Modifying the configuration

Configuration modification using the snapmirror.conf file

You control the synchronous SnapMirror operation using the snapmirror.conf file. In the basic configuration, you use a dash (-) to specify the default SnapMirror settings. You modify the basic configuration by substituting configuration arguments for synchronous SnapMirror operation.

Note
See the na_snapmirror.conf(5) manual page for detailed descriptions of the SnapMirror arguments.

The following sections describe changing the configuration the snapmirror.conf file to control synchronous SnapMirror behavior:

Controlling how quickly data becomes visible on the destination filer

With Synchronous SnapMirror, changes to the source volume do not show immediately on the destination volume, even though the changes have been mirrored. The changes are shown only after the source filer takes an automatic snapshot of the source volume. This happens every three minutes by default. You can change the interval for automatic snapshots, but performance can degrade if you set smaller intervals because more snapshots are taken more often. The smallest interval you can set is 30 seconds.

To change the interval for automatic snapshots, complete the following step.

Step	Action
1	On the administration host, edit the snapmirror.conf file on the destination filer to either increase or decrease the `visibility_interval` argument. See the na_snapmirror.conf(5) man page for details. Example If you want automatic snapshots taken every five minutes, you change the entry in the snapmirror.conf file to the following: src_filer:vol1 dest_filer:vol2 visibility_interval=300 sync

Step

Action

On the administration host, edit the snapmirror.conf file on the destination filer to either increase or decrease the visibility_interval argument.

See the na_snapmirror.conf(5) man page for details.

Example

If you want automatic snapshots taken every five minutes, you change the entry in the snapmirror.conf file to the following:

src_filer:vol1 dest_filer:vol2 visibility_interval=300 
sync

Controlling the synchronicity level to improve performance

Synchronous mirroring can affect the performance of the filers involved in the mirroring because of the potentially large number and frequency of write operations from clients. By default, for every write operation, the destination filer returns a write acknowledgement. You can improve performance by specifying a greater number of outstanding write operations before a write acknowledgement. However, this means the source and destination are out of synchronization between write acknowledgements.

To improve performance by decreasing the level of synchronicity, complete the following step.

Step	Action
1	On the administration host, edit the snapmirror.conf file on the destination filer to change the `outstanding` argument. Note The value for the outstanding argument can be in number of write operations (ops), number of milliseconds (ms), or number of seconds (s). See the na_snapmirror.conf(5) man page for details. Example 1 If you want to allow one outstanding write operation, you change the entry in the snapmirror.conf file to the following: src_filer:vol1 dest_filer:vol2 outstanding=1ops sync Example 2 If you want to allow outstanding write operations to collect for 20 ms, change the entry in the snapmirror.conf file to the following: src_filer:vol1 dest_filer:vol2 outstanding=20ms sync

Step

Action

On the administration host, edit the snapmirror.conf file on the destination filer to change the outstanding argument.

Note
The value for the outstanding argument can be in number of write operations (ops), number of milliseconds (ms), or number of seconds (s).

See the na_snapmirror.conf(5) man page for details.

Example 1

If you want to allow one outstanding write operation, you change the entry in the snapmirror.conf file to the following:

src_filer:vol1 dest_filer:vol2 outstanding=1ops sync

Example 2

If you want to allow outstanding write operations to collect for 20 ms, change the entry in the snapmirror.conf file to the following:

src_filer:vol1 dest_filer:vol2 outstanding=20ms sync

Improving data protection using checksum

By default, SnapMirror does not use a checksum algorithm to protect transmitted data, because the data has already had the RAID checksum recalculated when it arrived at the destination filer. If the error rate of your network is high enough to cause an undetected error, then you can improve data protection by using a cyclic redundancy check (CRC) checksum algorithm.

To add CRC checksum protection, complete the following step.

Step	Action
1	On the administration host, edit the snapmirror.conf file on the destination filer to change the `cksum` argument to crc32c. See the na_snapmirror.conf(5) man page for details. Example 1 If you want to implement CRC checksum protection, you change the entry in the snapmirror.conf file to the following: src_filer:vol1 dest_filer:vol2 cksum=crc32c sync

Step

Action

On the administration host, edit the snapmirror.conf file on the destination filer to change the cksum argument to crc32c.

See the na_snapmirror.conf(5) man page for details.

Example 1

If you want to implement CRC checksum protection, you change the entry in the snapmirror.conf file to the following:

src_filer:vol1 dest_filer:vol2 cksum=crc32c sync

Changing the TCP window size for a connection

You can improve SnapMirror performance by changing the TCP window size used for a connection between the source and destination filers. To change the TCP window size used for the connection, complete the following step.

Step	Action
1	Calculate a TCP window size that works well for the connection using the following fomula: WindowSize = (RoundTrip Delay) (DesiredRate) Example If your average round trip delay is 100 milliseconds and you want a rate of 10 Mbps, you should set the TCP window size to 125,000 bytes, as the following equation shows: 0.1 * 10,000,000/8 = 125,000
2	On the administration host, edit the snapmirror.conf file on the destination filer to change the `wsize` argument. See the na_snapmirror.conf(5) man page for details. Example If you want to decrease the TCP window size to 125,000 bytes, you change the entry in the snapmirror.conf file to the following: src_filer:vol1 dest_filer:vol2 wsize=125000 sync

Step

Action

Calculate a TCP window size that works well for the connection using the following fomula:

WindowSize = (RoundTrip Delay) (DesiredRate)

Example

If your average round trip delay is 100 milliseconds and you want a rate of 10 Mbps, you should set the TCP window size to 125,000 bytes, as the following equation shows:

0.1 * 10,000,000/8 = 125,000

On the administration host, edit the snapmirror.conf file on the destination filer to change the wsize argument.

See the na_snapmirror.conf(5) man page for details.

Example

If you want to decrease the TCP window size to 125,000 bytes, you change the entry in the snapmirror.conf file to the following:

src_filer:vol1 dest_filer:vol2 wsize=125000 sync