HP SAN Virtualization Services Platform - Manual (Page 238)

• The number of virtual disks on which you are creating PiTs.
• How much data is modified over a period of time (for example between successive PiT creations).
• How many PiTs are maintained on each virtual disk.
• Which virtual disks you are allocating from which storage pools.
• The frequency of PiT creation.
Built-in capacity protection
VSM features built-in protection mechanisms for storage pools running out of free capacity. The
objective of the protection mechanisms is to make sure that storage pools always have enough free
capacity to handle the expansion requests of snapshot temporary virtual disks (or else, write I/Os will
fail).
You can configure storage pool capacity thresholds both globally and for individual storage pools.
The global thresholds are configured by percentage (%) and total (GB) capacity for three individual
alert levels (Warning, Guard, and Emergency). The individual storage pool capacity alert thresholds
are configured by percentage (%) for a single alert level.
Virtual disk free capacity is calculated, per virtual disk, as the percentage ratio between the sum of
the free capacity in all storage pools from which the virtual disk is allocated and the virtual disk
capacity. Temporary virtual disks, which hold PiT modifications, inherit the virtual disk free capacity
value from the source virtual disk on which the PiT was created and for which the temporary virtual
disk holds data modifications.
Whenever VSM performs an operation that has to do with storage allocation, such as virtual disk
creation or virtual disk expansion, the virtual disk free capacity value of the involved object is evaluated
against three configurable levels: warning, guard, and emergency, configured in the Configuration
tab of the Options dialog box. Capacity is also evaluated against the individual alert threshold of
each storage pool involved in the operation. Alert notifications are triggered when the free space
drops below double the configured threshold for the storage pool. Actual preventive actions (such as
limiting expansions to 1 GB) are executed when the free space drops below the configured threshold
level.
VSM takes the following action when the virtual disk free capacity of any object reaches each of these
levels:
• Warning—A warning is added to the event log. This warning is repeated every 5 minutes. VSM
deletes PiTs maintained on mirror tasks on the virtual disk, leaving only the most recent PiT and
any PiTs that cannot be deleted because of snapshots using them.
• Guard—Mirror tasks running on the virtual disk are suspended so that new PiTs are not created.
Migration tasks are deleted. The oldest snapshot PiT on the virtual disk is deleted, unless there is
a snapshot using the PiT.
• Emergency—The snapshot service attempts to delete all PiTs on the virtual disk, beginning with
the oldest PiT, until it reaches a PiT that cannot be deleted because of a snapshot using the PiT.
To modify the global capacity thresholds:
1. From the Tools menu, select Options > General. The Options dialog box appears.
Using snapshots 238