HP StorageWorks 2300 Modular Smart Array reference guide (500911-001, January 2009)

HP StorageWorks 2300 Modular Smart Array reference guide 21
You must assign a name to an automatically added host to have the system retain it after a restart. Naming
hosts also makes them easy to recognize for volume mapping. A maximum of 63 names can be assigned.
The Configuration Viewpanel lists hosts by name, or if they are unnamed, by ID.
About volume mapping
Each volume has default host-access settings that are set when the volume is created; these settings are
called the default mapping. The default mapping applies to any host that has not been explicitly mapped
using different settings. Explicit mappings for a volume override its default mapping.
Default mapping enables all attached hosts to see a volume using a specified LUN and access permissions
set by the administrator. This means that when the volume is first created, all connected hosts can
immediately access the volume using the advertised default mapping settings. This behavior is expected by
some operating systems, such as Microsoft Windows, which can immediately discover the volume. The
advantage of a default mapping is that all connected hosts can discover the volume with no additional
work by the administrator. The disadvantage is that all connected hosts can discover the volume with no
restrictions. Therefore, this process is not recommended for specialized volumes such as payroll databases.
You can change a volume's default mapping, and create, modify, or delete explicit mappings. A mapping
can specify read-write, read-only, or no access through one or more controller host ports to a volume.
When a mapping specifies no access, the volume is masked. You can apply access privileges to one or
more of the host ports on either controller. To maximize performance, it is recommended to map a volume
to at least one host port on the controller that owns it. To sustain I/O in the event of controller failure, it is
recommended to map to at least one host port on each controller.
Continuing the example of the payroll volume, it could be mapped with read-write access for the Human
Resources host and be masked for all other hosts. An engineering volume could be mapped with read-write
access for the Engineering host and read-only access for other departments’ hosts.
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.
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