Creating And Scheduling Snapshots; Snapshot Overview - Compaq N2400 - TaskSmart - 1 GB RAM Administration Manual

Creating and Scheduling Snapshots

Snapshot Overview

Snapshots are copies of the data that is saved on virtual disks. Snapshots
initially take up very little space on the hard drive and can be created in mere
seconds. Snapshots are extremely beneficial. They can be used to immediately
recover a lost file or directory, to test a new application with important data
without affecting the "real" data, and as a source of data for backups.
Snapshots are a temporary backup of your data and are not meant to be a
permanent form of the data.
Snapshots work by mapping blocks of virtual disks and intercepting virtual
disk writes. Each initial write to a data block on the virtual disk causes that
write to perform a copy of the original data before replacing it with the new
data being written. This action is known as a copy-out and preserves a copy of
the virtual disk as it was when the snapshot was created. The copy is available
until the snapshot is deleted.
When using snapshots, performance of the virtual disk may be affected,
depending on the rate that your data is changing and the number of snapshots
kept for each virtual disk. Read performance of the virtual disk remains
constant, regardless of the presence of snapshots. Read performance of the
snapshot is identical to that of the virtual disk. Write performance, however,
may vary. Each initial write to a virtual disk area causes a copy-out to the
snapshot, and the initial write is slower than if a snapshot is not being used.
Copy-out is not performed on subsequent writes to the same virtual disk block,
so write performance is unaffected after the initial write to each block.
Predicting the exact effect of snapshots on any particular virtual disk is
difficult, because several variables are involved. These variables include the
type of applications accessing the data and the rate of change of the files on
the virtual disk. When a high percentage of writes is made to the same area, as
when a file is constantly rewritten, the area is called write locality. Virtual
disks with high write locality experience less performance degradation due to
snapshots.
Administrative Procedures 2-15