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Controlling instant snapshot synchronization

  Note   Synchronization of the contents of a snapshot with its original volume is not possible for space-optimized instant snapshots.

The commands in this section cannot be used to control the synchronization of linked break-off snapshots.

By default, synchronization is enabled for the vxsnap reattach, refresh and restore operations on instant snapshots. Otherwise, synchronization is disabled unless you specify the syncing=yes attribute to the vxsnap command.

The following table shows the commands that are provided for controlling the synchronization manually.

The vxsnap snapwait command is provided to wait for the link between new linked break-off snapshots to become ACTIVE, or for reattached snapshot plexes to reach the SNAPDONE state following resynchronization. See Creating and managing linked break-off snapshot volumes, Reattaching an instant snapshot and Reattaching a linked break-off snapshot volume 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.