Veritas™ File System 5.0.1 Administrator's Guide

To create a backup using a snapshop file system

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

# mount -F vxfs -o snapof=/home,snapsize=100000 \

/dev/vx/dsk/fsvol/vol1 /backup/home

# cd /backup

# find home -ctime -7 -depth -print | cpio -oc > /dev/rmt/0m

# umount /backup/home

To do a level 3 backup of /dev/vx/dsk/fsvol/vol1 and collect those files

that have changed in the current directory:

# vxdump 3f - /dev/vx/dsk/fsvol/vol1 | vxrestore -xf -

To do a full backup of /home, which exists on disk /dev/vx/dsk/fsvol/vol1,

and use dd to control blocking of output onto tape device using vxdump:

# mount -F vxfs -o snapof=/home,snapsize=100000 \

/dev/vx/dsk/fsvol/vol1 /backup/home

# vxdump f - /dev/vx/dsk/fsvol/vol1 | dd bs=128k > \

/dev/rmt/0m

Snapshot file system performance

Snapshot file systems maximize the performance of the snapshot at the expense

of writes to the snapped file system. Reads from a snapshot file system typically

perform at nearly the throughput rates of reads from a standard VxFS file system.

The performance of reads from the snapped file system are generally not affected.

However, writes to the snapped file system, typically average two to three times

as long as without a snapshot. This is because the initial write to a data block

requires reading the old data, writing the data to the snapshot, and then writing

the new data to the snapped file system. If there are multiple snapshots of the

same snapped file system, writes are even slower. Only the initial write to a block

experiences this delay, so operations such as writes to the intent log or inode

updates proceed at normal speed after the initial write.

Reads from the snapshot file system are impacted if the snapped file system is

busy because the snapshot reads are slowed by the disk I/O associated with the

snapped file system.

The overall impact of the snapshot is dependent on the read to write ratio of an

application and the mixing of the I/O operations. For example, a database

Online backup using file system snapshots

Snapshot file system performance