RN314 - want to limit number of daily snapshots kept

Yes, once free space falls below 5%, the oldest automatic snapshots will be deleted to free up some space until more than 5% is free again.

Welcome to the forum!

Note also that the NAS will automatically prune snapshots - http://kb.netgear.com/app/answers/detai ... management

Monthly snapshots are kept forever (which in my personal view is a mistake). But there aren't that many to manually delete.

There are two downsides with using snapshots on shares with a lot of data churn. You mentioned one - if there is a lot of data churn in the share, then the space used goes up The second is that the changing files can be become fragmented. For instance, if you have an SQL database in the share that is always being updated, taking snapshots will fragment the database file. So If you have a lot of data churn on a share, you might want to turn snapshots off altogether.

At the other extreme - if there is no churn at all in the main share, then snapshots are free. they take essentially no disk space. Similarly, if files are only added (but not renamed or deleted), snapshots are also free.

I am thinking that if snapshots need to be deleted after two weeks, then either the isn't enough disk capacity (and you should add a third drive) or snapshots likely need to be turned off for some shares.

Well, it's really the CoW that's causes the "fragmentation", since a file that may have miraculously been written to contiguous blocks will no longer be once even one of its blocks is updated. But it is of course true that snapshots can't be enabled without also having CoW enabled. :)

I didn't realize monthly snapshots were retained indefinitely. Might make me rethink using weekly instead. But that does make me curious how to adjust the retention rules, as mdgm mentioned. (Whoops! That showed up in the RSS feed but not in the web site... "This percentage can be adjusted via SSH.")

If CoW is disabled on a share when you take a snapshot it will do CoW at that point in time.

Also good to know. But it is possible to have CoW enabled without snapshots. That's the basic point I wanted to make. The block behavior is driven by CoW.

btaroli wrote:
Also good to know. But it is possible to have CoW enabled without snapshots. That's the basic point I wanted to make. The block behavior is driven by CoW.

Yes.

-if bitrot protection is enabled and snapshots are enabled then CoW is enabled all the time
-if bitrot protection is disabled and snapshots are enabled, then CoW is enabled before a snapshot is taken, and disabled immediately afterwords.

In the latter case, "enabling" CoW means that the snapshot and the main share start out both pointing to the same underlying data when the snapshot is taken. If CoW were disabled, then the snapshot would start out as an independent copy. Future changes to the main share can still increase fragmentation, and will increase the amount of real disk space taken up by the snapshots.

Either way, it is the data churn that actually causes the fragmentation. If files are not updated (but are only added/deleted/renamed or completely re-written) then CoW will not increase fragmentation.