VERITAS File SystemÖ 3.5 (HP OnlineJFS/JFS3.5) AdministratorÆs Guide (December 2002)

Chapter 6

Online Backup

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, allowing backups to proceed at the full speed of the standard file system.

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

Differences Between Snapshots and Storage Checkpoints

While snapshots and Storage Checkpoints both create a point-in-time image of a file system and only the

changed data blocks are updated, there are significant differences between the two technologies:

• Snapshots require a separate device for storage. Storage Checkpoints reside on the same device as the

original file system.

• Snapshots are read-only. Storage Checkpoints can be read-only or read-write.

• Snapshots are transient. Storage Checkpoints are persistent.

• Snapshots cease to exist after being unmounted. Storage Checkpoints can exist and be mounted on their

own

• Snapshots track changed blocks on the file system level. Storage Checkpoints track changed blocks on

each file in the file system.

• Although there can be more than one snapshot of a file system, they are all based on a single, parent file

system. Storage Checkpoints can be based on other Storage Checkpoints.

Storage Checkpoints also serve as the enabling technology for two other VERITAS features: Block-Level

Incremental Backups and Storage Rollback, which are used extensively for backing up databases. See

“Storage Checkpoints” on page 51 for more information.