Related Topics; About Volume Cache Options; Using Write-Back Or Write-Through Caching - HP P2000 Reference Manual

A LUN identifies a mapped volume to a host. Both controllers share a set of LUNs, and any unused LUN
can be assigned to a mapping; however, each LUN can only be used once per volume as its default LUN.
For example, if LUN 5 is the default for Volume1, no other volume in the storage system can use LUN 5 as
its default LUN. For explicit mappings, the rules differ: LUNs used in default mappings can be reused in
explicit mappings for other volumes and other hosts.
TIP: When an explicit mapping is deleted, the volume's default mapping takes effect. Therefore, it is
recommended to use the same LUN for explicit mappings as for the default mapping.
Volume mapping settings are stored in disk metadata. If enough of the disks used by a volume are moved
into a different enclosure, the volume's vdisk can be reconstructed and the mapping data is preserved.
P2000 G3 MSA System models use Unified LUN Presentation (ULP), which can expose all LUNs through all
host ports on both controllers. The interconnect information is managed in the controller firmware. ULP
appears to the host as an active-active storage system where the host can choose any available path to
access a LUN regardless of vdisk ownership. When ULP is in use, the system's operating/redundancy
mode is shown as Active-Active ULP. ULP uses the T10 Technical Committee of INCITS Asymmetric Logical
Unit Access (ALUA) extensions, in SPC-3, to negotiate paths with aware host systems. Unaware host
systems see all paths as being equal.

Related topics

• Using the Provisioning Wizard
• Changing a volume's default mapping
• Changing host mappings
• Viewing information about a volume
(page 99)

About volume cache options

You can set options that optimize reads and writes performed for each volume.

Using write-back or write-through caching

CAUTION: Only disable write-back caching if you fully understand how the host operating system,
application, and adapter move data. If used incorrectly, you might hinder system performance.
You can change a volume's write-back cache setting. Write-back is a cache-writing strategy in which the
controller receives the data to be written to disks, stores it in the memory buffer, and immediately sends the
host operating system a signal that the write operation is complete, without waiting until the data is actually
written to the disk. Write-back cache mirrors all of the data from one controller module cache to the other.
Write-back cache improves the performance of write operations and the throughput of the controller.
When write-back cache is disabled, write-through becomes the cache-writing strategy. Using write-through
cache, the controller writes the data to the disks before signaling the host operating system that the process
is complete. Write-through cache has lower write operation and throughput performance than write-back,
but it is the safer strategy, with minimum risk of data loss on power failure. However, write-through cache
does not mirror the write data because the data is written to the disk before posting command completion
and mirroring is not required. You can set conditions that cause the controller to change from write-back
caching to write-through caching.
In both caching strategies, active-active failover of the controllers is enabled.
You can enable and disable the write-back cache for each volume. By default, volume write-back cache is
enabled. Because controller cache is backed by super-capacitor technology, if the system loses power,
data is not lost. For most applications, this is the correct setting.
If you are doing random access to this volume, leave the write-back cache enabled.
The best practice for a fault-tolerant configuration is to use write-back caching.
24 Getting started
on page 57
(page 64) or explicit mappings
on page 73
(page 95), snapshot
(page 65)
(page 96), host (page 100), or all hosts