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

See “Thin provisioning and operating system interaction” (page 70) for more information about
how various operating systems treat thin virtual disks.
Thin virtual disks are:
• Allocated from only one storage pool
• Initial real allocated capacity
0.5 GB < real capacity < smallest of {32 GB or 10% of virtual capacity}◦
◦ The minimum initial allocated capacity is 1 GB
◦ For a 1 TB thin virtual disk, the maximum initial allocation is 32 GB (1,024 GB * 0.10
= 102 GB; this is greater than the 32 GB maximum initial allocation)
◦ For a 250 GB thin virtual disk, the maximum initial allocation is 25 GB (250 GB * 0.10
= 25 GB)
• Additional “on-demand” real capacity—smallest of:
Current real capacity◦
◦ 5% virtual capacity
◦ 20 GB
Example
The following example shows a 200 GB virtual disk and how the initial real and additional real
capacity is calculated. It is important to understand the free space calculation of the storage pool.
Real capacityCalculation
4 GB4 GB = less than 10% of 200 GB —>Initial real capacity
8 GB4 GB = current real capacity —>Additional real capacity (1)
16 GB4 GB = current real capacity —>Additional real capacity (2)
26 GB4 GB = 5% of 200 GB —>Additional real capacity (3)
26 GB4 GB = 5% of 200 GB —>Additional real capacity (4)
In general, a thin virtual disk has similar performance characteristics to those of a regular or thick
virtual disk; however, when additional capacity is required by a thin virtual disk, additional time
is needed. This may be observed after creation when random writes may trigger many expansions,
and is less likely to occur after a virtual disk has been used for a while.
Migrating a virtual disk to a thin virtual disk requires a host-based application, such as Backup,
Robocopy, or MirrorUX, that identifies the blocks or files currently in use by the file system to
perform the copy. SVSP can migrate a thick virtual disk to a thick virtual disk and add thin capacity
to it.
Virtual disks with dynamically allocated capacity 31