HP StorageWorks SAN Virtualization Services Platform Best Practices Guide (5697-0935, May 2011)

have this pattern. Although SVSP uses redirect-on-write snapshots (PiTs), SVSP still must perform a
segment read followed by a segment write to compose the new data into a segment on first write
to that segment after the snapshot (PiT) is created.
Using PiT storage allocation:
• Allocation is from the same storage pool as the original virtual disk
• Initial allocation = 0.5 GB
• Additional on-demand capacity is smallest among:
Current temporary virtual disk size◦
◦ 5% of original virtual disk
• Capacity cannot grow more than original virtual disk
Figure 12 (page 28) shows that PiTs can be created either automatically or manually. PiTs are not
mountable virtual disks, but by creating a snapshot of a PiT virtual disk, it may be used by another
host without impacting activity to the source virtual disk.
Figure 12 PiT creation
Example
The following example shows how a PiT uses capacity as it grows. It is important to understand
this and monitor the pool to avoid oversubscription. The PiT generally grows over time, so if you
create a 100 GB virtual disk, the application may only initially use 0.5 GB of capacity. As more
capacity is used, SVSP automatically allocates another 0.5 GB, and continues to do so for growth.
Temporary virtual disk capacity100 GBVirtual disk capacity
0.5 GB0.5 GBInitial capacity
1 GB0.5 GBAdditional capacity (1)
2 GB1 GBAdditional capacity (2)
4 GB2 GBAdditional capacity (3)
28 Virtual disks