http://www.netapp.com/us/media/tr-3326.pdf

http://backdrift.org/man/netapp/man5/na_snapmirror.conf.5.html

http://hd.kvsconsulting.us/netappdoc/733docs/html/ontap/onlinebk/GUID-47724B21-7C7E-4AC9-A497-9505B42C27AF.html

With SnapMirror Sync and SnapMirror Semi-Sync, changes to the source volume do not show up immediately on the destination volume, even though the changes have been replicated. The changes are first shown after the source system creates an automatic Snapshot copy of the source volume.

Snapshot copies created on the source volume are immediately propagated to the destination volume. The automatic Snapshot copies are created every three minutes by default.

To change the interval for automatic Snapshot copies, change the visibility_interval in the

snapmirror.conf file; however, performance can degrade if set to a smaller interval because frequent

Snapshot copies cause additional processing such as cache consistency checking on the destination system.

There is also a disadvantage in setting this to a large value. When the connection between the systems has an outage and SnapMirror goes into asynchronous mode, SnapMirror uses the last common Snapshot copy to transition into synchronous mode. This means that all data from the last valid common Snapshot copy will need to be replicated from the source to the destination storage system. If the visibility_interval is set to a large value, a large amount of data might have to be transferred and it might take longer time for SnapMirror to transition into synchronous mode. For these reasons, keep the default value of three minutes.

visibility_interval

The value for this argument is a number optionally followed by the suffixes: s (seconds), m (minutes) or h (hours). If a suffix is not specified, value is interpreted as seconds. This argument controls the amount of time before an automatic snapshot is created on the source volume that is synchronously mirrored. The value is the number of seconds between automatically created snapshots. The default value is 3 minutes. A small number here can negatively affect the performance of the mirror. This argument is ignored for asynchronous mirrors.

visibility_interval={xs | xm | xh}

Determines the amount of time before an automatic Snapshot copy is created on the source volume that is being replicated using synchronous or semi-synchronous SnapMirror. When replicating synchronously or semisynchronously, using SnapMirror, changes to the source volume do not show immediately on the destination volume, even though the changes have been replicated. The changes are shown only after the source system takes an automatic Snapshot copy of the source volume. This event occurs every three minutes by default.

You can change the interval for automatic Snapshot copies, but performance can degrade if you set smaller intervals because Snapshot copies are taken more often. The smallest interval you can set is 30 seconds.

Note: The s, m, and h suffixes specify seconds, minutes, and hours, respectively.