292 Implementing the IBM Storwize V5000 Gen2 with IBM Spectrum Virtualize V8.1
Having cache-disabled volumes makes it possible to use the native copy services in the
underlying RAID array controller for MDisks (LUNs) that are used as the IBM Spectrum
Virtualize image mode volumes. Using IBM Spectrum Virtualize Copy Services rather than
the underlying disk controller copy services gives better results.
Cache characteristics of a volume can have any of the following settings:
readwrite. All read and write I/O operations that are performed by the volume are stored
in cache. This is the default cache mode for all volumes.
readonly. All read I/O operations that are performed by the volume are stored in cache.
disabled. All read and write I/O operations that are performed by the volume are not
stored in cache. Under normal conditions, a volume’s read and write data is held in the
cache of its preferred node, with a mirrored copy of write data that is held in the partner
node of the same I/O Group. With cache disabled volume, the I/Os are passed directly
through to the back-end storage controller rather than being held in the node’s cache.
6.1.4 Mirrored volumes
The mirrored volume feature provides a simple RAID 1 function, so a volume has two physical
copies of its data. This approach enables the volume to remain online and accessible even if
one of the MDisks sustains a failure that causes it to become inaccessible.
The two copies of the volume often are allocated from separate storage pools or by using
image-mode copies. The volume can participate in FlashCopy and remote copy relationships.
It is serviced by an I/O Group, and has a preferred node.
Each copy is not a separate object and cannot be created or manipulated except in the
context of the volume. Copies are identified through the configuration interface with a copy ID
of their parent volume. This copy ID can be 0 or 1.
This feature provides a point-in-time copy function that is achieved by “splitting” a copy from
the volume. However, the mirrored volume feature does not address other forms of mirroring
that are based on remote copy, which is sometimes called
IBM HyperSwap, that mirrors
volumes across I/O Groups or clustered systems. It is also not intended to manage mirroring
or remote copy functions in back-end controllers.
Note: Having cache-disabled volumes makes it possible to use the native copy services
in the underlying RAID array controller for MDisks (LUNs) that are used as IBM
Spectrum Virtualize image mode volumes. Consult IBM Support before turning off the
cache for volumes in production environment to avoid any performance degradation.
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