Monday, July 21, 2008

Storage Group not responding | Version Store Issues

This happend few days earlier on our exchange server... all users in a particular Storage Group were not able to connect to Outlook/OWA. The mails queued up for them on the server...
On looking at the server logs... there were lot of logon error messages... users were not able to logon to their mailbox... and at the beginning when it all started, there was one error message with Event Id 623. It says:

Information Store (2984) Storage Group 1: The version store for this instance (3) has reached its maximum size of 155MB. It is likely that a long running transaction is preventing cleanup of the version store and causing it to build up in size. Updates will be rejected until long-running transaction has been completely committed or rolled back.


The Version Store keeps an in-memory list of modifications made to the database. It gives ESE the ability to track and manage the current transactions. Thus the Version Store is where transactions are held in memory until they can be written to disk.

Event 623 is the result, typically of a long running transaction. The result of this long running transaction is to exhaust resources in the version store. As a result, the Version Store no longer reaps deleted records causing unneeded data, which is marked as deleted, to accumulate in the database. The accumulation of unneeded data can exacerbate performance problems which can lead to event id 623. No more transaction can continue until this is clear.

Thus we will see 623 event indicating that the maximum Version Store size has been reached . All the Write operations to the database will fail because there's no more version store space to record the operation.

Why this happens

This can happen for one of the two reasons:

1) In order to properly reconcile write-conflicts and properly support repeatable reads, a given entry in the version store cannot be cleaned up until it is older that the oldest active transaction.
2) Version Store cleanup simply cant keep up with the load on the machine.

Possible Causes

- Online Maintenance Tasks running at peak times.
- Backups running at peak times.
- Disk I/O performance.
- Large Mails

Any of these can add up simultaneously and add to the performance degrade of the server.

More details and troubleshooting on this.


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