NAS 1000s

39NAS 1000s Administration Guide

Persistent Storage Manager

Persistent Storage Manager lets the administrator make replicas, called snapshots, of disks in a

matter of seconds. Snapshots enable the creation of multipurpose virtual replicas of production

data without having to physically copy the data. They can be used to immediately recover a

lost file or directory, to test a new application with realistic data without affecting the "real"

data, and to serve as a source of data for backups. Snapshots record data changes on volumes

and are thus not a replacement for off-line backups.

This chapter covers the following items:

■ Operational Overview

■ Data Recovery

■ Snapshot (Persistent Image) Considerations

■ Accessing Persistent Storage Manager

Operational Overview

Each snapshot is a complete point-in-time representation of the data on the volumes. Each

snapshot requires only a fraction of the hard-drive capacity of the original data. PSM does not

keep all the data that was ever written. PSM maintains only the data required to maintain a

snapshot.

PSM works below the operating system as a Filter Driver at the Volume block level. PSM

maintains a library of snapshots, each representing a specific point-in-time. Snapshots can be

accessed by users, administrators, or any Windows application, and look just like the familiar

file/folder view.

With the first snapshot taken on a target volume, PSM establishes a cache file for that volume

within which PSM retains overwritten data required to build a snapshot. The cache file size is

based on a percentage of the volume it resides on and is configured through the WebUI; the

default is 10 percent. As soon as the first snapshot is taken, PSM starts monitoring all writes on

the target volume. When a write request occurs, PSM intercepts and pauses the write, reads the

data that is to be overwritten, and saves the data in a Diff Directory within the PSM-specific

cache file. After the original data is written to the Diff Directory, the new data is written on the

active volume. This process is referred to as "copy-on-write." Only the first write forces a

copy-out, subsequent writes to the same data block does not force a new copy-out, unless of

course a new snapshot is taken between the initial and subsequent write.

PSM can create and manage up to 250 snapshots system wide. A snapshot can cover several

volumes at once with an upper limit of 63 volumes within a single snapshot. However, when

reverting from a "grouped" snapshot, the revert is non-selective and it reverts all volumes

associated with the "grouped" snapshot.