Using a Snapshot File System for Backup

After a snapshot file system is created, the snapshot maintains a consistent backup of data in the snapped file system.

Backup programs (such as cpio) that back up a standard file system tree can be used without modification on a snapshot file system because the snapshot presents the same data as the snapped file system. Backup programs (such as vxdump) that access the disk structures of a file system require some modifications to handle a snapshot file system.

VxFS utilities recognize snapshot file systems and modify their behavior so that they operate the same way on snapshots as they do on standard file systems. Other backup programs that typically read the raw disk image cannot work on snapshots without altering the backup procedure.

These other backup programs can use the fscat command to obtain a raw image of the entire file system that is identical to an image obtainable by running a dd command on the disk device containing the snapped file system at the exact moment the snapshot was created. The snapread ioctl takes arguments similar to those of the read system call and returns the same results that are obtainable by performing a read on the disk device containing the snapped file system at the exact time the snapshot was created. In both cases, however, the snapshot file system provides a consistent image of the snapped file system with all activity complete---it is an instantaneous read of the entire file system. This is much different than the results that would be obtained by a dd or read command on the disk device of an active file system.

If you create a complete backup of a snapshot file system using a utility such as vxdump and later restore it, you must run the fsck command on the restored file system because the snapshot file system is consistent, but not clean. That is, the file system may have some extended inode operations to complete, but there should be no other changes. Because a snapshot file system is not writable, it cannot be fully checked, but the fsck -n command can be used to report any inconsistencies.