How to Use a Mirror to Make an Online Backup (Command Line)

Although DiskSuite is not meant to be a "backup product," it does provide a means for backing up mirrored data without unmounting the mirror or taking the entire mirror offline, and without halting the system or denying users access to data. This happens as follows: one of the submirrors is taken offline--temporarily losing the mirroring--and backed up; that submirror is then placed online and resynced as soon as the backup is complete.

You can use this procedure on any file system except root (/). Be aware that this type of backup creates a "snapshot" of an active file system. Depending on how the file system is being used when it is write-locked, some files and file content on the backup may not correspond to the actual files on disk.

Limitations

• If you use this procedure on a two-way mirror, be aware that data redundancy is lost while one submirror is offline for backup. A three-way mirror does not have this problem.

• There is some overhead on the system when the offlined submirror is brought back online after the backup is complete.

If you use these procedures regularly, put them into a script for ease of use.

The high-level steps in this procedure are:

• Write locking the file system (UFS only). Do not lock root (/).

• Using the metaoffline(1M) command to take one submirror offline from the mirror

• Unlocking the file system

• Backing up the data on the offlined submirror

• Using the metaonline(1M) command to place the offlined submirror back online

1. Before beginning, run the metastat(1M) command to make sure the mirror is in the "Okay" state.

   A mirror that is in the "Maintenance" state should be repaired first.

2. For all file systems except root (/), lock the file system from writes.

   # /usr/sbin/lockfs -w mount point

   Only a UFS needs to be write-locked. If the metadevice is set up as a raw device for database management software or some other specific application, running lockfs(1M) is not necessary. (You may, however, want to run the appropriate vendor-supplied utility to flush any buffers and lock access.)

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   Caution -

   Write-locking root (/) causes the system to hang, so it should never be performed.

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3. Take one submirror offline from the mirror.

   # metaoffline mirror submirror

   In this command,

   mirror	Is the metadevice name of the mirror.
   submirror	Is the metadevice name of the submirror (metadevice) being taken offline.

   Reads will continue to be made from the other submirror. The mirror will be out of sync as soon as the first write is made. This inconsistency is corrected when the offlined submirror is brought back online in Step 6.

   There is no need to run fsck(1M) on the offlined file system.

4. Unlock the file system and allow writes to continue.

   # /usr/sbin/lockfs -u mount point

   You may need to perform necessary unlocking procedures based on vendor-dependent utilities used in Step 2 above.

5. Perform a backup of the offlined submirror. Use ufsdump(1M) or your usual backup utility.

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   Note -

   To ensure a proper backup, use the raw metadevice, for example, /dev/md/rdsk/d4. Using "rdsk" allows greater than 2 Gbyte access.

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6. Place the mirror back online.

   # metaonline mirror submirror

   DiskSuite automatically begins resyncing the submirror with the mirror.

Example -- Using a Mirror to Make an Online Backup

This example uses a mirror named d1, consisting of submirrors d2 and d3. d3 is taken offline and backed up while d2 stays online. The file system on the mirror is /home1.

# /usr/sbin/lockfs -w /home1
# metaoffline d1 d3
d1: submirror d3 is offlined
# /usr/sbin/lockfs -u /home1
(Perform backup using /dev/md/rdsk/d3)
# metaonline d1 d3
d1: submirror d3 is onlined