ReadyNAS 312 Need Help Understanding Snapshots

Just want to share these articles below if in case have not yet came across with it since might provide you with usable info:

http://kb.netgear.com/app/answers/detai ... erating-my

http://kb.netgear.com/app/answers/detail/a_id/24478

http://www.netgear.com/images/ReadyNAS% ... -76104.pdf

Hi, yeah I did find those article, and they are helpful. I'm still just not grasping the concept of how much data a snapshot uses. Thank you.

Sharing these forum thread links below since its related to your concern. Hope that you will be enlightened:

https://www.readynas.com/forum/viewtopi ... 50&t=74657

viewtopic.php?f=7&t=76917

I initiated a chat support session and the person said Snapshots work like the Time Machine does in Mac. I haven't found any other information to support this, but if that's the case, then basically the first snapshot is a full "backup" and each subsequent snapshot is a differential "backup". I'm really hoping that's the case.

It's wrong to think of the snapshot as a backup. I think the support agent gave you misleading information at best.

The answer to your question depends on the file churn after the snapshot is created.

Let's use 100 10MB files in a share as an example, and assume no file fragmentation at the start.

When you first make a snapshot it actually takes no space. All the datablocks in the snapshot are shared by the main share and the snapshot.
So there is 1000MB total: 1000MB in common; 0 MB only in the snapshot; 0 MB only in the share.

Now imagine that fifty 10 MB files are deleted on the main share. The original datablocks remain in the snapshot, but are no longer shared.
So there is 1000MB total: 500MB in common; 500 MB only in the snapshot; 0 MB only in the share.

Furthermore, imagine that one file is completely changed in the main share. Again, the original datablocks end up in in the snapshot alone, and we have 10MB of more total space.
So there is 1010MB total: 490MB in common; 510 MB only in the snapshot; 10MB only in the share.

Now lets modify the first 5 MB of another file. That 5 MB can no longer be in common, total space goes up 5 MB. The version of the file in the main share is fragmented, the version in the snapshot is not fragmented.
So there is 1015MB total: 485MB in common; 515 MB only in the snapshot; 15MB only in the share.

Now lets add 10 new files of 10 MB each to the share. These aren't in the snapshot, but only in the share.
So there is 1115MB total: 485MB in common; 515 MB only in the snapshot; 115MB only in the share.

I can't tell you how helpful that is StephenB. Makes perfect sense and I think Netgear should append this to their KB articles.

Glad it helps. Of course tracking all this becomes more difficult as you create more snapshots.

But the principles are the same:
Whenever a shared datablock is deleted in the main folder, it becomes snapshot space.
Whenever a shared datablock is modified in the main folder, CoW creates a new copy of that datablock in the main folder, and the original ends up in one or more snapshots.

It's not a backup, because if there were data corruption in a shared block, then the snapshot and the main folder are both corrupted.

Very helpful. Main reason we like this is we use Previous Versions in Server 2008, and I can't tell you how many times people want to go back 3 hours for a Word document they messed up. If we did hourly snapshots this would be a lifesaver. I guess my only issue is you have to "clone" the entire snapshot to get a single file restored, but from my reading you just clone the snapshot to a new location and grab the file you need. Hopefully I'm understanding that correctly :-)

Previous versions works with these snapshots too. :-)

Oh, I mean we will no longer be in a server environment once we go to ReadyNAS. Just too few people to justify the added expense and overhead. Where are these previous versions being stored?