snapshot/snapclone

[Oleg Mochkin]

What is differences between vsnap and snapclone ? (related eva3k/eva5k). Oleg

[Uwe Zessin]

A 'vsnap' is a space-preserving snapshot. The vsnap is tied to the parent virtual disk and being presented by the same controller.

A snapclone starts out as a snapshot, but all data from the parent virtual disk is copied to form a completely new virtual disk. Once the copy is done it is no longer tied to the same controller like the parent virtual disk. You can also create the snapclone into a different disk group.

[Mike Naime]

In addition to what Uwe already provided.

Snapshot is available for use almost immediately. We use SNAPSHOT for backup purposes.

Snapclone is not available until ALL of the original data is coppied. On a LARGE LUN of 1+TB, this can require several hours to create depending on how busy the EVA is. (We never timed one)

[Uwe Zessin]

Mike,
what do you mean by 'not available'? I have not tried the snapclone in a while, but I am pretty sure you can present it to a host while the data is still copying. Granted, not all options are available (e.g. controller assignment), but that is clear, because some blocks are still linked to the parent vdisk.

[Andrew_168]

Hi,
You are quite correct a snapclone is available from the moment it is created, but it is referenced by the application as a snapshot until the clone process completes, also be aware snapshots are not the answer to all backup problems, as the child relies on the parent volume, if the parent is being thrashed by an application when a backup takes place this causes more I/O's on the parent this causes the application/backup to slow down. Snapclones/Clones are far better for backup if using a high speed device such as LT02.

[Uwe Zessin]

Hello Andrew,
I thought I have described the snapshot/snapclone like you did with the exception of talking about backup. Is there something wrong with it?

I wonder: have you measured the impact of a backup against a snapshot vs. the creation of a snapclone? The snapclone copy speed is about 100 GByte/hour, I have been told - that makes almost 28 MBytes/second and if you want to avoid the backup I/Os hitting the parent, you have to first wait until the snapclone is done. After the backup you have to get rid of the clone - have you tried to find out what is faster: deletion of a snapshot or the clone? (This is not a rethorical question)

Don't get me wrong: I don't want to argue that snapshots are better than snapclones.

[Mike Naime]

Uwe:

I did not say that the Snapclone could not be presented immediately to another sytem. I said that it would not be ready for backups immediately. You yourself stated that the snapclone coppied at about 100GB/hour. Therefore, If you wanted to backup to tape from your snapclone a 1TB disk, you would have to wait about 10 hours before the entire contents of the LUN was copied. Plus, you would need that extra TB of EVA space to do it with!


Here is our backup scheme:
A.) RMAN backup ==> Backup LUN on EVA.
B.) When RMAN backup is compleated.
C.) SNAP Backup LUN on EVA using no extra space.
D.) Present LUN to backup cluster for sending to tapes.
E.) Delete Snapshot to make space available for another PROD cluster.
F.) Repeat the next night.

Non DB drives use a similar Image/Incremental scheme to another LUN on the EVA.

Note: RMAN only backups USED Oracle space. Not the full Allocated space. This can save you TB of data on multiple clients!

So, I guess that you could say that we are actually running 2 backups every night. The first one goes Disk-2-Disk. The second one goes Disk-2-tapes (Onsite/Offsite) Where we backup they results of the first backup to tape.

[Uwe Zessin]

I'm sorry, Mike, it's all my fault :-(

I somehow have missed your comment that you use snapshots for backup and so I thought we were still discussing this from an 'abstract' level, but the arguments didn't make sense to me. Sigh.

[Mike Naime]

Uwe:

I believe that we where the first and possibly only VMS client to dedicate an entire EVA5000 with 240 spindles to backups. When we had it installed, the install folks said "You want to do what?"

[Uwe Zessin]

Cool! I guess they have been first-time visitors at your site? Else they should have known better...

Last week I have been on a HP event where one guy was talking about future products. He was very enthusiastic, because now he could store all his Britney Spears pictures at a cheap price ;-)