Veritas Volume Manager 5.0.1 Administrator's Guide, HP-UX 11i v3, First Edition, November 2009

Table 16-1
Kernel tunable parameters for VxVM (continued)
DescriptionParameter
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 that 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 this 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 this tunable 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 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_fmr_logsz
The maximum number of volumes that can be created on
the system. The minimum and maximum permitted values
are 1 and the maximum number of minor numbers
representable on the system.
The default value is 8388608.
vol_max_vol
533Performance monitoring and tuning
Tuning VxVM