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

NOTE: This recommendation has changed since earlier installations of SVSP. Be sure your
configuration matches this recommendation if running synchronous mirroring.
Working set recommendations
A working set is a description (maps within the DPM of 1 MB chunks of data) of all the storage
currently in use on a particular virtual disk. When this working set size exceeds the SVSP limits for
either synchronous mirroring or basic virtual disks, new maps must be imported from the VSM.
This is a relatively inexpensive operation and should not affect performance of the virtual disk
unless the churn rate is excessive.
For each DPM configuration, there are two synchronous mirror capacity values:
• Maximum capacity—The maximum virtual capacity that a DPM group can maintain in its
internal map table without additional update operations under normal conditions only.
• Safe capacity—The maximum virtual capacity that a DPM group can maintain in its internal
map table without additional update operations under normal and failover conditions.
The values in Table 3 (page 43) serve as a reference for sizing. They are based on lab testing
observations for synchronous mirror-only configurations. If you are experiencing issues with
throughput, excess mapping may be occurring above these numbers.
Table 3 DPM capacity values
Safe capacityMaximum capacityDPM configuration
15 TB30 TB1 DPM
30 TB60 TB1 DPM group (2 DPMs)
60 TB120 TB2 DPM groups (4 DPMs)
90 TB180 TB3 DPM groups (6 DPMs)
120 TB240 TB4 DPM groups (8 DPMs)
PiTs in conjunction with synchronous mirrors
A PiT can be created on a synchronous mirror task or on the virtual disk of a synchronous mirror
copy. When a PiT is created on a synchronous mirror task, one PiT is created on each member of
the mirror task at the exact same time, so that all the PiTs are consistent with each other, for example,
exact copies. When a PiT is created on virtual disk that is a member of a synchronous mirror task,
the PiT is only created for that virtual disk, not for any of the other copies in the group.
A member, either primary or secondary, can be used as a source for an asynchronous mirror or
as a source for a snapclone operation. However, members of a synchronous mirror cannot be
migrated using the migration feature. As an alternative, a task can be added using the location
to migrate to as destination for the task. Once synchronized, detach the task serving the old location,
and delete its destination virtual disk if no longer needed.
Migrating an existing synchronous mirror group
When using synchronous mirrors to protect multiple virtual disks that belong to the same application,
HP recommends all virtual disks be active on the same DPM. This is especially true for a stretched
domain, so all virtual disks of that application fail over together or not at all.
HP recommends a minimum of two setup virtual disks per site. This allows the active VSM server
to keep on working when the connection with the other site is broken. Because either VSM server
can be active, a copy of the setup virtual disk must reside on each site. Having mirrored copies
of the setup virtual disk on both sites is also required to prevent the passive VSM server from
becoming active when the connection between sites is broken.
Working set recommendations 43