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

Virtual disks with dynamically allocated capacity
Thin provisioning is the ability to provide more capacity to host servers than physically available
today and allows virtual disks to grow dynamically without impacting the operating system. This
allows the administrator to manage capacity as it is actually used instead of using it all at the time
of creation. Alerts and warnings allow the administrator to manage the resources needed for the
growth. It is necessary for an administrator to understand how these virtual disks operate in the
SVSP environment.
Thin provisioned virtual disk expansion is a two-step process:
1. Convert from thin to thick provisioned.
2. Convert back to thin provisioned and expand at the same time.
Using thin provisioning
Thin provisioning may be useful for:
• Sharing storage
• Increasing application capacity over time (long term)
• Managing oversubscribed hosts; because the host is not impacted and planning capacity is
easier
• Monitoring overall storage capacity
• Upgrading storage capacity when pools run out of capacity, most likely at a lower cost than
initially forecasted
Figure 14 (page 30) illustrates how thin provisioning works in the SVSP environment.
Figure 14 How thin provisioning works
Thin provisioned virtual disks require some initial planning. You must understand initial allocation
and potential growth requirements and monitor license use. Thin virtual disks are compatible with
snapshots, snapclones, migrations, and mirroring services and can be migrated to regular virtual
disks. It is also important to understand how operating systems or applications handle thin
provisioning. They may defeat the purpose of thin provisioning by filling up the space very quickly.
For example:
• Windows does not reuse deleted space until it uses up all of the free space on a virtual disk.
• Oracle formats a virtual disk the first time it uses it.
• A virtual disk format or erase operation may write zeroes to all blocks. This pattern is not
detected by SVSP at the present time, thus writing data to the blocks.
30 Virtual disks