Looking at the way VMware snapshots operate, there certainly are some best practices to consider when working with snapshots:
- Snapshots are NOT a backup. Even though some storage vendors will try and sell you that story: They really are not. Remember: The base disk is still the only place of most of the information (note the stressed "only");
- Snapshots have storage impact. Having snapshots require extra IOPS, removing snapshots requires even more IOPS. Limit the number of snapshots you have, and design your storage accordingly if you plan on using snapshots (make sure you have headroom for coping with snapshot impact);
- Snapshots grow. Remove snapshots as soon as possible. Very large snapshots can take days to commit back to the base disk (with days of storage performance impact (see 2));
- Snapshots are dynamically growing files. On block systems (iSCSI, FC) without VAAI support this can cause "LUN locking"; as the VMFS file system where the vDisks live is a shared file system, growing a file on disk requires all other hosts accessing that same LUN to be write disabled for a short period of time. If you have enough snapshots growing on a LUN, at some point the system will be impacted as locking keeps occurring constantly. VMware's VAAI (specifically the atomic locking feature) will solve these issues on block storage (which is why using VAAI is almost a must in View/vCloud Director environments that utilize linked clones).
The above tip was previously published in an issue of WServerNews, a weekly newsletter from TechGenix that focuses on the administration, management and security of the Windows Server platform in particular and cloud solutions in general. Subscribe to WServerNews today by going to http://www.wservernews.com/subscribe.htm and join almost 100,000 other IT professionals around the world who read our newsletter!
Mitch Tulloch is an eleven-time recipient of the Microsoft Most Valuable Professional (MVP) award and a widely recognized expert on Windows Server and cloud computing technologies. Mitch is also Senior Editor of WServerNews. For more information about him see http://www.mtit.com.