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Forum Discussion
ReadyNAS 312 Need Help Understanding Snapshots
We're about to pull the trigger on a ReadyNAS 312 with either two 2TB or 4TB drives. We have about 290GB of data files. Ideally we'd like to have daily snapshots, but I'm unsure how much data a snaps...
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.
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.
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