294 Implementing the IBM Storwize V5000 Gen2 with IBM Spectrum Virtualize V8.1
When there are two copies of a volume, one copy is known as the primary copy. If the primary
is available and synchronized, reads from the volume are directed to it. The user can select
the primary when the volume is created or can change it later.
Placing the primary copy on a high-performance controller maximizes the read performance
of the volume.
Write I/O operations data flow with a mirrored volume
For write I/O operations to a mirrored volume, the IBM Spectrum Virtualize preferred node
definition, with the multipathing driver on the host, is used to determine the preferred path.
The host routes the I/Os through the preferred path, and the corresponding node is
responsible for further destaging written data from cache to both volume copies. Figure 6-5
shows the data flow for write I/O processing when volume mirroring is used.
Figure 6-5 Data flow for write I/O processing in a mirrored volume
As shown in Figure 6-5, all the writes are sent by the host to the preferred node for each
volume (1). Then, the data is mirrored to the cache of the partner node in the I/O Group (2),
and acknowledgment of the write operation is sent to the host (3). The preferred node then
destages the written data to the two volume copies (4).
A volume with copies can be checked to see whether all of the copies are identical or
consistent. If a medium error is encountered while it is reading from one copy, it is repaired by
using data from the other copy. This consistency check is performed asynchronously with
host I/O.
Mirrored volumes use bitmap space at a rate of 1 bit per 256 KiB grain, which translates to
1 MiB of bitmap space supporting 2 TiB of mirrored volumes. The default allocation of bitmap
space is 20 MiB, which supports 40 TiB of mirrored volumes. If all 512 MiB of variable bitmap
space is allocated to mirrored volumes, 1 PiB of mirrored volumes can be supported.
Important: Mirrored volumes can be taken offline if no quorum disk is available. This
behavior occurs because the synchronization status for mirrored volumes is recorded on
the quorum disk.
To Next Page
Loading...
Need help?
General