Q&A Pool Snapshot. What you should know before implementing pool snapshot.

  • 3701522
  • 12-Oct-2006
  • 30-Apr-2013

Environment

Novell NetWare 6.5 Support Pack 5
Novell NetWare 6.5 Support Pack 4
Novell NetWare 6.5 Support Pack 3

Situation

Q&A: Questions Pool Snapshot. What you should know before implementing pool snapshot.
1. Can I create a snapshot of my cluster pool?
2. How many pool snapshots can I have?
3. How much space does a snapshot take up on the stored on pool?
4. How can I access the snapshot data?
5. How do I manage pool snapshots?
6. Are there any startup switches?

Resolution

Q&A: Answers Pool Snapshot.
In NetWare 6.5, Novell Storage Services supports pool snapshots to improve backup and restore services. A pool snapshot is a meta data copy of a storage data pool that preserves a point-in-time view of a data pool. When you back up a pool from its pool snapshot, your backup can capture every file in the pool, even those that are in use at the time.
Novell DocumentationNovell Storage Services Administration | Using NSS Pool Snapshots
1. Yes, but there are some limitations. You must store the snapshot data on your cluster pool which you are doing the snapshot of. This is necessary because the snapshot data only contains meta data and any original blocks of any changed data so it must be able to access the real data at all times. A snapshot pool is of no value without access to the real pool.

Note: In some instances it has been recommended that a snapshot be stored on a separate pool to avoid any problems with the snapshot. With clustering this isn't possible because a resource can only be active on one node at time. If the snapshot is on another cluster resource and it fails over it will not be able to access the real data, making the snapshot worthless.

If you decide to implement snapshots on a cluster make sure you have the latest support pack as there have been fixes to the MM.NLM that prevent issues with pool deactivation. It is also recommended that you test it in a lab before rolling it into production if possible.

2. You can have up to 500 snapshots stored on any given pool. The snapshot name can be between 2-16 characters in length. The biggest concern is disk space.
3. The average disk space requirements for a pool snapshot are 10 percent to 20 percent of the original pool size. The actual space depends on how many writes are made to the original volume and the time period that the snapshot exists before it's deleted or archived.
Recommendations: You can have several active snapshots but it's always recommended to delete the oldest ones first. We've seen some issues when having 6-10 active snapshots and trying to delete the most recent one first. This is because the most recent snapshot is where it's keeping track of the changed data so if the most recent one is deleted, it has to move that information back to a previous snapshot and we've seen the delete process hang up the console forever. Engineering is aware of this and is working on a solution.
When a snapshot is created, it will allocate 5000 blocks (by default) to store the meta data of the pool being snapped. If it needs more space, it will dynamically allocate 5000 more blocks. 5000 4KB blocks translates into 20 Megs so your snapshot pool data will increment in size by 20 megs each time it needs more blocks. This space is allocated on the internal volume of the NSS pool (stored-on pool). You may use NSS /SPACE to see how much pool space is available on the pool. You may see how large the snapshot pool has become in iManager. You may increase / decrease the amount of disk space it allocates with the following set parameter. Set Snapshot Allocate Block Count = 1000 - 50000.
4. You must first activate a snapshot. This is done by the following command: MM SNAP ACTIVATE 'SNAPNAME'. This will activate the snapshot and mount the volume with a ' _SV' appended to it. At this point the volume is active and anyone with rights to original volume should be able to access the volume with same rights as they have on the original volume. If multiple snapshots are activated, the snapshot volume name will be incremented. IE: share_sv001. These snapshot volumes can be renamed through NSSMU or iManager but are generally left as default. The Novell documentation talks about more recommendation regarding naming schemes for pool snapshots which might help out when creating multiple snapshots.

5. You have two options. iManager or Command Line.

help MM - will display command line options for pool snapshots which include the following.

MM SNAP CREATE poolname datapool snapname
MM SNAP DELETE snapname
MM SNAP ACTIVATE snapname
MM SNAP DEACTIVATE snapname
MM SNAP LIST
Netware 6.5 SP3 MM.NLM will have a couple new command line switches.
MM SNAP SHRED SNAPNAME = ON
MM SNAP SHRED SNAPNAME= OFF
Please refer to
NOVL100864for specific issue addressed by new parameters.
6. There were a couple switches added (July 6, 2006) to help with pools which couldn't be activated because of an abend on a server or node and the pool snapshot data had become corrupt. Pool snapshot data doesn't use the NSS journaling so if the server abends while having pending IO writes to the snapshot data, there could be some inconsistencies preventing the NSS pool from activating.
Startup switch which can be added to AUTOEXEC.BAT where SERVER.EXE loads.
Example:
SERVER.EXE /NOSNAP
Another switch was added to the MM SNAP DELETE ALL command to eliminate the prompt. /y
Example:
MM SNAP DELETE ALL /Y

Additional Information


Formerly known as TID# 10096484

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