Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

465Performance monitoring and tuning
Tuning VxVM
tunable is used by utilities performing operations such as resynchronizing mirrors or
rebuilding RAID-5 columns.
The default for this tunable is 50 ticks.
Increasing this value results in slower recovery operations and consequently lower system
impact while recoveries are being performed.
vol_fmr_logsz
The maximum size in kilobytes of the bitmap that Non-Persistent FastResync uses to track
changed blocks in a volume. The number of blocks in a volume that are mapped to each
bit in the bitmap depends on the size of the volume, and this value changes if the size of
the volume is changed. For example, if the volume size is 1 gigabyte and the system block
size is 1024 bytes, a vol_fmr_logsz value of 4 yields a map contains 32,768 bits, each
bit representing one region of 32 blocks.
The larger the bitmap size, the fewer the number of blocks that are mapped to each bit.
This can reduce the amount of reading and writing required on resynchronization, at the
expense of requiring more non-pageable kernel memory for the bitmap. Additionally, on
clustered systems, a larger bitmap size increases the latency in I/O performance, and it
also increases the load on the private network between the cluster members. This is
because every other member of the cluster must be informed each time a bit in the map is
marked.
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.