HP LeftHand P4000 SAN Solutions - Manual (Page 147)

Figure 71 Write patterns and parity in Network RAID-6 (Dual Parity)
1. P1 is parity for data blocks A, B, C, D
2. P2 is parity for data blocks E, F, G, H
3. P3 is parity for data blocks I, J, K, L
4. P4 is parity for data blocks M, N, O, P
Provisioning snapshots
Snapshots provide a copy of a volume for use with backup and other applications. You create
snapshots from a volume on the cluster.
Snapshots are always thin provisioned. Thin provisioning snapshots saves actual space in the SAN,
while letting you have more snapshots without the concern of running out of cluster space.
Snapshots can be used for multiple purposes, including:
•
Source volumes for data mining and other data use
•
Source volumes for creating backups
•
Data or file system preservation before upgrading software
•
Protection against data deletion
•
File-level restore without tape or backup software
Snapshots versus backups
Backups are typically stored on different physical devices such as tapes. Snapshots are stored in
the same cluster as the volume. Therefore, snapshots protect against data deletion, but not device
or storage media failure. Use snapshots along with backups to improve your overall data backup
strategy.
At any time you can roll back to a specific snapshot. When you do roll back, you must delete all
the snapshots created after that snapshot. Also, using an iSCSI initiator, you can mount a snapshot
to a different server and recover data from the snapshot to that server.
The effect of snapshots on cluster space
Snapshots take up space on the cluster. Because snapshots are a thin provisioned space, they save
space compared to a full provisioned space.
Provisioning snapshots 147