The Exchange Server Version Store keeps an in-memory list of modifications made to the database. It provides ESE the ability to track and manage the current transactions (A transaction is a series of operation that are treated as indivisible). In other word Version Store is where transactions are held in memory until they can be written to disk.

If suppose that transaction need to rollback it looks in the version store to get the list of operations it performed and it rollback the transaction by performing the inverse of all the operations. Suppose that if two different sessions try to modify the same record, the version store notice and reject the second modification.

Some of the Exchange Server VERSION STORE ERRORS

• Information Store (3852) c69f3875-ef9c-4152-8d85-e636f17c4d2f: The version store for this instance (0) has reached its maximum size of 108Mb. It is likely that a long-running transaction is preventing cleanup of the version store and causing it to build up in size.
• Event 623 is the result error. As a result of that version store no longer reaps deleted records causing unneeded data, which is marked as deleted, to gather in the database.
• 623 events indicate that the maximum Version Store size has been reached because of unresponsive transaction
• Storage Group not responding.
• The version store cannot grow because it is receiving out-of-memory errors from the operating system.
• Event ID 624, 1022, 1097 are a result of Version Store entries not getting cleaned up, either not at all or not in a timely enough fashion.

This can happen for one of the two reasons:

•  Version Store cleanup simply cant keep up with the load on the machine.

•  In order to properly settle 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.

You can determine the size of the Version Store by the msExchESEParamMaxVerPages on the "Storage Group" object in Active Directory. Possible Causes for this would be Backups running at peak times, Disk I/O performance, Large Mails, Online Maintenance Tasks running at peak times. Any of these can add up simultaneously and add to the performance degrade of the server.

You can use Exchange server repair and recovery tool to short out the problem of version store and to enhance the performance of Exchange server.