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

Keep in mind that production cycles are not the same across all customer environments and that
process controls differ.
If you do not have precise answers for the above considerations (knowing that there is another
threshold alarm preset at 80%), do the following:
1. Record the current capacity and time.
2. Set the virtual disk alarm threshold initially at 10% above the current capacity.
3. When the alarm triggers, record the time.
4. Use the growth rate of the individual virtual disk to determine where to set the storage pool
limits.
Please keep in mind that some operating system and file system combinations allocate more space
than others when creating a new file system. For those types of file systems (regardless of whether
it is for production or testing), set your thresholds at no lower than 50% and apply the above rules.
A ThP pool is likely shared by several V-VOLs, so it is preferable that the storage pool capacity is
much larger than a single free V-VOL capacity.
SVSP ThP limitations and guidelines
Thin virtual disks with SVSP v3.0 support all of the SVSP features including:
• PiT creation
• Snapshot creation
• Snapclones
• Migration
• Synchronous mirroring
• Asynchronous mirroring
• VSS
Controlling the run-away process
Controlling the run-away process results in space allocations and better control file system (FS)
fragmentation. An approach to control run-away file system allocations and avoid file system
fragmentations with large virtual disks with ThP virtual disk LUs (but not limited to this implementation)
is explained in the following subsections.
If, for example, a 2 TB virtual disk (traditional or ThP) is presented to a file system, the file system
can have the entire 2 TB address range to use. Over time, files are added, deleted, or purged.
The file system will typically attempt to allocate the next contiguous space for the new files.
Challenges
• Defragmenting a ThP virtual disk is not recommended because it may result in unnecessary
page allocation.
• It can be very difficult to predict whether a user may or may not cause a ThP virtual disk page
allocation by mistake. Those mistakes can be quite difficult to correct after they occur.
• There are no safeguards concerning run-away applications space allocation, so be careful
while using ThP virtual disks.
Proposed solution
Use the following solution for those file systems that can grow their virtual disks on the fly (for
example, Windows file systems, most of UNIX advanced file systems, and Linux advanced file
systems) where large virtual disks are presented by ThP to be used for an application’s life cycle
over time. (Example: A 2-TB ThP virtual disk to be used over the course of the next 24 months).
1. Configure the ThP virtual disk normally for the full 2 TB size.
2. Present the 2-TB V-VOL to the host.
74 Thin provisioning and operating system interaction