20 snapshot limit per filesystem?

Lowell Gilbert freebsd-questions-local at be-well.ilk.org
Mon May 23 06:17:12 PDT 2005


Rich Winkel <rich at math.missouri.edu> writes:

> Hi, I was just wondering how much overhead would be incurred by increasing
> the 20 snapshot limit on mksnap_ffs ?

Unfortunately, I don't see a way to give a short answer to that
question.  On a fairly static filesystem, the answer would be quite
small.  On a more active filesystem, the number (and size) of
different files being modified is more relevant.

I tried to do some calculations on my own system, but I kept finding
factors that I had overlooked.  In practice, it was easier to make my
snapshots and see how much disk space and CPU time were used over time.

> I use hard links to get snapshot-like functionality under 4.x.  I can
> recover accidentally deleted files for up to 30 days.  I was hoping
> I could switch to snapshots without crimping this strategy...

You could always try it and see what happens.  Or you could change
your strategy slightly.  I keep weekly snapshots for a few weeks, but
daily snapshots only stick around for a week.  If you did something
like that, you could also keep hourly snapshots for the last few
hours.  For most users, this kind of approach would be much more
likely to have an accidentally deleted file in a snapshot.

> By the way, how do snapshots interface with user disk quotas?  I assume
> files which exist only in a snapshot aren't counted by the quota system.

Right.  


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