Manage mailbox database copies

APPLIES TO: 2016 2019 Subscription Edition

In Exchange Server, you can use the Exchange Management Console (EAC) or the Exchange Management Shell to add mailbox database copies after a database availability group (DAG) is created, configured, and populated with Mailbox server members.

Managing database copies

After multiple copies of a database are created, you can use the EAC or the Exchange Management Shell to do the following tasks:

• Monitor the health and status of each copy.
• Do other management tasks associated with database copies. For example:
  • Suspend or resume a database copy.
  • Seed a database copy.
  • Monitor database copies.
  • Configure database copy settings
  • Remove a database copy.

Suspending and resuming database copies

For a variety of reasons, such as performing planned maintenance, you might need to suspend and resume continuous replication activity for a database copy. In addition, some administrative tasks, such as seeding, require that you first suspend a database copy. We recommend that you suspend all replication activity when the path for the database or its log files is being changed. You can suspend and resume database copy activity by using the EAC, or by running the Suspend-MailboxDatabaseCopy and Resume-MailboxDatabaseCopy cmdlets in the Exchange Management Shell. For detailed steps about how to suspend or resume continuous replication activity for a database copy, see Suspend or resume a mailbox database copy.

Seeding a database copy

Seeding, also known as updating, is when a blank database or a copy of the production database, is added to the target copy location on another Mailbox server in the same DAG as the active database. This database becomes the baseline database for the copy maintained by that server.

Depending on the situation, you can seed a database by using an automatic process or a manual process that you initiate. When a database copy is added, the copy is automatically seeded, if the target server and its storage are properly configured. To manually seed a database copy and don't want automatic seeding to occur when creating the copy, you can use the SeedingPostponed parameter on the Add-MailboxDatabaseCopy cmdlet.

Database copies rarely need to be reseeded after the initial seeding. However, if reseeding is necessary, or to manually seed a database copy instead of having the system automatically seed the copy, you have two options:

• Use the Update Mailbox Database Copy wizard in the EAC.
• Use the Update-MailboxDatabaseCopy cmdlet in the Exchange Management Shell.

Before seeding a database copy, you must first suspend the mailbox database copy. For detailed steps about how to seed a database copy, see Update a mailbox database copy.

After a manual seed operation is complete, replication for the seeded mailbox database copy is automatically resumed. If you don't want replication to automatically resume, you can use the ManualResume parameter on the Update-MailboxDatabaseCopy cmdlet.

Choosing what to seed

When you perform a seed operation, you can choose to:

• Seed the mailbox database copy.
• Seed the content index catalog for the mailbox database copy.
• Seed both the database copy and the content index catalog copy.

The default behavior of the Update Mailbox Database Copy wizard and the Update-MailboxDatabaseCopy cmdlet is to seed both the mailbox database copy and the content index catalog copy.

To seed just the mailbox database copy without seeding the content index catalog, use the DatabaseOnly parameter on the Update-MailboxDatabaseCopy cmdlet.

To seed just the content index catalog copy, use the CatalogOnly parameter on the Update-MailboxDatabaseCopy cmdlet.

Selecting the seeding source

You can use any healthy database copy as the seeding source for another copy of that database. This option is particularly useful when you have a DAG extended across multiple physical locations.

For example, consider the following four-member DAG deployment:

• MBX1 and MBX2 are located in Portland, Oregon.
• MBX3 and MBX4 are located in New York, New York.
• A mailbox database named DB1 is active on MBX1.
• There are passive copies of DB1 on MBX2 and MBX3.

When adding a copy of DB1 to MBX4, you can use the copy on MBX3 as the source for seeding. This option avoids seeding over the wide area network (WAN) link between Portland and New York.

To use a specific copy as a source for seeding when adding a new database copy, you can do the following steps:

• Use the SeedingPostponed parameter on the Add-MailboxDatabaseCopy cmdlet to add the database copy. Otherwise, the database copy is explicitly seeded using the active copy of the database as the source.

• You can specify the source server to use in the Update Mailbox Database Copy wizard in the EAC, or you can use the SourceServer parameter on the Update-MailboxDatabaseCopy cmdlet to specify the desired source server for seeding.

  In the previous example, you would specify MBX3 as the source server. Otherwise, the database copy is explicitly seeded from the active copy of the database.

Seeding and networks

In addition to selecting a specific source server for seeding a mailbox database copy, you can also use the Exchange Management Shell to specify which DAG networks to use. You can override the DAG network's compression and encryption settings during the seed operation.

You can specify the networks to use for seeding by using the Network parameter on the Update-MailboxDatabaseCopy cmdlet and specify the DAG networks that you want to use. If you don't use the Network parameter, the system uses the following default behavior for selecting a network to use for the seeding operation:

• If the source server and target server are on the same subnet and a replication network that includes the subnet is configured, the replication network is used.

• If the source server and target server are on different subnets, even if a replication network containing those subnets is configured, the client (MAPI) network is used for seeding.

• If the source server and target server are in different datacenters, the client (MAPI) network is used for seeding.

At the DAG level, DAG networks are configured for encryption and compression. The default settings use encryption and compression only for communications on different subnets. If the source and target are on different subnets and the DAG is configured with the default values for NetworkCompression and NetworkEncryption, you can override these values by using the NetworkCompressionOverride and NetworkEncryptionOverride parameters on the Update-MailboxDatabaseCopy cmdlet.

Seeding process

When you begin a seeding process by using the Add-MailboxDatabaseCopy or Update-MailboxDatabaseCopy cmdlets, the following tasks are performed:

1. Database properties from Active Directory are read to validate the specified database and servers, and to verify that the source and target servers are running Exchange Server, they're both members of the same DAG, and that the specified database isn't a recovery database. The database file paths are also read.

2. Preparations occur for reseed checks from the Microsoft Exchange Replication service on the target server.

3. The Microsoft Exchange Replication service on the target server checks for the presence of database and transaction log files in the file directories read by the Active Directory checks in step 1.

4. The Microsoft Exchange Replication service returns the status information from the target server to the administrative interface from where the cmdlet was run.

5. If all preliminary checks pass, you're prompted to confirm the operation before continuing. If you confirm the operation, the process continues. If an error is encountered during the preliminary checks, the error is reported and the operation fails.

6. The seed operation is started from the Microsoft Exchange Replication service on the target server.

7. The Microsoft Exchange Replication service suspends database replication for the active database copy.

8. The Microsoft Exchange Replication service updates the state information for the database to reflect a status of Seeding.

9. If the target server doesn't already have the directories for the target database and log files, they're created.

10. A TCP request to seed the database is passed from the Microsoft Exchange Replication service on the target server to the Microsoft Exchange Replication service on the source server. This request and the subsequent communications for seeding the database occur on a DAG network configured as a replication network.

11. The Microsoft Exchange Replication service on the source server initiates an Extensible Storage Engine (ESE) streaming backup via the Microsoft Exchange Information Store service interface.

12. The Microsoft Exchange Information Store service streams the database data to the Microsoft Exchange Replication service.

13. The database data is moved from the source server's Microsoft Exchange Replication service to the target server's Microsoft Exchange Replication service.

14. The Microsoft Exchange Replication service on the target server writes the database copy to a temporary directory located in the main database directory called temp-seeding.

15. The streaming backup operation on the source server ends when the end of the database is reached.

16. The write operation on the target server completes, and the database is moved from the temp-seeding directory to the final location. The temp-seeding directory is deleted.

17. On the target server, the Microsoft Exchange Replication service proxies a request to the Microsoft Exchange Search service to mount the content index catalog for the database copy, if it exists. If there are existing out-of-date catalog files from a previous instance of the database copy, the mount operation fails, which triggers the need to replicate the catalog from the source server. Likewise, if the catalog doesn't exist on a new instance of the database copy on the target server, a copy of the catalog is required. The Microsoft Exchange Replication service directs the Microsoft Exchange Search service to suspend indexing for the database copy while a new catalog is copied from the source.

18. The Microsoft Exchange Replication service on the target server sends a seed catalog request to the Microsoft Exchange Replication service on the source server.

19. On the source server, the Microsoft Exchange Replication service requests the directory information from the Microsoft Exchange Search service and requests that indexing is suspended.

20. The Microsoft Exchange Search service on the source server returns the search catalog directory information to the Microsoft Exchange Replication service.

21. The Microsoft Exchange Replication service on the source server reads the catalog files from the directory.

22. The Microsoft Exchange Replication service on the source server moves the catalog data to the Microsoft Exchange Replication service on the target server using a connection across the replication network. After the read is complete, the Microsoft Exchange Replication service sends a request to the Microsoft Exchange Search service to resume indexing of the source database.

23. If there are any existing catalog files on the target server in the directory, the Microsoft Exchange Replication service on the target server deletes them.

24. The Microsoft Exchange Replication service on the target server writes the catalog data to a temporary directory called CiSeed.Temp until the data is completely transferred.

25. The Microsoft Exchange Replication service moves the complete catalog data to the final location.

26. The Microsoft Exchange Replication service on the target server resumes search indexing on the target database.

27. The Microsoft Exchange Replication service on the target server returns a completion status.

28. The final result of the operation is passed to the administrative interface from which the cmdlet was called.

Configuring database copies

After a database copy is created, you can view and modify its configuration settings when needed. You can view some configuration information by examining the Properties page for a database copy in the EAC. You can also use the Get-MailboxDatabase and Set-MailboxDatabaseCopy cmdlets in the Exchange Management Shell to view and configure database copy settings. For example, replay lag time, truncation lag time, and activation preference order. For detailed steps about how to view and configure database copy settings, see Configure mailbox database copy properties.

Using replay lag and truncation lag options

Mailbox database copies support the use of a replay lag time and a truncation lag time, both of which are configured in minutes. Setting a replay lag time enables you to take a database copy back to a specific point in time. Setting a truncation lag time enables you to use the logs on a passive database copy to recover from the loss of log files on the active database copy. Because both of these features result in the temporary buildup of log files, using either of them affects your storage design.

Replay lag time

Replay lag time is a mailbox database copy property that specifies the amount of time, in minutes, to delay log replay for the database copy. The replay lag timer starts when a log file is replicated to the passive copy and successfully passes inspection. By delaying the replay of logs to the database copy, you have the capability to recover the database to a specific point in time in the past. A mailbox database copy configured with a replay lag time greater than zero is referred to as a lagged mailbox database copy, or simply, a lagged copy.

A strategy that uses database copies and the litigation hold features in Exchange Server can provide protection against a range of failures that would ordinarily cause data loss. However, these features can't provide protection against data loss due to logical corruption. Although logical corruption is rare, it can cause data loss. Lagged copies are designed to prevent loss of data due to logical corruption. Generally, there are two types of logical corruption:

• Database logical corruption: The database pages checksum matches, but the data on the pages is wrong logically. This situation occurs when ESE attempts to write a database page. Although the operating system returns a success message, the data is either never written to the disk or it's written to the wrong place. This condition is referred to as a lost flush. To prevent lost flushes from losing data, ESE includes a lost flush detection mechanism in the database along with a page patching feature (single page restore).

• Store logical corruption: Data is added, deleted, or manipulated in a way that the user doesn't expect. Non-Microsoft applications generally cause these cases. It's generally considered corruption in the sense that the user views it as corruption. The Exchange store considers the transaction that produced the logical corruption to be a series of valid MAPI operations. The litigation hold feature in Exchange Server provides protection from store logical corruption (because it prevents content from being permanently deleted by a user or application). However, there might be scenarios where a user mailbox becomes so corrupted that it would be easier to restore the database to a point in time prior to the corruption, and then export the user mailbox to retrieve uncorrupted data.

The combination of database copies, hold policy, and ESE single page restore leaves only the rare but catastrophic store logical corruption case. Your decision on whether to use a database copy with a replay lag (a lagged copy) depends on which non-Microsoft applications you use and your organization's history with store logical corruption.

If you choose to use lagged copies, be aware of the following implications for their use:

• The replay lag time is an administrator-configured value. By default, replay lag time is disabled.

• The replay lag time setting has a default setting of zero days, and a maximum setting of 14 days.

• Lagged copies aren't considered highly available copies. Instead, they're designed for disaster recovery purposes, to protect against store logical corruption.

• The greater the replay lag time set, the longer the database recovery process. It might take several hours to recover a database due to:

  • The number of log files to be replayed.
  • The speed at which your hardware can replay them.

• We recommend that you determine whether lagged copies are critical for your overall disaster recovery strategy. If using them is critical to your strategy, we recommend using multiple lagged copies, or using a redundant array of independent disks (RAID) to protect a single lagged copy, if you don't have multiple lagged copies. If you lose a disk or if corruption occurs, you don't lose your lagged point in time.

• Lagged copies can't be patched with the ESE single page restore feature. If a lagged copy encounters database page corruption (for example, a -1018 error), the copy needs to be reseeded. Reseeding loses the lagged aspect of the copy.

If you want the database to replay all log files and make the database copy current, then activating and recovering a lagged mailbox database copy is an easy process. If you want to replay log files up to a specific point in time, the process is more difficult because you have to manually manipulate log files and run Exchange Server Database Utilities (Eseutil.exe).

For detailed steps about how to activate a lagged mailbox database copy, see Activate a lagged mailbox database copy.

Truncation lag time

Truncation lag time is the property of a mailbox database copy that specifies the time in minutes to delay log deletion for the database copy after the log file has been replayed into the database copy. The truncation lag timer starts when a log file has been replicated to the passive copy, successfully passed inspection, and has been successfully replayed into the copy of the database. By delaying the truncation of log files from the database copy, you have the capability to recover from failures that affect the log files for the active copy of the database.

Database copies and log truncation

Log truncation works the same in Exchange 2016 and Exchange 2019 as it did in Exchange 2010. Truncation behavior is determined by the replay lag time and truncation lag time settings for the copy.

The following criteria must be met for a database copy's log file to be truncated when lag settings are left at their default values of 0 (disabled):

• The log file is successfully backed up or circular logging is enabled.
• The log file must be below the checkpoint (the minimum log file required for recovery) for the database.
• All other lagged copies inspected the log file.
• All other copies (except lagged copies) replayed the log file.

The following criteria must be met for truncation to occur for a lagged database copy:

• The log file must be below the checkpoint for the database.
• The log file must be older than ReplayLagTime + TruncationLagTime.
• The log file is truncated on the active copy.

In Exchange Server, log truncation doesn't occur on an active mailbox database copy when one or more passive copies are suspended. If planned maintenance activities are going to take an extended period of time (for example, several days), you might have considerable log file buildup. To prevent the log drive from filling up with transaction logs, you can remove the affected passive database copy instead of suspending it. When the planned maintenance is completed, you can re-add the passive database copy.

Exchange Server now has a feature called loose truncation that is disabled by default. During normal operations, each database copy keeps logs that need to be shipped to other database copies until all copies of a database confirm:

• They replayed the log files (passive copies).
• They received the log files (lagged copies) .

This behavior is the default log truncation behavior. If a database copy goes offline for some reason, the log files begin accumulating on the disks used by the other copies of the database. If the affected database copy remains offline for an extended period, this can cause the other database copies to run out of disk space.

Truncation behavior is different when loose truncation and circular logging are enabled. Each database copy tracks its own free disk space and applies loose truncation behavior if free space gets low.

• For the active copy, the oldest straggler (the passive database copy that is farthest behind in log replay) is ignored and truncation respects the oldest remaining passive copies. The active database copy is where global truncation is calculated.

• For a passive copy, if space gets low, it independently truncates its log files using the configured parameters described later in the Registry Value table. The passive copies attempt to respect the truncation decision made on the active copy. Despite the implication of the name MinCopiesToProtect, Exchange only ignores the oldest known straggler at the time truncation is run.

When the offline database is brought back online, its missing log files deleted from the other healthy copies, and its database copy status is FailedAndSuspended. In this event, if Autoreseed is configured, the affected copy is automatically reseeded. If Autoreseed isn't configured, the database copy needs to be manually seeded by an administrator.

If circular logging is disabled, loose truncation respects any backups taken. Loose truncation doesn't remove log files that aren't backed up.

Truncation is a recommended feature for preferred architecture where backups aren't used and circular logging is enabled.

The required number of healthy copies, the free disk space threshold, and the number of logs to keep are all configurable parameters. By default, the free disk space threshold is 204800 MB (200 GB), and the number of logs to keep is 100,000 (100 GB) for passive copies, and 10,000 (10 GB) for active copies.

Enabling loose truncation and configuring loose truncation parameters is performed by editing the Windows registry on each DAG member. There are three registry values that can be configured, that are all stored under HKLM\Software\Microsoft\ExchangeServer\v15\BackupInformation. The BackupInformation key the following DWORD values don't exist by default and must be manually created. The DWORD registry values under BackupInformation are described in the following table:

When using the LooseTruncation_MinLogsToProtect registry value, the behavior is different for active and passive database copies

• Active: The number of extra logs retained preceding those logs required by the protected passive copies and the required range of the active copy.
• Passive: The number of logs maintained from the latest available log. One tenth of this number is also used to maintain logs before the required range of this passive copy.

The two limits are in place to ensure that lagged database copies don't take up too much space, since their required range is typically very large.

Database activation policy

There are scenarios where you might want to create a mailbox database copy and prevent the system from automatically activating that copy. For example, after a failure:

• You deploy one or more mailbox database copies to an alternate or standby datacenter.
• You configure a lagged database copy for recovery purposes.
• You're doing maintenance or a server upgrade.

In each of the preceding scenarios, you have database copies that you don't want the system to activate automatically. To prevent the system from automatically activating a mailbox database copy, you can configure the copy to be blocked (suspended) for activation.

This configuration allows the system to maintain the currency of the database through log shipping and replay, but prevents the system from automatically activating and using the copy. An administrator must manually activate copies blocked for activation.

You can set the DatabaseCopyAutoActivationPolicy parameter to Blocked for:

• An entire server by using the Set-MailboxServer cmdlet.
• An individual database copy by using the Set-MailboxDatabaseCopy cmdlet.

For more information about configuring database activation policy, see Configure activation policy for a mailbox database copy.

Effect of mailbox moves on continuous replication

On a very busy mailbox database with a high log generation rate, there is a greater chance for data loss if replication to the passive database copies can't keep up with log generation. One scenario that can introduce a high log generation rate is mailbox moves. Exchange Server includes a Data Guarantee API that's used by services such as the Exchange Mailbox Replication service (MRS) to check the health of the database copy architecture based on the value of the DataMoveReplicationConstraint parameter that was set by the system or an administrator. Specifically, the Data Guarantee API can be used to:

• Check replication health: Confirms that the prerequisite number of database copies is available.

• Check replication flush: Confirms that the required log files are replayed against the prerequisite number of database copies.

When executed, the API returns the following status information to the calling application:

• Retry: Signifies that there are transient errors that prevent a condition from being checked against the database.

• Satisfied: Signifies that the database meets the required conditions or the database isn't replicated.

• NotSatisfied: Signifies that the database doesn't meet the required conditions. In addition, information is provided to the calling application as to why the NotSatisfied response was returned.

The value of the DataMoveReplicationConstraint parameter for the mailbox database determines how many database copies should be evaluated as part of the request. The DataMoveReplicationConstraint parameter has the following possible values:

• None: When you create a mailbox database, this value is set by default. When this value is set, the Data Guarantee API conditions are ignored. This setting should be used only for mailbox databases that aren't replicated.

• SecondCopy: This is the default value when you add the second copy of a mailbox database. When this value is set, at least one passive database copy must meet the Data Guarantee API conditions.

• SecondDatacenter: When this value is set, at least one passive database copy in another Active Directory site must meet the Data Guarantee API conditions.

• AllDatacenters: When this value is set, at least one passive database copy in each Active Directory site must meet the Data Guarantee API conditions.

• AllCopies: When this value is set, all copies of the mailbox database must meet the Data Guarantee API conditions.

Check Replication Health

When the Data Guarantee API is executed to evaluate the health of the database copy infrastructure, several items are evaluated.

In all scenarios, the passive database copy must meet the following conditions:

• Be healthy.

• Have a replay queue within 10 minutes of the replay lag time.

• Have a copy queue length less than 10 logs.

• Have an average copy queue length less than 10 logs. The average copy queue length is computed based on the number of times the application queried the database status.

Check Replication Flush

The Data Guarantee API can also be used to validate that a prerequisite number of database copies have replayed the required transaction logs. This is verified by comparing the last log replayed timestamp with that of the calling service's commit timestamp (in most cases, this is the timestamp of the last log file that contains required data) plus an extra five seconds (to deal with system time clock skews or drift). If the replay timestamp is greater than the commit timestamp, the DataMoveReplicationConstraint parameter is satisfied. If the replay timestamp is less than the commit timestamp, the DataMoveReplicationConstraint isn't satisfied.

Before moving large numbers of mailboxes to or from replication databases within a DAG, we recommend that you configure the DataMoveReplicationConstraint parameter on each mailbox database according to the following table:

Balancing database copies

Due to the inherent nature of DAGs, as the result of database switchovers and failovers, active mailbox database copies change hosts several times throughout a DAG's lifetime. As a result, DAGs can become unbalanced in terms of active mailbox database copy distribution. The following table shows an example of a DAG that has four databases with four copies of each database (for a total of 16 databases on each server) with an uneven distribution of active database copies.

In the preceding example, there are four copies of each database, and therefore, only four possible values for activation preference (1, 2, 3, or 4). The Preference count list column shows the count of the number of databases with each of these values. For example, on EX3, there are 13 database copies with an activation preference of 1, two copies with an activation preference of 2, one copy with an activation preference of 3, and no copies with an activation preference of 4.

As you can see, this DAG isn't balanced in terms of the number of active databases hosted by each DAG member, the number of passive databases hosted by each DAG member, or the activation preference count of the hosted databases.

You can use the RedistributeActiveDatabases.ps1 script to balance the active mailbox databases copies across a DAG. This script moves databases between their copies in an attempt to have an equal number of mounted databases on each server in DAG. If required, the script also attempts to balance active databases across sites.

The script provides two options for balancing active database copies within a DAG:

• BalanceDbsByActivationPreference: When this option is specified, the script attempts to move databases to their most preferred copy (based on activation preference) without regard to the Active Directory site.

• BalanceDbsBySiteAndActivationPreference: When this option is specified, the script attempts to move active databases to their most preferred copy, while also trying to balance active databases within each Active Directory site.

After running the script with the first option, the preceding unbalanced DAG becomes balanced, as shown in the following table.

As shown in the preceding table, this DAG is now balanced in terms of number of active and passive databases on each server and activation preference across the servers.

The following table lists the available parameters for the RedistributeActiveDatabases.ps1 script.

RedistributeActiveDatabases.ps1 examples

This example shows the current database distribution for a DAG, including preference count list.

RedistributeActiveDatabases.ps1 -DagName DAG1 -ShowDatabaseDistributionByServer | Format-Table

This example redistributes and balances the active mailbox database copies in a DAG using activation preference without prompting for input.

RedistributeActiveDatabases.ps1 -DagName DAG1 -BalanceDbsByActivationPreference -Confirm:$False

This example redistributes and balances the active mailbox database copies in a DAG using activation preference, and produces a summary of the distribution.

RedistributeActiveDatabases.ps1 -DagName DAG1 -BalanceDbsByActivationPreference -ShowFinalDatabaseDistribution

Monitoring database copies

You can view a variety of information, including copy queue length, replay queue length, status, and content index state information, by examining the details of a database copy in the EAC. You can also use the Get-MailboxDatabaseCopyStatus cmdlet in the Exchange Management Shell to view a variety of status information for a database copy.

For more information about monitoring database copies, see Monitor database availability groups.

Removing a database copy

A database copy can be removed at any time by using the EAC or by using the Remove-MailboxDatabaseCopy cmdlet in the Exchange Management Shell. After removing a database copy, you must manually delete any database and transaction log files from the server from which the database copy is being removed. For detailed steps about how to remove a database copy, see Remove a mailbox database copy.

Database switchovers

The Mailbox server that hosts the active copy of a database is referred to as the mailbox database master. The process of activating a passive database copy changes the mailbox database master for the database and turns the passive copy into the new active copy. This process is called a database switchover. In a database switchover, the active copy of a database is dismounted on one Mailbox server and a passive copy of that database is mounted as the new active mailbox database on another Mailbox server. When performing a switchover, you can optionally override the database mount dial setting on the new mailbox database master.

You can quickly identify which Mailbox server is the current mailbox database master by reviewing the right-hand column under the Database Copies tab in the EAC. You can perform a switchover by using the Activate link in the EAC, or by using the Move-ActiveMailboxDatabase cmdlet in the Exchange Management Shell.

There are several internal checks performed before a passive copy is activated. In some cases, the database switchover is blocked or canceled. In other cases, you can use cmdlets to move or skip over some checks.

• The status of the database copy is checked. If the database copy is in a failed state, the switchover is blocked. You can override this behavior and bypass the health check by using the SkipHealthChecks parameter of the Move-ActiveMailboxDatabase cmdlet. This parameter lets you move the active copy to a database copy in a failed state.

• The active database copy is checked to see if it's currently a seeding source for any passive copies of the database. If the active copy is currently being used as a source for seeding, the switchover is blocked. You can override this behavior and bypass the seeding source check by using the SkipActiveCopyChecks parameter of the Move-ActiveMailboxDatabase cmdlet. This parameter allows you to move an active copy that's being used as a seeding source. Using this parameter causes the seeding operation to be cancelled and considered failed.

• The copy queue and replay queue lengths for the database copy are checked to ensure their values are within the configured criteria. Also, the database copy is verified to ensure that it isn't currently in use as a source for seeding. If the values for the queue lengths are outside the configured criteria, or if the database is currently used as a source for seeding, the switchover is blocked. You can override this behavior and bypass these checks by using the SkipLagChecks parameter of the Move-ActiveMailboxDatabase cmdlet. This parameter allows a copy to be activated that has replay and copy queues outside of the configured criteria.

• The state of the search catalog (content index) for the database copy is checked. If the search catalog isn't up to date, is in an unhealthy state, or is corrupt, the switchover is blocked. You can override this behavior and bypass the search catalog check by using the SkipClientExperienceChecks parameter of the Move-ActiveMailboxDatabase cmdlet. This parameter causes this search to skip the catalog health check. If the search catalog for the database copy you're activating is in an unhealthy or unusable state and you use this parameter to skip the catalog health check and activate the database copy, you need to either crawl or seed the search catalog again.

When you perform a database switchover, you also have the option of overriding the mount dial settings configured for the server that hosts the passive database copy being activated. Using the MountDialOverride parameter of the Move-ActiveMailboxDatabase cmdlet instructs the target server to override its own mount dial settings and use the settings specified by the MountDialOverride parameter.

For detailed steps about how to perform a switchover of a database copy, see Activate a mailbox database copy.