Veritas File System 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Online Backup Using File System Snapshots
Performance of Snapshot File Systems
Chapter 6106
Performance of Snapshot File Systems
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, however, 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 application running an online transaction processing (OLTP)
workload on a snapped file system was measured at about 15 to 20 percent slower than a file system that was
not snapped.