During the migration of a large Exchange store from one machine to another, the information store was suddenly unavailable! Checking the event logs, the first error reported was Event 1159:
Event Type: Error
Event Source: MSExchangeIS
Event Category: General
Event ID: 1159
Description: Database error 0xfffffd9a occurred in function JTAB_BASE::EcUpdate while accessing the database "<DatabaseName>".
The store was manually remounted ok and everything was back to normal, but why did it dismount in the first place?
It turns out this can happen if you’re moving mailboxes when a Backup is taking place! Unfortunately moving mailboxes and backups are both processes performed out of hours, so are likely to coincide.
What happens is that there is a limit of 1008 ESE (Extensible Storage Engine) transaction log files. When the number of uncommitted ESE log files reaches 1008, Exchange dismounts all of the information stores in the storage group!
The suggested solution – don’t run backups and large mailbox transfers at the same time!
There’s a knowledge base article here explaining the problem: