Using a Snapshot File System for Backup

Once a snapshot file system is created, it can be used to perform a consistent backup of the snapped file system. Backup programs that function using the standard file system tree (such as cpio) can be used without modification on a snapshot file system, since the snapshot presents the same data as the snapped file system. Backup programs that access the disk structures of a VxFS file system need to be snapshot-aware to work correctly with a snapshot file system. The VxFS utilities understand snapshot file systems and make suitable modifications in their behavior so that their operation on a snapshot file system is indistinguishable from that on a normal file system.

Other backup programs that normally read the raw disk image cannot work on snapshots without modification. These programs can use the fscat command to obtain a raw image of the entire file system identical to that which would have been obtained by a dd of 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 as would have been obtained 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 a marked contrast to the results that would be obtained by a dd or read of the disk device of an active file system.

If a complete backup of a snapshot file system is made through a utility such as vxdump and is later restored, it will be necessary to fsck the restored file system because the snapshot file system is only consistent and not clean. The file system may have some extended inode operations that must be completed, though there should be no other changes. Since the snapshot file system is not writable, it cannot be fully checked. However, the fsck -n command can be used to report any inconsistencies.

Creating a Snapshot File System

A snapshot file system is created by using the -o snapof= option of the mount command. The -o snapsize= option may also be required if the device being mounted does not identify the device size in its disk label, or if a size smaller than the entire device is desired. Use the following syntax to create a snapshot file system:

mount -F vxfs -o snapof=special|mount_point,snapsize=snapshot_size \ snapshot_special snapshot_mount_point

The snapshot file system must be created large enough to hold any blocks on the snapped file system that may be written to while the snapshot file system exists. If a snapshot file system runs out of blocks to hold copied data, it will be disabled and all further access to the snapshot file system will fail.

During a period of low activity when the system is relatively inactive (for example, on nights and weekends), the snapshot only needs to contain two to six percent of the blocks of the snapped file system. During a period of higher activity, the snapshot of an "average" file system might require 15 percent of the blocks of the snapped file system, though most file systems do not experience this much turnover of data over an entire day. These percentages tend to be lower for larger file systems and higher for smaller ones. You should manage the blocks allocated to the snapshot based on such things as file system usage and duration of backups.

	CAUTION: Any existing data on the disk used for the snapshot is overwritten and lost.

	NOTE: A snapshot file system ceases to exist when unmounted. If remounted, it will be a fresh snapshot of the snapped file system. A snapshot file system must be unmounted before the corresponding snapped file system can be unmounted. If you try to unmount a file system that has an existing snapshot, umount will fail with a "Device Busy" message, and fuser will not indicate the problem.

Making a Backup

Here are some typical examples of making a backup of a 300,000 block file system named /home (which exists on disk /dev/dsk/c0t0d0) using a snapshot file system on /dev/dsk/c0t1d0 with a snapshot mount point of /backup/home:

• To back up files changed within the last week using cpio:

  # mount -F vxfs -o snapof=/dev/dsk/c0t0d0, \ snapsize=100000 /dev/dsk/c0t1d0 /backup/home # cd /backup # find home -ctime -7 -depth -print | \ cpio -oc > /dev/rmt/0m # umount /backup/home

• To do a full backup of /dev/dsk/c0t0d0 and use dd to control blocking of output onto tape device using vxdump:

  # vxdump f - /dev/rdsk/c0t0d0 | dd bs=128k > /dev/rmt/0m

• To do a level 3 backup of /dev/dsk/c0t0d0 and collect those files that have changed in the current directory:

  # vxdump 3f - /dev/rdsk/c0t0d0 | vxrestore -xf -

• To do a full backup of a snapshot file system:

  # mount -o snapof=/dev/dsk/c0t0d0,snapsize=100000 \ /dev/dsk/c0t1d0 /backup/home # vxdump f - /dev/rdsk/c0t1d0 | dd bs=128k > /dev/rmt/0m

The vxdump utility will ascertain whether /dev/rdsk/c0t1d0 is a snapshot mounted as /backup/home and do the appropriate work to get the snapshot data through the mount point.