Veritas Volume Manager 5.1 SP1 Administrator"s Guide (5900-1506, April 2011)

Table 15-1
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
497Performance monitoring and tuning
Tuning VxVM