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

334 Administering volume snapshots
Creating instant snapshots
break-off snapshot volumes” on page 321, “Reattaching an instant snapshot” on page 327
and “Reattaching a linked break-off snapshot volume” on page 328 for details.
Improving the performance of snapshot synchronization
Two optional arguments to the -o option are provided to help optimize the performance of
synchronization when using the
make, refresh, restore and syncstart operations:
iosize=size Specifies the size of each I/O request that is used when synchronizing
the regions of a volume. Specifying a larger size causes
synchronization to complete sooner, but with greater impact on the
performance of other processes that are accessing the volume. The
default size of 1m (1MB) is suggested as the minimum value for high-
performance array and controller hardware. The specified value is
rounded to a multiple of the volume’s region size.
slow=iodelay Specifies the delay in milliseconds between synchronizing successive
sets of regions as specified by the value of
iosize. This can be used
to change the impact of synchronization on system performance. The
default value of iodelay is 0 milliseconds (no delay). Increasing this
value slows down synchronization, and reduces the competition for I/
O bandwidth with other processes that may be accessing the volume.
Options may be combined as shown in the following examples:
# vxsnap -g mydg -o iosize=2m,slow=100 make \
source=myvol/snapvol=snap2myvol/syncing=on
# vxsnap -g mydg -o iosize=10m,slow=250 syncstart snap2myvol
Note: These optional parameters only affect the synchronization of full-sized instant
snapshots. They are not supported for space-optimized snapshots.
Listing the snapshots created on a cache
To list the space-optimized instant snapshots that have been created on a cache object, use
the following command:
# vxcache [-g diskgroup] listvol cache_object
The snapshot names are printed as a space-separated list ordered by timestamp. If two or
more snapshots have the same timestamp, these snapshots are sorted in order of
decreasing size.
Tuning the autogrow attributes of a cache
The highwatermark, autogrowby and maxautogrow attributes determine how
the VxVM cache daemon (
vxcached) maintains the cache if the autogrow feature has
been enabled and
vxcached is running: