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480 Performance monitoring and tuning
Tuning VxVM
Since the region size must be the same on all nodes in a cluster for a shared
volume, the value of the vol_fmr_logsz tunable on the master node overrides
the tunable values on the slave nodes, if these values are different. Because the
value of a shared volume can change, the value of vol_fmr_logsz is retained
for the life of the volume.
In configurations which have thousands of mirrors with attached snapshot
plexes, the total memory overhead can represent a significantly higher
overhead in memory consumption than is usual for VxVM.
The default value of this tunable is 4KB. The maximum and minimum permitted
values are 1KB and 8KB.
Note: The value of this tunable does not have any effect on Persistent
FastResync.
vol_max_vol
The maximum number of volumes that can be created on the system. This value
can be set to between 1 and the maximum number of minor numbers
representable in the system.
The default value for this tunable is 16777215.
vol_maxio
The maximum size of logical I/O operations that can be performed without
breaking up the request. I/O requests to VxVM that are larger than this value are
broken up and performed synchronously. Physical I/O requests are broken up
based on the capabilities of the disk device and are unaffected by changes to this
maximum logical request limit.
The default value for this tunable is 256 sectors (256KB).
Note: The value of voliomem_maxpool_sz must be at least 10 times greater
than the value of vol_maxio.
If DRL sequential logging is configured, the value of voldrl_min_regionsz
must be set to at least half the value of vol_maxio.
vol_maxioctl
The maximum size of data that can be passed into VxVM via an ioctl call.
Increasing this limit allows larger operations to be performed. Decreasing the
limit is not generally recommended, because some utilities depend upon