Warning: Shadow Copies for Shared Folders are not a replacement for backup

Shadow Copies for Shared Folders are a great feature that can give your users access to previous files in minutes, as opposed to hours or even days (as it is takes normally to reclaim a file from the tape in an corporate environment).

One word of caution though: Shadow Copies for Shared Folders (SCSF) are not a substitute for backup. The same remark applies to any other type of shadow copies created through the default shadow copy provider (volsnap.sys). So, in addition to enabling SCSF you also need to rely on the good old backup to make sure you don’t lose any data.

The reason for this warning is that the shadow copies can go away during exceptional circumstances. This doesn’t happen normally, but here are some exceptions:
1) You do not have enough free space on the Shadow Storage disk. If there is not enough free space to keep the differences between shadow copies, then old shadow copies are simply deleted to make room for the new ones. Before enabling Shadow Copies for Shared Folders, make sure that you have enough space. At the very minimum several hundred of megabytes, but several Gigabytes is probably recommended. You can also control the maximum percentage can be consumed by SCSF. You can use the VSSADMIN command or the “Shadow Copies” tab in explorer to configure this maximum percentage/space to be used, so SCSF won’t eat all your free space.
2) Disk corruption (due to the underlying hardware). While this is very unlikely to happen, it can happen any time… Nobody is immune to that, including shadow copies, which usually will get deleted automatically when a disk corruption is detected.
3) In very rare situations, on high-end systems, shadow copies for shared folders might be lost due to very high I/O activity. This issue was experienced by very few customers, and addressed by the latest VSS cumulative hotfixes: KB 833167 and KB 887827. See their associated web pages for more details. If you ever experienced the issues described in these hotfixes, you need to call Microsoft PSS to get a download location. The phone call is free.

[update: Adding an observation: the fact that shadow copies can go away applies only to shadow copies created with the default shadow copy provider. If you use a third-party shadow copy provider (hardware shadow copies for example) then you might have different lifetime guarantees.]

Comments (3)

  1. Anonymous says:

    There is an other reason to continue making the traditional back-ups: software errors can never be excluded.

    This is true for any replication mecanism. I have seen many sites go down including the back-up site because a software error caused a data base corruption on the main site which immediately caused by replication the same error to be propagated to the back-up system. That’s what called a rolling disaster.

    In fact application software companies like SAP do not recommend synchronized replication, but slightly differed replication so that if a data corruption occurs on the main site you get a chance to check what happened before applying the data replication to the back-up system.

  2. Anonymous says:

    Even human error can have a disastruous effect when a data/file replication. If you delete a very important file, in many cases there is no easy to "cancel" the replication even after you realized your mistake!

  3. Anonymous says:

    Studies have shown that human error—primarily accidental file deletion or modification—causes over one-third…