Recommended Reading: Shadow copies may be lost when you defragment a volume
http://support.microsoft.com/kb/312067/ – MS document on VSS 16KB cluster best practice
Well, after seeing System Error 25 VolSnap too many times and realizing each time it appears all Volume Shadow Copy snapshots were being dumped, I decided to dig into this. Why? Because those VSS snapshots are very valuable safety nets that users rely on. Things like redirected My Documents …etc are highly valuable and worth having an extra version to fall back to.
What I discovered is that whenever a defragmenter runs against the default 4 KB (SBS/Win2k3) cluster size these events occur. I experienced this testing out Diskeeper and then PerfectDisk (Raxco) but also see this when running the native Microsoft defragmenter. Further research shows that the cluster size needs to be 16 KB for VSS.
So next time you create a data partition my best practice advise to you is to raise the cluster size up from the default to 16 KB (or a greater size).
Although this blog was written a couple of years ago for Windows Server 2003 this still applies in the Server 2008 product. How do I know? I asked the Server 2008 VSS team in their blog at the time of launch earlier this year. What was amazing is that they gave me the impression by the tone of their response that didn’t seem to understand that this issue was relevant or significant.
Here’s the 3/24/2008 response I got from Greg Jacklin via Jim Benton:
“Although volsnap and defrag/ntfs do their best to get along, it is still
best to format with a 16K cluster size for performance.
BTW if you think your heard about this before but you can’t remember where perhaps it was via Wayne Small or Microsoft. Below are these historical references:
- Hack#95 – Work with Shadow Copies
- KB833167 – A Volume Shadow Copy Service (VSS) update package is available for Windows Server 2003
There are some other planning & “hot fix” articles from MS that address some aspects of snapshot dumping; nevertheless, remember that the root cause is that your cluster size isn’t large enough.
- Default cluster size for FAT and NTFS
- How Shadow Copies for Shared Folders Work
- Volume Shadow Copy Service Technical Reference (Oct’03)
- Best Practices for Using Volume Shadow Copy Service with Exchange Server 2003 (for larger than SBS environs)
- A Volume Shadow Copy Service (VSS) update package is available for Windows Server 2003 (Oct ’05)
- Time-out errors occur in Volume Shadow Copy service writers, and shadow copies are lost during backup and during times when there are high levels of input/output