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 will typically perform at nearly the throughput of reads from a normal VxFS file system, allowing backups to proceed at the full speed of the VxFS file system.

The performance of reads from the snapped file system should not be affected. Writes to the snapped file system, however, typically average two to three times as long as without a snapshot, since the initial write to a data block now requires a read of the old data, a write of the data to the snapshot, and finally the write of the new data to the snapped file system. If multiple snapshots of the same snapped file system exist, writes will be even slower. Only the initial write to a block suffers this penalty, however, so operations like 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, since the snapshot reads are slowed by all of 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. As an example, Oracle running an OLTP workload on a snapped file system was measured at about 15 to 20 percent slower than a file system that was not snapped.