers in our farm fail? Have we tested our site switchover capability? Do we have site switchover capability? We have a joke in support that whenever an IT Professional makes a critical mistake resulting in significant downtime it’s referred to as an RGE, or a “Resume Generating Event.” Having a firm understanding of Exchange, what it relies on to function, its built-in data retention/recovery features, and the various supported paths it provides for recovery can help you ensure your company or clients are well prepared for the worst Murphy’s Law can throw at you. This information, combined with a sound testing practice, are vital for all Exchange Professionals.

Golden Rule; never have a critical workload in production without an escalation path.

What is important to backup for Exchange?

It would be easy to purchase a backup product said to be made for Exchange, kick off a backup, and tell yourself you have done the due diligence. If the product truly is as advertised, you might genuinely be adequately prepared for all manner of failure and recovery scenarios. Much like “script kiddies” are ridiculed by the hacking community for actually understanding what they’re working with, running wizards and hoping for success is not helpful when you encounter an issue. This is why understanding what is actually being backed up and how it relates to Exchange is so important.

Below are the relevant items you should be backing up on an Exchange server

• Mailbox Databases
• Public Folder Databases (if running Exchange 2010)
• Installed Certificates
• IIS/Web.Config file customizations
• Voicemail directory (if utilizing Unified Messaging)
• Any third-party tools

Mailbox and Public Folder Databases are by far the most important of any of these items, as they hold the contents of all mailbox and Public Folder data. The methods used to capture, store, and potentially restore this data is where you will find the differentiation between backup solution vendors. Later in this module, I will discuss the best means of backing up Exchange databases, but it’s well understood that the most important piece of your Exchange deployment is the data held within these files. The below files make up an Exchange database:

• Database file (.edb)
• Transaction Logs (.log)
• Checkpoint File (.chk)
• System/Reservation files (.jrs)

Most people will only concern themselves with the above files, since they can be easily gathered via a backup program. However, it is important to have an exported copy of all added SSL certificates used for Exchange, including the Private Keys. I do not mean the self-signed certificates added by the Exchange installer, but any certificated added by an Administrator. While it’s true many of these can be re-requested, doing so can take valuable time. It is also discouraged to rely on the response time of third-party Certificate Authority if you’re in the middle of an Exchange outage. Keep the certificate in a safe location that is easily accessible.

Should the environment be utilizing any IIS customizations such as custom OWA login pages, modified web.config files, be sure to take note of these and backup those changes/settings/files in a secure location. Some companies choose to implement IIS-based redirection to simplify the OWA URL (removing the requirement specify the /owa path for instance). These configurations should be properly documented so they can be easily re-implemented after an Exchange server recovery. This ensures the end user experience remains consistent.

Lastly, any third-party plug-ins, add-ins, extensions, or applications should either be backed up (if they include their own database such as SQL for storing configuration information) or included in the disaster recovery plans. Whether it be an IIS extension, Transport Agent, Anti-SPAM, Anti-Virus, or email archiving program, ensuring you are well-equipped to reinstall and reconfigure properly (in a possibly sleep-deprived state working an Exchange disaster recovery all-nighter) will drastically reduce the time of your outage.

Traditional Backup options

A “Traditional Backup” in this context, is any backup methodology reliant upon copying Exchange database information and later restoring it. I intentionally speak in generic terms because there are many ways of achieving this goal. You could use Windows Server Backup to take a VSS Full backup of local Exchange database files in a single server environment. You could use a centralized backup server with an agent installed on each Exchange server to take a VSS Differential backup, copying the data across the network to a centralized storage repository. You could use a virtualization-based backup program at the hypervisor host level (having VSS tools integrated within the virtual machine) to backup a virtual machine. You could even use the very low-tech method of dismounting the database and manually copying the databases files (the database must be dismounted for the backup to be in a consistent state, as this method is not using the VSS subsystem). The last method is usually a last ditch effort to get a copy of the database files, typically done when the backup solution isn’t working properly.

With each option the goal is the same, get a consistent/non-corrupted copy of the database which can be placed into a “Clean Shutdown” state and mounted for recovery purposes. When discussing types of traditional backups, the differentiators typically relate to the handling of transaction logs. Are they truncated or left in-place? Below are the common backup types:

Normal (or Full) – Backs up Exchange data store in its entirety. Results in transaction log truncation

Copy – Backs up same data set as Full except no marking takes place to give Incremental/Differential backups context. Does not result in transaction log truncation. Typically done as an out of routine backup operation.

Incremental – Backs up data set that has changed since a Full or Incremental backup. Results in transaction log truncation. A restore would require each Incremental backup since the most recent Full backup, as well as the Full backup.

Differential – Backs up data set that has changed since the last Full backup. A restore would require one Full and one Differential backup. Does not result in transaction log truncation.

Regardless of which backup type or application used, it must be “Exchange-Aware” and utilize the Microsoft Volume Shadow Copy Service (VSS). In simplistic terms, VSS ensures that active workloads may be backed up in a consistent and stable manner, without requiring downtime. It does this by providing a framework of the following components:

• VSS Service within Windows
• Requestor: Backup Application
• Writer: An applications VSS component
• Provider: Holds backup data within software or hardware

Framework

Ultimately, this means that not only must your backup application support VSS (which almost all do), it must also be “Exchange-Aware” in that it is able to communicate with the Exchange Information store to quiesce IO, create a VSS snapshot, resume IO, and request that the Information Store truncate the required transaction logs.

Transaction Logging and Recovery

To fully comprehend log truncation during normal operations and backup/restore scenarios, a fundamental knowledge of Exchange database transaction logging is required. This will also help the reader in understanding Database Availability Group log shipping and truncation.

Regardless of which Exchange client is used, after connecting through the various Exchange services and worker processes, a user ultimately accesses their mailbox via the Information Store process. The Exchange Information Store (formerly the Store.exe process, but with Exchange 2013 onward, Microsoft.Exchange.Store.Worker.exe) is where the Exchange Database Cache exists. A separate worker process runs for every database on a server. The cache holds all currently active Exchange database transactions in memory, such as new mail, deletions, modifications, etc. This cache can become quite large but this is by design as keeping transactions in memory is much faster than fetching and updating database pages from disk. When the cache does read/write to the database (.edb), it does so in 32 KB pages.

It’s important to understand that clients connect to the cache, not the actual .edb file. No client ever directly accesses the database (.edb) or any log files (.log). Instead, all connections occur in memory to the database cache. Of course, if the database (.edb) file becomes locked due to another process (such as anti-virus or backup programs), the Information Service will eventually be unable to communicate with it and the database will eventually dismount.

When discussing backups, transaction logs are extremely important, so it’s vital to understand which role they play in database transactions. As transactions occur in the cache, they create a series of transaction records or log data, which fills a log buffer. Once a log buffer is full (1 MB), it is flushed to disk to create a 1 MB transaction log file. This process repeats as more transactions occur in cache and the log buffer fills and is written to disk. The currently active transaction log file is always E0n.log, where n represents the number of the log stream for that database.

As transaction log files are written to disk, the transactions in cache might yet still not be committed to the actual database (.edb) file. This process is referred to as “write-ahead logging” (In fact, technically the transactions are written to the logs before the user sees the change). This is because writes to the database are random IO while creating a transaction log is a sequential IO. Since the data being written to the database could be located anywhere in a very large .edb file, the operation on disk is usually random. On the other hand, creating a 1 MB transaction log takes a single new sequential write operation to disk. With rotational media, this distinction becomes important as Seek Time contributes to disk latency. Sequential IO is very low impact on a disk subsystem while random IO is more burdensome. Writing the transactions sequentially to logs instead of the database (.edb) file allows the transactions to be captured efficiently (in cache and on disk in the transaction log) while also reducing random IOPS. The focus on trading random IO for sequential IO by using memory has contributed to the gradual reduction in the product’s IO requirement since Exchange 2007.

When are the transactions written to the database (.edb) file? After a predetermined amount of transaction logs are generated, the transactions in cache are flushed to the database (.edb) file. The predetermined amount is called the Checkpoint Depth Target and is tracked by the Checkpoint File (.chk). The Checkpoint File monitors the logs with the oldest outstanding uncommitted page. Databases which have no replicas/copies have a Checkpoint Depth Target of 20 transaction logs whereas databases with passive copies (DAG) have a target of 100 transaction logs. This fact will become relevant when I discuss log truncation, especially in a DAG.

Of course, there is another reason that Exchange writes transactions to the log files before being committed to the database (.edb) file. By quickly committing transactions to disk (via transaction logs), it means that the transactions exist in two locations; memory and disk. If a failure occurs that causes the Information Store to terminate unexpectedly, the most recent transactions are still available and can be replayed into the database from the transaction logs after the database is mounted and to bring the database up-to-date.

For a long time, it was recommended to place your Exchange database file (.edb) onto different spindles than your transaction logs. This is still the recommendation when only one copy of a database exists (non-DAG protected). In fact, this is not for performance reasons but to assist recovery in the event of a storage failure. Say you lost your database drive, leaving you only with the transaction logs. Technically, if you still had every transaction log present since the database was first created, you could use ESEUTIL to generate a new database and commit every transaction from the logs into it. However, this is not usually the case. People usually resort to an Exchange-Aware backup, which leads us to why an Exchange-Aware backup truncates log files. When a Full Exchange-Aware backup is performed against a database, the .edb file is copied to the backup location, as well as any transaction logs present for the database. With these files, the database can be restored and the database can be brought into a Clean Shutdown state, meaning all transactions in the logs have been committed to the .edb file and the database can be mounted. As the backup completes, it sends a command to the ESE database engine stating that any transaction logs older than a certain point can be truncated (deleted). These logs are no longer required on the Exchange server because we now possess a copy of them in the backup set.

Back to our scenario where the .edb file was lost due to a storage failure. We would simply restore the database and transaction logs from our Exchange-Aware backup, and replay both the restored and whatever logs are available into the .edb file. If a complete set of logs are available, the result should be that Exchange could replay a continuous stream of transaction logs from the point where the backup was created to the time when the failure occurred and result in no data loss.

This brings me to the question: “Why can’t I use Circular Logging to delete my transaction logs automatically so they don’t fill up my drive?” First, we need to understand that Circular Logging is a property on a database. When circular logging enabled, the Information Store deletes transaction log files as soon as the transactions contained within them have been committed from cache to the .edb file. A much smaller set of logs is maintained, as the set never grows over time because logs are continuously deleted.

The command to enable circular logging is as follows:

[PS] C:\> Set-MailboxDatabase –Identity <DatabaseFileName> -CircularLoggingEnabled $True

The question then comes into play as to why not enable circular logging for every database? Having read the chapter up to this point, the answer should be obvious. If your database subsequently becomes unusable due to a failure, no transaction logs will be available for replay into the database, as all but the most recent logs will have been automatically deleted by Exchange. Since all transaction logs must be replayed into the database in their proper sequence, your only choice would be to restore the most recent database backup, and lose all Exchange transactions that occurred between the time of the backup and the time of failure.

It is at this point that I should define the differences between traditional (JET) Circular Logging and Continuous Replication Circular Logging (CRCL). What I described above is JET Circular Logging and is used on standalone Exchange Servers, or databases on Exchange Servers which do not have replicated copies. CRCL is used when a database has replicated copies in a DAG and is enabled using the same command. When using traditional backup methods, neither should be enabled as the backup program’s consistency check will fail due to missing logs.

In my opinion, JET Circular Logging should only ever be enabled in a lab or when you are in dire need of disk space during a recovery or during transaction-intense operations such as moving mailboxes. This is because if you encounter a failure and lose the database, you will lose all transactions that occurred between the last backup and the failure. CRCL should only be enabled in a DAG environment where Exchange Native Data Protection is being used, as traditional backups are not performed. In this configuration, since backups are not performed, no transaction logs will ever be truncated and database drives would ultimately reach capacity.

The logic used to determine when logs will be truncated is as follows (for more information, see this reference):

For truncation to occur on highly available (non-lagged) mailbox database copies, the answer must be “Yes” to the following questions:

Has the log file been backed up, or is CRCL enabled?

Is the log file below the checkpoint?

Do the other non-lagged copies of the database agree with deletion?

Has the log file been inspected by all lagged copies of the database?

For truncation to occur on lagged database copies, the answer must be “Yes” to the following questions:

Is the log file below the checkpoint?

Is the log file older than ReplayLagTime + TruncationLagTime?

Is the log file deleted on the active copy of the database?

Simply put, the DAG ensures a transaction log has been inspected by all database copies in the DAG before it can be deleted. The log file must also be below the Checkpoint Depth Target, which is 100 transaction logs in DAG-protected databases. Knowing this target is important for understanding expected behavior when verifying transaction log truncation after a successful Exchange-Aware backup. On a standalone database there will always be ~20 transaction logs and on a DAG-protected database there will always be ~100 transaction logs, even after a successful Exchange-Aware backup. This is because only logs that have reached the Checkpoint Depth Target are eligible for truncation. On several occasions I’ve been asked why transaction logs within a DAG were not truncating after a successful backup because there always seemed to be transaction logs in the log directory. The short answer is there will always ~100 logs in these directories. If you want to verify successful log truncation following a backup in a DAG, I recommend the following article. Similarly, in lab environments truncation may not occur because the database is new and has yet to generate 100 transaction logs, so there is nothing to truncate.

Understanding the inner workings of Exchange databases helps to determine why a backup might not be successful and the steps needed to remedy it. As mentioned above, utilizing traditional backups in a DAG present their own unique challenges; namely, transaction logs not truncating due to replication issues between nodes.

Traditional backup considerations

The most important phase of planning a backup solution should be defining your availability and data retention requirements. Much of this was covered in the High Availability chapter. Namely, how long can you be down and how much data can you afford to lose? This is commonly defined as Recovery Time Objective (RTO) and Recovery Point Objective (RPO). Relative to Exchange, we are speaking of how long users are unable to access the email system and how much data can the business afford to lose to remain within acceptable measures.

While most Exchange High Availability planning discussions centre around the Preferred Architecture, your backup solution design can have a significant impact to your overall sizing and design. For instance, Microsoft recommends database sizes no greater than 200 GB for standalone (non-DAG) and no greater than 2 TB for highly available databases (DAG). I feel it’s important to understand the reasoning behind these numbers, and many customers either follow the recommendation blindly or ignore it while not understanding the logic behind it. Making that mistake can drastically affect the availability of your solution. The 200 GB recommendation for standalone servers (which will be using traditional backup methods) is to ensure the backups are able to complete in a reasonable amount of time and capable of being restored in a reasonable amount of time. I’ve seen far too many customers with databases sometimes over 1 TB in size that have issues with their backup jobs taking forever to complete. However, the most serious scenarios involve the extended outages resulting from restoring such a large backup set in a reasonable amount of time. It’s less common for multiple databases to become lost/corrupted (assuming they’re wisely placed on differing storage from one another) therefore it makes more sense to have multiple smaller databases than fewer larger databases. Should a database be lost/corrupted, fewer users are affected and for less time, as the smaller database is more quickly restored from traditional backup. While the 200 GB recommendation for standalone servers is dictated by backup/restore times, the 2 TB recommendation for DAG-protected databases is related to re-seed times. If an active database copy fails in a DAG, a passive copy will be brought online within seconds. Therefore, even when utilizing traditional backups in a DAG, the restore time of a database is negligible. Having a database no greater than 2 TB allows the seeding/reseeding operation to complete within an operationally reasonable amount of time. If seeding occurs at 20 MB/sec then a 2TB database would take 23 hours to reseed while an 8TB database would take almost 4 days. All this time, your availability has been reduced as one of your passive copies is unavailable until seeding completes.

Here we have two examples of how your backup requirements could potentially dictate an important core design change (database size, mailbox layout, and general performance load). Whether you implement a standalone server or a DAG utilizing traditional backups, database size plays an important part to the design and operation of the solution. Equally important is database file location, and whether database (.edb) and log (.log) files should be isolated from one another on different physical spindles. The answer is quite simple; if on a standalone server, the .edb and .log files should be on separate spindles (truly separate spindles, not separate volumes/partitions on the same RAID array/LUN) and if using a DAG they can reside on the same set of spindles. I like to explain to customers that you should never allow the failure if a single disk bring down a database, so be aware of your failure domains when placing your database files. When discussing RAID during design discussions, many will hear Microsoft discussing JBOD storage options for Exchange and forget it is only supported when utilizing a DAG and the database has three highly available (not-lagged) copies. If you have a standalone server, or a DAG deployment where your database has less than threw highly available copies, then you must still use RAID (or your underlying SAN/intelligent storage solution) for protection.

The selection of your backup solution itself and the questions you must ask. As mentioned, the backup application must be “Exchange-Aware” and fully capable of interacting with the VSS and ESE subsystems. This ensures the data is consistent and the transaction logs are properly truncated upon completion. Performing a “flat-file” backup, or simply copying the database as if it were a simplistic file on the server, will not only result in a database file you may not be able to restore from, but also a server that will likely run out of disk space as no log truncation will occur.

Lastly, it should go without saying but an untested backup an untrusted backup. I recommend regularly monitoring and restoring a backup set to an alternate location then mounting it in a Recovery Database to verify the database files can in fact be mounted. Practicing and documenting this process will make your team/customer more capable of satisfying recovery Service Level Agreements (SLAs) as well as ensure there are no flaws in the backup software itself.

Exchange Native Data Protection

Ten years ago, the idea of not performing traditional backups of an Exchange server was absurd to everyone but the Microsoft Exchange think tank in Redmond Washington. However, Office 365 as we know it would not be possible without Exchange Native Data Protection. At one time in my career, I worked for a hosted email archiving company, and it was there I learned the struggle of implementing a traditional backup solution in such an environment. Imagine customers who require email on hold be available, redundant, and easily accessible. Imagine those same customers who also require email to immediately be purged and not recoverable once it has fallen out of hold; usually because of a legal matter that has ended and they wish for the email to no longer be discoverable. Using traditional backup methodologies, how would you accomplish keeping copies of backed up data while simultaneously ensuring all remnants of a particular mail item were purged in an automated fashion? A script that touches every instance of a backup, mounts the backup, and purges the relevant items? It’s quite unfeasible and certainly impossible at the scale of Office 365 or even large on-premises companies handling their own mail retention.

This retention conundrum is one of many challenges that spurred the creation of Exchange Native Data Protection. A means of using the built-in Exchange features to create redundant copies of Exchange data, retain it for specified time periods, purge it as required, and easily search/extract it upon request. This is accomplished without the use of a traditional backup, meaning Exchange database files are never backed up using VSS or any kind of snapshot. This has the added benefit of not requiring lengthy backup and restore job times. This was another hurdle to overcome to allow Exchange to provide mailboxes 50 GB and above while not worrying about lengthy backup/restore times of such mailboxes.

The following Exchange features make Exchange Native Data Protection possible:

Single Item Recovery

Lagged Database Copies

Recovery Databases

In-Place Hold

Knowledge of how these features work is required if you are to design or support an environment which uses Exchange Native Data Protection. The DAG provides redundancy for databases, allowing up to 16 copies of each database (although 3-5 is more practical due to the bandwidth each copy consumes) on servers placed in one or more datacenters that can, in turn, be geographically dispersed. Should one database copy fail, or should one datacenter fail, an up-to-date copy of that database can be mounted automatically on another server with little to no data loss. In a way, the DAG replaces the need for RAID as a single disk failure will not make a database unavailable. Obviously, this also improves availability as even the failure if a single server will not make a database unavailable.

Single Item Recovery (SIR) ensures that items which have been deleted/purged by end users can still be recovered up to the DeletedItemRetention property of a mailbox database. For instance, if the value of DeletedItemRetention is set to 365 days, no matter what action a user takes on a message, it can still be retrieved from the Recoverable Items folder for 365 days after deletion, even if the user selects and then purges the item using the Recover Deleted Items feature. SIR thus avoids the resource-intense need to restore an entire database from backup media, just to recover one item. Single Item Recovery also allows preservation and recovery of mail in its original form, part of what provides Exchange with the ability to claim that its data is stored in an immutable format. To utilize SIR, a user must simply select the “Recover Deleted Items” option in either OWA or Outlook to recover the deleted email themselves. If the mail has already been purged, an administrator must utilize Exchange Management Shell or Exchange Admin Center to perform an eDiscovery search of the user’s mailbox. While somewhat burdensome, it is certainly easier and quicker than restoring from a traditional backup.

Normally, the replication that occurs within a DAG ensures that database copies are kept up-to-date as close as possible, in terms of content, to the active database copy. Lagged database copies represent a moving point-in-time of the data in the database. This is accomplished by establishing an artificial delay on when a database’s transaction logs are replayed into the database’s .edb file, meaning the replay queue for a lagged database copy always contains the transaction logs that lie outside the set of transactions that have taken place during the replay lag period. In effect, the lagged database copy is a snapshot of what the database looked like at the replay lag period (up to 14 days), thus providing the answer to the question of “how do we recover from logical corruption events?” or “how do we recover from massive data loss?” To activate a Lagged Database Copy, one must simply remove all transaction logs after and up to the point where the loss occurred and then activate the copy. Another use case for Lagged Copies is to restore them to a Recovery Database and extract the data that was lost from them.

A Recovery Database is similar to a Recovery Storage Group used in older versions of Exchange. Traditionally, this database is used as a location for restoring Exchange database files from an Exchange-Aware Backup. The restored database could then be mounted and the data restored as needed. This method was commonly used when needing to recover mail items that had been deleted or during various disaster recovery scenarios. To make use of a Recovery Database, simply create the Recovery Database object, copy/restore database files to its location, mount the database files, and leverage the New-MaiboxRestoreRequest Cmdlet to restore content from the Recovery Database into either the original mailbox or a mailbox of your choosing.

In-Place Hold is an evolution of Litigation Hold. Although both types of hold allow data to be kept for an indefinite period, an in-place hold can apply a filter to make the hold more granular so that only important data is kept. Either hold type can replace the need to retain backup tapes for extended periods for compliance purposes. The ability to hold data based on a time and content define query is what achieved feature parity with third party email archiving and compliance solutions.

Is Exchange Native Data Protection for everyone? Let’s look at the technical requirements of implementing it. It will require the implementation of a DAG, which itself isn’t a tall order to fill. However, to do without a traditional backup Microsoft recommends three highly available (non-lagged) database copies and an additional lagged copy for each database. This means a minimum of four Exchange servers deployed in a DAG, deployed across two physical locations as to avoid all nodes being within the same datacenter/failure domain. This will be the requirement that keeps most smaller-to-medium sized businesses away from Exchange Native Data Protection. They will instead implement traditional backups or consider moving to Office 365 where the operational complexities are Microsoft’s problems.

In my opinion, if you wish to stay on premises but are looking to offer a level of feature/capability parity to Exchange Online, then Exchange Native Data Protection should be your path forward. If you’re wishing to deliver mailboxes that will truly reach 10 GB or greater and you have a large number of users, operationally it simply isn’t practical to run traditional backups. Your backup sets will likely reach the point where they’re unable to compete in a reasonable amount of time, and you need to resort to a complex backup strategy to maintain operations. Of course, the first rule of designing an IT solution should be that it be only as complex as it needs to be, since complexity is the enemy of availability.

Disaster Recovery Overview

I classify Exchange disasters into their most common occurrences, database-level, server-level, and site/organization-level. Let us first define each of these failure scenarios. All important at-rest information in an Exchange environment can be found within a database. Therefore, the integrity and recoverability of the database is how Exchange data retention can be measured. Regardless of the Retention Policies, In-Place Holds, or Single Item Recovery settings in place, none will matter if the database is lost with no possibility for recovery. Many Exchange admins will go years without the need to completely recover a Mailbox Database, as most restoration scenarios are to restore a subset of information within a database (usually accidental deletions by end users). We call these disaster scenarios because the risk of complete loss of data within a database (potentially housing hundreds of mailboxes) and the inaccessibility of mailbox data can have a devastating effect on a business. These risks are mitigated by either performing traditional backups of the database files or implementing a DAG with a Lagged copy.

Server-Level failures are most commonly the result of either operating system or hardware failures. Failed motherboard, failed controller, no boot, file system corruption, and power failure are among the possible causes of an Exchange server-level failure. For a standalone environment, it means a complete Exchange outage, but for a properly designed multi-server environment it’s a fairly easy risk to mitigate. With a simplistic two-server DAG you achieve a measure of protection against a single server failure. For years, in the world of Microsoft enterprise administration, having a two-node Windows Failover Cluster in the same rack, connected to a shared SCSI storage enclosure was considered adequate protection against a server failure. At the very least, it afforded you the ability to perform maintenance on one server while the other handled the production workload. In the modern IT world, having multiple shared failure domains (same rack, same storage enclosure, even the same datacenter) is considered poor practice in terms of availability and data recoverability. The last 15 years’ worth of natural disasters have taught American businesses the value of redundant datacenters, preferably in different geographic regions.

Regardless of the disaster category, 20 years of Exchange Server feature enhancement have given the product many built-in capabilities to enable recovery from the worst of RGE’s (Resume Generating Events). Let us first discuss database recoverability, a topic that will be aided by our understanding of the content previously covered in this chapter regarding Exchange Database Transaction logging.

Database Recovery

An Exchange database, while running and allowing users to access the mailbox data it hosts, is said to be in a “Mounted” state. While in a “Dismounted” state, no data within the database can be accessed by end users. Exchange databases rely on the Microsoft Exchange Information Store Service to function, should the service stop, all databases on that server will dismount. Some databases on a server can be in the mounted status while others are in the dismounted status, as the state of each database is managed independently of each other. In Exchange 2010 and older there was a single process associated with the Information Store Service regardless of the number of databases on a server, which was of course the infamous store.exe process. I say infamous because unknowing admins would commonly decry the service for using almost all the RAM on the server. Of course, they didn’t realize that was exactly what it was designed to do. The job of the Exchange Database Cache (Store.exe) was to handle all transactions within each database on a server, while being as efficient as possible. Exchange database cache efficiency could be defined as doing as much in memory as possible while going to disk as seldom as possible, since doing so reduces costly IO to disk. Since Exchange assumes it is the only production application on the server (as it should be in an Enterprise environment), it sees no issue with using the majority of the RAM. Exchange 2010 would use approximately 70% of installed RAM (plus ~2 GB), so it was normal to see store.exe consuming ~70 GB of RAM on a system with 96 GB of memory. Exchange 2013 and newer instead have a separate worker process (Microsoft.Exchange.Store.Worker.exe) for each database copy present on the server.

Now understanding how the databases and their cache operates, let us focus on how this data can become corrupted. Generally speaking, there are two types of corruption that can occur within a mailbox database: physical and logical. Physical corruption is things like corrupt ESE database pages, bad checksums, missing transaction log information, or anything that can result in a “Dirty Shutdown.”

The Extensible Storage Engine (ESE) or “JET Blue” is a data storage technology from Microsoft that is used in Exchange Server, Active Directory, Branch Cache, and Windows Search among others. It’s important to understand that ESE is a physical data storage technology and exists at a level below the applications running on it. Therefore, ESE has no understanding of the data within the database and only concerns itself with organizing, accessing it, and validating its integrity. At the ESE level, there is no understanding of what an email or mailbox is; only tables, pages, rows, etc. The utility used to repair physical corruption in an ESE database is ESEUTIL, which has no understanding for the Exchange data within the .edb file, but only the physical integrity of the file. This is why it’s possible to use ESEUTIL to repair physical corruption but still have a database file that is unusable to the Exchange Server application. I’ve worked several cases where I was able to bring a database from a Dirty Shutdown to a Clean Shutdown state, yet when mounting the database it would immediately dismount. Usually the Windows Application logs will indicate that after mounting the database, the Information Store Service was unable to interpret the Exchange information within it. This is where the line between physical and logical meet. While the ESE understands the database at the physical level, the Microsoft Exchange Information Store accesses the data physically but handles all logical interpretation of the data. Corruption at the logical level is commonly leftover remnants of physical corruption. Corrupted physical database pages that removed logical references to mailbox data; typically resulting in search/display issues within the mailbox (or simply missing items).

Common causes of physical corruption:

• Hardware failure (power loss, component failure, etc.)
• Storage failure resulting in file system corruption
• Information Store Worker process crash
• Anti-V locking/corrupting .edb file
• Common causes of logical corruption:
• Aftereffects of physical corruption
• Faulty Outlook add-ins
• Faulty mobile clients

So let’s say you encounter a physical corruption event, either via hardware failure, misbehaving anti-virus, etc. You’re left with a database which is in a Dirty Shutdown state and need to understand how to proceed forward. You must first determine if the database file (.edb) is in a Clean Shutdown or Dirty Shutdown state by issuing the below command:

[PS] C:\> ESEUTIL /mh Database1.edb

If the database is listed as in a Clean Shutdown yet cannot be mounted, you should look towards Exchange/Active Directory-related issues as the culprit. The Windows Application log will typically indicate the cause. However, if state is listed as Dirty Shutdown (and attempts to mount it fail) then the cause is likely a missing transaction log file or physical corruption in the .edb file. Missing transaction logs can be detected by running the below command (I like to think of /ml as standing for “Missing Logs”, as in “go check for missing logs”), which will parse through all present transaction logs in the log stream, indicating if any are missing or damaged/corrupted.:

[PS] C:\> ESEUTIL /ml E00

If any logs are missing/damaged/corrupted, the next course of action should be to restore the .edb file from backup. The .edb file needs the transactions held within them to get into a consistent state and without them, the only course of action is a restore or an ESEUTIL /p (more on that later). Should all transaction logs be present and healthy, you may check for physical corruption to the .edb file itself as the cause of the Dirty Shutdown. This can be done by issuing using the below command to check for bad checksums:

[PS] C:\> ESEUTIL /k Database1.edb

If bad checksums are detected, the potential for recovery is unlikely. Also, it’s highly recommended to perform a chkdsk on the volume where the .edb files resided before proceeding further, to ensure the file system which the database resides on is healthy before proceeding with further recovery steps. If you’re following Microsoft’s recommendation to store Exchange databases on ReFS volumes, this step may be unneeded as it has built-in recovery mechanisms. Either way, should bad checksums be detected, the .edb file should be restored from backup. However, another option exists, the infamous ESEUTIL /p.

Running ESEUTIL /p against a .edb file is considered by many to be “the nuclear option” of database recovery. In simplistic terms, a /p will remove anything and everything from an ESE database that it does not recognize, in an effort to get it to mount. If this is merely a few missing transaction logs which were keeping the database from mounting, then there’s likely no major risk. However, what if the corrupted portions of the .edb file were part of an important table or index? I’ve performed over one hundred ESEUTIL /p operations (in classes, labs, and unfortunately production) and in 9 out of 10 scenarios, nothing drastic happens afterwards. However, I’ve also seen a 200 GB .edb file be reduced to 500 MB after running a /p, so understand the risks involved with the process. In practice, a /p should only be run when it is your last resort. You either don’t have a backup or the only one you have is corrupted. Unfortunately, if you search online for phrases like “Dirty Shutdown” or even “ESEUTIL”, often times the first result you’ll find will be a forum or blog post with someone saying “just run an ESEUTIL /p and the database should mount.” This is akin to a Doctor recommending chemotherapy for a mole after only 30 seconds of questioning over the phone. The potential destructive consequences are far too risky to be used haphazardly.

So let’s say you’ve had to resort to running a /p and the database successfully mounted. Unfortunately your work is not done, if you wish to be in a supported state. In 2015, Microsoft announced a New Support Policy for Repaired Exchange Databases. This policy states that any Exchange database which has had an ESEUTIL /p run against it must immediately be vacated (move all mailboxes to a different database) for it to be supported. By running the below command against a dismounted database, you can determine if a /p has been run against it:

[PS] C:\> ESEUTIL /mh Database1.edb

If the “Repair Count” is anything other than 0, it means a /p has been run and this database must be vacated. After moving all mailboxes from the database, the database can then be deleted.

Let’s now discuss repairing logical corruption, which may be present after a physical corruption event. Before Exchange 2010 SP1 you would use ISINTEG, an aptly named Exchange Information Store utility (not an ESE tool) used to repair logical corruption in an Exchange database. While the database needed to be mounted while the tool was run, access to it was prevented while the scan was being performed. In Exchange 2010 SP1, the New-MailboxRepairRequest Cmdlet replaced ISINTEG as the means for repairing logical corruption in mailboxes. The most beneficial thing about the Cmdlet is that it can be run against individual mailboxes, and while that mailbox will be made inaccessible to the end user during the repair, other mailboxes on the database are not affected.

The format of the Cmdlet is as follows:

[PS] C:\> New-MailboxRepairRequest -Mailbox <Mailbox Identifier> -CorruptionType <Corruption Type>

The four core corruption types to choose from (via TechNet) are:

• SearchFolder
• AggregateCounts
• ProvisionedFolder
• FolderView

However, in Exchange 2013 many other types were added:

• ReplState
• MessagePTAGCn
• MessageID
• RuleMessageClass
• Restriction Folder
• FolderACL
• UniqueMidIndex
• CorruptJunkRule
• MissingSpecialFolders
• DropAllLazyIndexes
• ImapID
• ScheduledCheck
• Extension1
• Extension2
• Extension3
• Extension4
• Extension5

These corruption types are not exposed via TechNet, as Microsoft has added several that only they utilize in Office 365. While there are no support statements related to using these corruption types, I usually recommend that only the original four be used, at least for an initial run. The status and result of the request can be monitored using the Get-MailboxRepairRequestCmdlet. As a repair request suspends access to the mailbox from all clients for the duration of its execution, it’s important to schedule it accordingly. The time needed to complete the repair will depend on the size of the mailbox and the performance of the server. I’ve personally seen a repair finish in less than a minute for a lab environment with small mailboxes, and take over an hour for larger mailboxes.

While Microsoft requires mailboxes to be vacated from a database after an ESEUTIL /p has been performed, I personally recommend, in addition, to execute a repair request with the original four corruption types against all affected mailboxes (after moving them to a new database). This can be run during off hours.

You might ask why Microsoft recommends moving mailboxes to overcome corruption events when a mailbox move is typically seen as a migration or operational activity. The answer becomes clearer when you realize what’s actually happening under the hood as a mailbox is moved. A mailbox move is really a copy event, where the raw mailbox data is extracted from the source mailbox and inserted into a new mailbox on the destination database. Once complete, the original mailbox is placed into a disconnected state where it will eventually be deleted (based on the value of the MailboxRetention parameter on the mailbox database). Think of this as you hand writing a rough draft of an essay and then writing it again in its final form, removing any errors, signs of erasure, or veering off of the lines. Instead, only the pure source material survives the process and you’re left with a clean final product. With a mailbox move, the act of moving will ignore bad/corrupted items and place mailbox data into new tables/rows/etc. within the newly created mailbox. This process removes corruption and could even be considered a sound operational practice for larger mailboxes that may be more susceptible to corruption. While it may not necessarily be required, as mentioned, I like to also run a mailbox repair request after moving a mailbox (if the purpose of the move was to remove corruption).

In practice, when troubleshooting display, search, or the inexplicable mailbox issue, I typically first recommend a repair request. If that does not resolve the issue, I then attempt to move the mailbox to a new database. In some rare cases (usually after a corruption or dirty shutdown event), a mailbox move may hang or stall. This may be caused by the mailbox becoming quarantined. One useful addition of the New-MoveRequest Cmdlet is the aptly named –ForceOffline parameter (Exchange 2013/2016 only), which forced the mailbox offline for the duration of the move; similar to how mailbox moves functioned prior to Exchange 2010. Therefore, if you encounter a mailbox move that keeps stalling or failing, and the logs (Get-MoveRequest | Get-MoveRequestStatistics –IncludeReport | FL) do not indicate anything obvious like the –BadItemLimit being exceeded, then forcing the move offline may allow it to complete.

In the worst case scenarios, you may genuinely encounter a mailbox can neither be moved nor can a repair request be successfully completed against it. In such cases, I’ve even tried running an offline defrag on the database where it resides, though that’s not a guarantee either. In these instances, you may actually need to export the mailbox data to .PST, disable the mailbox (keeping the AD user object in-tact and the mailbox in a disconnected state), creating a new “clean” mailbox, and importing the data from .PST. Obviously, a restore from backup would be preferred but I’ve encountered scenarios where this was either not possible or not practical (out of date backup usually).  The outline for this process is as follows:

• Export mailbox contents to .PST either with New-MailboxExportRequest or manually using Outlook
• Do not forget calendar items
• Disable the mailbox using the Disable-Mailbox Cmdlet
• Do not mistake Disable-Mailbox with Remove-Mailbox, which also deletes the AD user object
• Create a new mailbox for the AD user using the Enable-Mailbox Cmdlet
• Import the mailbox data from .PST using New-MailboxImportRequest

While certainly a procedure of last resort, it’s actually fairly simplistic. However, remember that in addition to retaining the data, remember to capture the settings of the mailbox as well. Custom proxy addresses, storage/transport limits, or policies must be noted and reconfigured on the mailbox afterwards.

Server Recovery

In my experience, individual server recovery is more commonly seen in smaller environments. Mostly due to no other servers being available to move databases to (DAG) and keep the environment functional. Sadly, due to inexperience and ignorance in smaller environments, they’re more likely to spend hours troubleshooting the server, even if it’s desperately too far gone through the eyes of an expert. They would be much better off, and have services restored must quicker, by simply rebuilding the box. It’s a discussion I commonly have with customers in my support role. When does it make more sense to reinstall and be done in a couple hours, vs spending many hours with support in hope that you can save the existing installation? In the famous words of Bones from Star Trek, “He’s dead Jim.”

Below are the common causes which would justify an Exchange Server recovery:

• Corrupted OS/No Boot scenario
• File system corruption resulting in application binary corruption
• Misconfiguration in underlying software (IIS/.NET/UCMA/TCPIP/etc.)
• Complete loss of server (fire/flood/storage failure/etc.)

As I covered at the beginning of this chapter, the entire operating system is not typically backed up, as the Exchange Product team has never encouraged “image-based backups.” Therefore when a server fails, the recommendation is to either move resources to another Exchange Server (assuming a DAG) and build a new server in its place, or to perform a Disaster Recovery Installation of Exchange. This is sometimes referred to as a “Recover Server Installation”, after the “Setup /m:RecoverServer” command-line switch used in the procedure. Of the various Exchange Disaster Recovery scenarios, the Disaster Recovery Installation is the most intimidating to the uneducated, namely because it reinstallation. To people who are only in charge of administering a product, the idea of formatting the operating system, installing Exchange via command-line, and restoring databases might seem daunting. However, this process is actually quite easy (assuming your Active Directory environment is healthy and your backup of the database files are consistent).

The process for performing a Disaster Recovery Installation of Exchange is as follows. This process has gone largely unchanged since Exchange 2003:

• An Exchange server installation is lost or beyond repair
• The system is reinstalled with the same Operating System and brought back to the same patch level as the previous server (DO NOT attempt to gracefully uninstall Exchange or attempt to disjoin the server from the domain; I recommend a format of the OS and Exchange application partitions)
• The Computer Account for the Exchange Server should be reset
• Configure the Computer Name on the new installation the same as the previous server
• Rejoin the computer to the domain
• Install Exchange prerequisites including the same CU version of Exchange that was previously installed (the same CU version as what was previously installed should be used)
• Open an elevated Command Prompt on the server to be recovered and run the below command:
• Setup /m:RecoverServer /IAcceptExchangeServerLicenseTerms
• Note: If Exchange is installed in a location other than the default location, you must use the /TargetDir switch to specify the location of the Exchange binary files. If you don’t use the /TargetDir switch, the Exchange files are installed in the default location (%programfiles%\Microsoft\Exchange Server\V15).
• This causes the Exchange installation to look to Active Directory and pull the Exchange Server configuration that was previously present onto this server
• Once completed and the server is restarted, any databases will be dismounted
• At this time, restore databases from backup
• Import and assign any previously installed certificates
• Apply any previously configured customizations to the Operating System and/or IIS
• The beauty of this approach is that all server-related configuration information, such as virtual directory URL’s, Outlook configuration information, and customizations will be retained. Essentially, anything configured using the Set-ExchangeServer, Set-MailboxServer, Set-TransportServer, and Set-ClientAccessServer Cmdlets or the relevant Set-*VirtualDirectory Cmdlets, among others. However, any customizations to IIS, such as HTTP redirection or web.config file settings will require reconfiguration, as those settings are not stored in Active Directory and therefore cannot be recovered. Common mistakes are trying to uninstall Exchange or disjoining the computer from the domain (assuming the installation was still reachable). This will result in the Exchange installation being removed from Active Directory and the link between the computer account and Active Directory being broken respectively.

In some scenarios, I’ve actually seen where the Exchange computer account has a trust relationship or permissions issue with Active Directory and you encounter trouble with the /RecoverServer approach. Fortunately, since Exchange 2010 and the advent of Database Portability, a Mailbox Database can be mounted on any Exchange Server in the organization. Therefore, even if you do not have a DAG in the environment, a database can be mounted on a newly installed server in a recovery scenario.

The process for this would be as follows:

• Install and patch Windows by running Windows Updates on a new server with a new name
• Install Exchange prerequisites
• Install new Exchange Server (do not use /RecoverServer option as this will be a new Exchange Server installation)
• Configure all URLs, certificates, or customizations to match previous server (most can be found using ADSIEDIT if server is no longer accessible)
• Create new Mailbox Database, dismount new database, remove all database files, copy-paste backed up (original) database files to location
• Run following command to mark new database as able to be overwritten: Set-MailboxDatabase –Identity <NewDBName> –AllFilerestore $True
• Mount database
• Re-home mailboxes to the new database (which will map the AD user accounts to the disconnected mailboxes in the restored .edb file) by running “Get-Mailbox –Database <OldDBName>| Set-Mailbox –Database <NewDBName>”
• Do the same for any Arbitration mailboxes which were housed on the database by running “Get-Mailbox –Database <OldDBName> –Arbitration | Set-Mailbox –Arbitration –Database <NewDBName>”

Using this method, services are restored within the 1-2 hours it takes to deploy a Windows OS, install Exchange binaries, and restore/copy database files. All Exchange configuration tasks are fairly straightforward, especially if scripted/automated.

This brings me back to my previous statements around troubleshooting a problematic Exchange Server vs a rebuild. I’m a strong proponent of simplicity and uptime, as I would think all administrators would be. Unfortunately, some people fall into the trap of digging too deep into the weeds, trying for hours on end to troubleshoot an issue which may have been a fluke. Maybe it’s the inner Engineer in all of us wish to get down to the bottom of every issue. However, the business ultimately cares more about uptime than our own individual curiosities. Microsoft has even admitted that in Office 365, they spend little time troubleshooting individual servers, instead opting to “log occurrence, move resources off, shoot it in the head, and rebuild via automation” as one Program Manager put it. They only begin deep troubleshooting if an issue is systemic, and avoid “diving into the weeds” on issues which may simply be a fluke. So I can only recommend practicing the above server recovery procedures in a lab so you can give accurate information to your customers/supervisors regarding your recovery options during a disaster.

About the Author:

I work for Dell as a Technical Program Manager in our Global Services organization. I work with New Product Introduction readiness and strategy, as well as serve as a technical subject matter expert for Exchange, Office 365, Azure, and other cloud technologies.

Reference:

Higginbotham, A. (2018). Exchange Server Backup, Restore, and Disaster Recovery.  Available at: https://ashdrewness.wordpress.com/2018/10/10/exchange-server-backup-restore-and-disaster-recovery/ [Accessed: 4th April 2019].