VERITAS Volume Manager 3.1 Administrator's Guide

Volume Manager Operations
FastResync (Fast Mirror Resynchronization)
Chapter 3 117
missed when a mirror is offline/detached/shapshotted, and then applying
only those updates when the mirror returns, considerably reduces the
time to resynchronize the volume.
The basis for this change tracking is the use of a bitmap. Each bit in the
bitmap represents a contiguous region (an extent) of a volume’s address
space. This contiguous region is called the region size. Typically, this
region size is one block; each block of the volume is represented by one
bit in the bitmap. However, a tunable called vol_fmr_logsz is provided,
which can be used to limit the maximum size (in blocks) of the FMR map.
When computing the size of the map, the algorithm starts with a region
size of one, and if the resulting map size is less than the vol_fmr_logsz,
then the computed value becomes the map size. If the size is larger than
vol_fmr_logsz, an attempt is made to accommodate vol_fmr_logsz
with a region size of two, and so on until the map size is less than the
vol_fmr_logsz tunable.
For example:
volume size = 1G
vol_fmr_logsz = 4
on a system with a block size of 1024 bytes, that is 4*1024=4096 bytes or
4096*8 = 32768 bits.
Thus, for a 1G volume, a region size of one is 4096 bits, which is less than
four blocks, so the map size is 4096 bits or 512 bytes.
Note that if the size of the volume increases, this computation is redone
to ensure the map size does not exceed the vol_fmr_logsz tunable.
Persistent Versus Non-Persistent Tracking
For VxVM 3.1, the FMR maps are allocated in memory. They do not
reside on disk or persistent store, unlike a DRL. Therefore, if the system
crashes, this information is lost and the entire length of the volume must
be synced up. One advantage of this approach is that the FMR updates
(updates to this map) do not cost anything in terms of performance, as no
disk updates must be done. However, if the system crashes, this
information is lost and full resynchronization of mirrors is once again
necessary.
Snapshot(s) and FMR
To take advantage of the FMR delta tracking when using snapshots, use
the new snapshot option. After a snapshot is taken, the snapshot option