When you run Disk Defragmenter on a volume with shadow copies activated, all or some of your shadow copies may be lost, starting with the oldest shadow copies.

After the shadow copies are lost, events that are similar to the following may be logged in the System log:

Event Type: Warning
Event Source: VolSnap
Event Category: None
Event ID: 24 User: N/A
Computer: production
Description: There was insufficient disk space on volume C: to grow the shadow copy storage for shadow copies of C:. As a result of this failure all shadow copies of volume C: are at risk of being deleted.
For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.
Data: 0000: 00000000 00580003 00000000 80060018 0010: 00000002 c000007f 0000003f 00000000 0020: 00000000 00000000 You may have several event ID 33 logged both before and after the event ID 24 is logged.

Event Type: Information
Event Source: VolSnap
Event Category: None
Event ID: 33 User: N/A
Computer: production
Description: The oldest shadow copy of volume C: was deleted to keep disk space usage for shadow copies of volume C: below the user defined limit.
For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.
Data: 0000: 00000000 00580002 00000000 40060021 0010: 00000001 00000000 00000040 00000000 0020: 00000000 00000000

The System Shadow Copy provider uses a copy-on-write mechanism that operates at a 16-KB block level. This is independent of the file system's cluster allocation unit size. If the file system's cluster size is smaller than 16 KB, the System Shadow Copy provider cannot easily determine that disk defragmentation I/O is different from typical write I/O, and performs a copy-on-write operation. This might cause the Shadow Copy storage area to grow very quickly. If the storage area reaches its user-defined limit, the oldest shadow copies are deleted first.

If the file system's cluster size is 16 KB or larger, the provider can recognize disk defragmentation I/O and handle it correctly. Microsoft recommends that you use a 16-KB or larger cluster allocation unit size when you format the volume if you plan to defragment volumes that are used for shadow copies of shared folders, .