HP JFS 3.3 and HP OnLineJFS 3.3 VERITAS File System 3.3 System Administrator's Guide

90 Chapter4
Online Backup
Performance of Snapshot File Systems
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.