VERITAS Volume Manager 3.5 Administrator's Guide (September 2002)

Tuning VxVM
294 VERITAS Volume Manager Administrator’s Guide
vol_fmr_logsz
The maximum size in kilobytes of the bitmap that Non-Persistent FastResync uses to
track changedblocks in a volume. Thenumber of blocks in avolume 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 is the bitmap size, the fewer the number of blocks that are mapped to each
bit.This canreduce the amountofreadingand writing requiredonresynchronization,
at the expense of requiring more non-pagable kernel memory for the bitmap.
Additionally, on clustered systems, a larger bitmap size increases the latency in I/O
performance, and italso increasesthe loadon the privatenetwork 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 or until FastResync is turned on for the volume.
In configurations whichhave thousandsof mirrorswith attachedsnapshot 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 4 KB. The maximum and minimum permitted
values are 1 and 8 KB.
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.