HP P6000 Continuous Access Software - Manual (Page 273)

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If the disk group has insufficient space to increase the capacity of demand-allocated snapshots,
the snapshots will automatically be invalidated, but the source virtual disks will continue
accepting requests.
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Snapshots count against the maximum number of virtual disks per array.
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You can perform an instant restore of a snapshot of a mirrorclone.
Snapshots cannot be created when the disk to be replicated is:
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A snapshot
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In the process of normalizing or being deleted. See Normalization .
Snapshots per virtual disk
In HP XCS controller software, the maximum number of snapshots per virtual disk varies with the
size of the disk. This is because the total snapshot size (per disk) cannot exceed 15 TB.
Virtual disk snapshot estimator
Snapshots (max) Source size (TB)
16 0 < to 0.94
15 0.95 to 1.00
14 1.01 to 1.07
13 1.08 to 1.15
12 1.16 to 1.25
11 1.26 to 1.36
10 1.37 to 1.50
9 1.51 to 1.67
8 1.68 to 1.88
7 1.89 to 2.00
Snapshot types (allocation policy)
Snapshot types (allocation policy) specifies how the storage system allocates space in a disk group
for a snapshot. Values are:
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Demand allocated. The space allocated for the snapshot can automatically change from an
initial minimum amount, up to the full capacity of the original (active) virtual disk.
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Fully allocated. The space allocated for the snapshot is initially set to, and remains fixed at,
the full capacity of the source (active) virtual disk.
Demand-allocated snapshots
When a snapshot is demand allocated, the storage system initially allocates only a small amount
of space for the snapshot, just enough to store point-in-time information and pointers to data on
the source. As data on the source is over-written, the controller increases the allocated space for
the snapshot and copies the original (point-in-time) data from the source to the snapshot.
If all the original data on the source is over-written, the controller increases the allocated space on
the snapshot by an amount equal to the full size of the source.
The size of the disk group in which the source and snapshot are located must be sufficient to handle
increases in snapshot size, whenever the increases might occur. Insufficient space in the disk group
can not only prevent the controller from increasing the space allocation, but it can also prevent
writes to both the source and snapshot.
Virtual disk concepts 273