Online and Offline Defragmentation – What it does ??
Online defragmentation is an automated defragmentation process as a part of its scheduled maintenance which we specify on a mailbox store level in ESM where the actual database size remains the same wherein Offline defragmentation is the process we use to shrink the database size removing white spaces using the eseutil /d switch.
Things to remember about online maintenance.
Does not reduce the physical size of the database
Online defragmentation runs for 15 minutes at the end of the database maintenance cycle (assuming that a task has completed).
By default, maintenance runs from midnight to 4 a.m.
There are 11 task that is perfromed by online maintenance along with the defragmentation. They are
1. Purge the indexes on the mailbox and public folder stores.
2. Perform tombstone maintenance on mailboxes and public folders.
3. Remove expired messages from the dumpster for the mailbox and public folder stores.
4. Remove expired messages from public folders.
5. Remove deleted public folders with tombstones over 180 days old.
6. Clean up message conflicts within public folders.
7. Update server version information on public folders.
8. Check for and remove duplicate site folders on public folder stores.
9. Clean up deleted mailboxes on mailbox stores.
10. Check the message table for orphaned messages (messages with a reference count of 0).
11. Perform an online defragmentation of the store.
Reduces the physical size of the database
When you run eseutil /d against a database to defragment it, the streaming file associated with the database is automatically defragmented. If you do not want the streaming file to be defragmented, include the /i option.
Time that the defragmentation will take will depend on the amount of white space in the database, as well as the size of the transactions recorded in the database, and your hardware specifications.