Veritas Storage Foundation 5.1 SP1 Advanced Features Administrator"s Guide (5900-1503, April 2011)

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.
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
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.
About snapshot file system disk structure
A snapshot file system consists of:
■ A super-block
■ A bitmap
■ A blockmap
■ Data blocks copied from the snapped file system
The following figure shows the disk structure of a snapshot file system.
Administering snapshot file systems
Snapshot file system performance
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