Snap shot policy Diffrence ?

[HENRYPAN2]

Good Morning DOT Gurus,

Would you please advice what’s the difference between the 2 snap shot policy below:

4 hourly + 6 daily + 13 weekly or 90 daily

Thanks in advance & looking forward to hear from you.

Henry PAN

[aborzenkov]

Well … I am not sure I understand the question. Could you explain what is unclear to you?

[HENRYPAN2]

Ha-ha

aborzenkov,

It's claer. Just wish to know the diffrence.

Which policy will use less disk space ?

Which policy will be better for what kind of restore ?

Thanks

Henry

[aborzenkov]

It is not possible to give blanket answer. Less snapshots will likely take up less space, and the longer snapshot is kept the more difference is accumulated (i.e. snapshot “grows”). But again - it is possible that there is large amount of transient data generated at specific (time of) day which is captured by frequent snapshots, but weekly snapshot capture mostly the same static information and so does not take any space at all.

YMMV … you have to experiment what is better suited for your specific conditions.

[scottgelb]

snap delta is your friend to see the overhead between snaps and also the active file system.  If you have 4 weeklys, then usually the nightlys during the week don't free much space when deleted since changed blocks are also in the longer retention weekly snaps.  There is some variation depending on when changes/deletes are made.  At a lot of my customers there is a lot of append data, so snapshots don't take much overhead at all...they could have 90 snaps or 200 snaps and their snapshot overhead would be the same since new blocks to the active file system won't grow the snapshots.  snap reclaimable is also a really nice command...you can use it to see that if you delete weekly 2 and 3 that you might get 10GB back, but if you delete weekly 2,3 and 4 you get 800GB back to let you see what combination of snapshots or single snapshot gets you back for space.