Transaction log replay: The foundation for DAG replication in C#.NET

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Transaction log replay: The foundation for DAG replication
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Two consequences usually result from the heavy load generated on the target server . First, all of the servers that host copies of the target database will be very busy and users might experience performance degradation until the mailbox moves taper off toward the end . Second, Exchange might put the mailbox move requests into a StalledDueToHA status and not resume the moves until server load decreases and the replication queues clear . You can see the StalledDueToHA status by using the G et-MoveRequestStatistics cmdlet to interrogate the current move queue . The stalled status indicates that Exchange has had to back off processing of move requests until the replay queue clears to reduce the number of outstanding transaction logs and ensure the preservation of high availability . For these reasons, it is important to schedule mailbox moves at a time when server load is light and the servers are able to cope with the demand generated by moving data from source to target servers . In addition, you should spread the load of mailbox move requests so that no one database becomes the target of more than three or four concurrent moves .
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It s important to know that EMC shows a static snapshot of replication activity that is valid only for the time that EMC last fetched information about a database. To know what the current activity is, you have to click the Refresh option to force EMC to execute the Get-MailboxDatabase Status command and retrieve updated status information about the selected database. Sometimes it is more convenient to see what s happening with replication using the Get-MailboxDatabaseCopyStatus cmdlet.
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Get-MailboxDatabaseCopyStatus Identity 'DB1\ExServer2'
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Name --------DB1\ExServer2
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Status -----Healthy
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CopyQueue Length ----------1
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ReplayQueue Length ----------3
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LastInspectedLogTime ------------------3/15/2010 9:23:10 AM
ContentIndex State -------------Healthy
Apart from noting that the copy and replay queue lengths are within acceptable limits and that the last inspected log is reasonably recent (at times of low user activity, replication activity can be limited to the log files generated every 15 minutes as a result of log roll ), the other interesting item that we see from this output is that the content index is functioning properly for the copy. This indicates that items are being properly indexed as transaction logs are replayed into the database copy. It s important that context indexing works as the Active Manager takes the health of the index into account when it considers which copies are candidates for activation following an outage.
8
444 8 Exchange s Search for High Availability
Before the Information Store replays transaction logs into a database copy, it verifies the checksum and database signature of the logs to ensure that they are valid. This is equivalent to running the ESEUTIL against the log using the /K switch. If the checksum fails, Exchange recopies that log. If Exchange fails to copy and validate the log a third time, it is an indication that something very bad is happening within the storage subsystem to cause a physical corruption and you might need to reseed the database.
Many problems will be limited to a single mailbox
It s worth emphasizing at this point that the way Exchange 2010 performs continuous background maintenance to fix problematic pages in the database coupled with the validation of transaction logs during the replication process does a good job of preventing physical corruption arising from storage problems from infecting multiple database copies . If a page becomes corrupt and cannot be fixed by background maintenance, it will be detected when the replication process attempts to validate the transaction log that contains the corrupted page . It s possible that logical corruption caused by a software bug might sneak through, as this might not result in a bad checksum . If this happens, the new schema makes it likely that the problem will affect a single mailbox (in the past, a logical corruption in a table could affect many mailboxes) and you can recover from a backup or a lagged database copy .
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