Replicating a volume

Description

Replicate a volume if you are in any situation described in "Purposes of SnapMirror." You can replicate a volume at any time. After you replicate a volume, you have a mirror that contains the same data as the source volume.

Prerequisites

The following prerequisites must be met before you can replicate a volume:

You must purchase the SnapMirror license. If the source volume and the mirror are on different filers, you must purchase a license and enter the SnapMirror license code for each filer. Refer to "Enabling services" for information about how to enter a license code.
You must create an off-line volume to be used as the mirror. SnapMirror does not automatically create a volume. Refer to "Creating volumes" for information about how to create a volume; refer to "Making a volume inactive" for information about how to take a volume off-line.
The source volume must be on-line. Refer to "Reactivating an off-line volume" for information about how to put a volume on-line.
The source volume must not be a mirror.
The capacity of the mirror must be greater than or equal to the capacity of the source volume. The configuration of the volumes, however, can be different. For example, the RAID group size can be different for the two volumes. Refer to "Adding disks to a volume" for information about how to add disks to a volume.
The mirror must not be the root volume.

Restrictions

Each filer supports up to four simultaneous volume copy operations. For information about volume copy operations, refer to "Number of volume copy operations SnapMirror generates."

Cautions

SnapMirror creates snapshots in the source volume, which are copied to the mirror. Do not delete these snapshots because incremental changes to the mirror depend on the snapshots. If the filer cannot find the required snapshot, it cannot perform incremental changes to the mirror.
Refer to "Snapshots created during data replication" for an example about how snapshots are created on both the source volume and the mirror.
The mirror must have snapmirrored status. Do not disable the snapmirrored status using the volume options command. Otherwise, the mirror becomes a regular, writable volume. Disable the snapmirrored status only when you no longer need to mirror incremental changes from the source volume.

Recommendations

Follow these recommendations to minimize confusion or to replicate data efficiently:

When specifying the mirror update schedule in the /etc/snapmirror.conf file, stagger the update times instead of starting multiple mirror updates at the same time. If the filer does not have enough resources to perform all scheduled mirror updates, it postpones some updates until it has enough resources to do so. As a result, the filer might need to perform subsequent updates at times that are different from those you specify in the /etc/snapmirror.conf file.
During data replication, the filer copies data from all snapshots from the source volume. Therefore, the filer must preserve all snapshots when data replication is in progress, even though the snapshot schedule might call for the deletion of some snapshots. If you want the filer to delete snapshots at the exact time specified by the snap sched command, schedule the mirror updates to be different from the snapshot deletion times.
The filer transfers data faster if the source volume and the mirror have the following characteristics:
- They contain disks of the same size.
- They contain RAID groups of the same size.
- They contain the same number of RAID groups.
If the source volume has quotas enabled and you want to apply the same quota restrictions to the mirror after converting the mirror to a regular volume, make sure that the filer on which the mirror resides contains a copy of the /etc/quotas file from the source filer. Whenever you make a change to the /etc/quotas file on the source filer, make the same change to the copy of the /etc/quotas file on the destination filer.

Steps

Follow these steps to replicate a volume.


Step	Action
1	Add the host name of the destination filer to the /etc/snapmirror.allow file on the source filer.
2	Edit the /etc/snapmirror.conf file on the destination filer to specify the volume to be replicated and the schedule at which the mirror is updated.
3	Enter the `vol snapmirror on` command on both the source filer and destination filer to enable SnapMirror. Result The filer reads the /etc/snapmirror.conf file. If the filer is a destination filer specified in the /etc/snapmirror.conf file, it establishes a connection with the source filer at the time of the scheduled mirror update. If a baseline version of the mirror does not exist, the filer takes a snapshot of the source volume at the time of the scheduled mirror update and transfers all data in the snapshot from the source volume to the mirror. The filer does not display any messages if it can transfer the data successfully. You can check the data replication status following the instructions in "Checking data replication status." The filer makes subsequent updates to the mirror according to the schedule specified in the /etc/snapmirror.conf file. Note: The `vol snapmirror` `on` command does not persist across filer reboots. Put the command in the /etc/rc file if you want the command to remain in effect after the filer is rebooted. If the /etc/rc file does not contain a `vol snapmirror` command, data replication is disabled.
4	If the source volume and the mirror reside on different filers, and if the source volume has quotas enabled, copy the /etc/quotas file from the source filer to a file on the destination filer. Whenever you make a change to the /etc/quotas file on the source filer, make the same change to the copy on the destination filer so that the destination filer always contains a record of all the quota entries used by the source filer.

Part No. 210-02334
File: mirror8.htm
Last updated: 02/24/99 12:17:14