Release Notes

31 Dell PS Series Snapshots and Clones: Best Practices and Sizing Guidelines | BP1027
Total Snapshot Reserve = (∆ snapshot 1) + (∆ snapshot 2) + (∆ snapshot 3) + … (∆ snapshot n)
For example, if 1 GB of data changes between snapshots of a volume and the administrator wishes to retain
5 snapshots, then at least 5 GB of snapshot reserve is needed. Because the data change rate may fluctuate
slightly. Allocate slightly more than 5 GB to the snapshot reserve to ensure that all five are retained.
7.3 Clones versus snapshots
Because snapshots share data with a base volume, snapshots are not a replacement for backups. If the base
volume is lost, all associated snapshots would be lost too. Snapshots can be used to facilitate more efficient
backups (to tape or disk backup targets), especially when used with application-aware frameworks like
Microsoft’s VSS, which allow the volume and application to remain online during a backup.
PS Series snapshots and clones can enhance the recovery process by allowing an administrator to quickly
rollback a volume to a specific time, or even access a single file on a snapshot or clone volume for fine grain
recovery. A typical use case for snapshots is to allow system administrators to respond to and recover from
storage loss and data corruption situations overnight. Clones can be used for longer term recovery or for test
and development purposes since they are a full independent copy of the source volume.
7.4 Potential causes of unexpected snapshot growth
When a volume is defragmented, data is moved in the file system to ensure that large files are contiguous.
This prevents the file system from having to search the disk to access a complete file. The process of
defragmenting typically results in a large amount of the volume being modified; not the actual data contained
in the files, but simply the way the files are stored. When a snapshot is performed before defragmenting, the
PS series snapshot reserve may rapidly grow to as high as 100% depending on how severely the disk is
defragmented.
Some applications also defragment or reorganize database files on a schedule. These operations can lead to
a similar effect as defragmenting the file system. Large amounts of data may be modified or deleted leading
to a higher amount of snapshot reserve being required to retain the number of desired snapshots.
Similarly, using a backup application that resets the archive bit on files also causes the file system to be
modified resulting in a greater consumption of snapshot reserve space. Many modern backup applications
use a database (or other method) to track which files have already been backed up and modified files that
need to be backed up, preventing the application from needing to modify the archive bit.
Windows 2003 systems (or systems that were upgraded from Windows 2003) may have the
NtfsDisableLastAccessUpdate feature enabled (its value in the Windows Registry set to 0). When this feature
is enabled, the NTFS file system will update the timestamp on the file when it is opened, even if it is not
modified. This results in the file system, and therefore the page of the PS Series volume, to be modified,
forcing snapshot reserve to retain the old version. Setting this value to 1 disables the feature (default for
Windows 2008 and later).
See the following Microsoft Knowledgebase article for more information:
http://technet.microsoft.com/en-us/library/cc758569(WS.10).aspx