Hardware manual

Group Administration Snapshot management
11–2
About snapshot reserve allocation
Before you can create snapshots of a volume, you must allocate snapshot reserve for the volume. Snapshot reserve
is consumed from the pool where the volume resides.
You can allocate snapshot reserve when you create a volume, or
you can modify a volume’s properties to change
the snapshot reserve. Snapshot reserve is a percentage of the volume reserve. Because the volume reserve for a
thin-provisioned volume changes as volume usage increases, the snapshot reserve for a thin-provisioned volume
also changes.
The group generates event messages when the amount of free snapshot reserve falls below a use
r
-defined
threshold. Depending on the policy that you set for snapshot space recovery, the group preserves snapshot reserve
as described in About snapshot reserve settings.
About snapshot access controls
Online snapshots are seen on the network as iSCSI targets. It is important to protect your snapshots from
unauthorized and uncoordinated access by iSCSI initiators.
Note: When a
snapshot is online and accessible, a user or application can change the contents of the snapshot. If
this happens, the snapshot no longer represents a point-in-time copy of a volume and has limited use for
data recovery.
All iSCSI target security mechanisms apply to snapshots, including acces
s control records, which prevent
unauthorized iSCSI initiator access to a volume and its snapshots. See iSCSI target security.
About snapshot reserve settings
There are group-wide default values for the following snapshot reserve settings, unless you explicitly change them
for a volume:
• Snapshot re
serve – Amount of space, based on a percentage of the volume reserve, that the group allocates to
snapshots. When you create a volume, you can specify the snapshot reserve percentage for the volume.
Otherwise, the group applies the group-wide default value. You can modify the snapshot reserve value.
• Snapshot spac
e r
ecovery policy – Action the group takes when a new snapshot exceeds snapshot reserve:
– Delete the oldest snapshots to fre
e space for new snapshots.
– Set the volume (and snapshots) offline.
If a snapshot has active iSCSI connections, the group closes
the connections before deleting the snapshot.
Note: In some cases, you
might want to preserve the data in a snapshot that might be at risk of deletion. To
preserve the data in a snapshot, you can clone the snapshot. See Cloning a snapshot to create a new
volume on page 1
1-9.
• Snapshot space warning per
centage – Percentage of the snapshot reserve, when reached by in-use snapshot
reserve, results in an event message. The default is 90% of the snapshot reserve.
For example, if snapshot reserve space is 200 MB and the warning level is
90%, a warning occurs when in-use
snapshot reserve equals or exceeds 180 MB.