Restructuring a cascade

You might restructure a cascade to balance loading of your systems; to use a storage system or volume for a different purpose; or to perform upgrades, maintenance, or repairs.

Considerations

For example, in the cascade structure that follows, you might want to make systemD:vol1 a destination of systemM:vol1 instead of a destination of systemC:vol1.

Figure 1. Restructuring the relationship of the destinations in a cascade

Steps

1. On the destination system, change the /etc/snapmirror.conf file to indicate the new source for the destination.

   Example

   systemM:vol1 systemD:vol1 - 35 * * 1,2,3,4,5

2. If...	Then...
   The newest Snapshot copy on the destination exists on the source.	Use the following command to update the destination from the new source.snapmirror update -S source_volume dest_system:dest_volume For example:snapmirror update -S systemM:vol1 systemD:vol1
   The newest Snapshot copy on the destination does not exist on the source.	Perform one of the following tasks. Update the new source from the original source using the snapmirror update command. Wait for the destination to update. Make the destination writable using the snapmirror break command. Then resynchronize the destination with the new source using the snapmirror resync command.

3. Release the former source using the following command:snapmirror release source_volume [[dest_system:]dest_volume]

   Example

   systemC> snapmirror release systemC:vol1 systemD:vol1

Disconnecting a destination from a cascading series

Figure 2. Disconnecting a destination in a cascade

For the configuration depicted in the preceding diagram, suppose that from systemB you enter the following command:snapmirror release vol1 systemC:vol1

These results follow.

• systemA:vol1 continues to be the source for the destination systemB:vol1.
• systemC:vol1 no longer copies from systemB:vol1. SnapMirror retains Snapshot copies for systemC and below.
• If systemC requests an update from systemB, the destination is reestablished if it is still not writable and the base Snapshot copy still exists on the source.
• systemD:vol1 still copies systemC:vol1.
• All the destinations that depend on systemL:vol1 continue functioning as before.

You can check that the destination was released by running the snapmirror destinations command on systemA, as follows.

systemA> snapmirror destinations -s systemA:vol1

Volume Snapshot                     Destination
vol1   systemB(0015269532)_vol1.37  systemB:vol1
vol1   systemL(0015269532)_vol1.42  systemL:vol1->systemM:vol1->systemXvol1->systemY:vol1
vol1   systemL(0015269532)_vol1.42  systemL:vol1->systemM:vol1->systemXvol1->systemZ:vol1
vol1   systemL(0015269532)_vol1.42  systemL:vol1->systemM:vol1->systemN:vol1

Note: If you want to permanently release a destination, you need to either delete the entry in the /etc/snapmirror.conf file or comment out the entry by preceding it with a pound sign (#). Otherwise, SnapMirror continues to try to update the destination.