[BBLISA] ZFS question

[Rudie, Tony]

I'll just outline what Network Appliance equipment does, for contrast:

For auto-clearing, there are hierarchical types of snaps.  Hourlies, nightlies, and weeklies.  You define how many of each to keep before letting them roll of the back

For space consumption, each volume has a "snap reserve" associated with it.  One way to think of it is that the files fill up the volume from one end, and the snaps fill it up from the other, and the snap reserve determines how you report on it.  If, for example, snap reserve is set to 20%, and the volume has 19% of its space taken up by snaps and 70% of its space taken up by files, df will report as follows:

Vol1  87.5% 
Vol1/.snapshot   95%

If the same volume has 70% as files and 21% as snaps, df will say:

Vol1  88.7%
Vol1/.snapshot   105%

Because, in the second case, the snaps went over the edge of the snap reserve and started using up space in what is being reported on as file space.  Nothing breaks in this case, it just looks odd.

If you change the snap reserve on the volume to 21%, but still have 21% snap, 70% data, df looks like this:

Vol1  89.8%
Vol1/.snapshot   100%

More than you wanted to know, but...


 - Tony Rudié 

-----Original Message-----
From: bblisa-bounces at bblisa.org [mailto:bblisa-bounces at bblisa.org] On Behalf Of Edward Ned Harvey
Sent: Thursday, October 25, 2007 9:53 PM
To: bblisa at bblisa.org
Subject: RE: [BBLISA] ZFS question

I agree, the thing that is causing annoyance for Theo is in fact something
I've come to accept as normal.

It doesn't often matter, but yes it is distinctly possible for snapshots to
take up too much space and fill the filesystem.  When that happens (and it's
of course bound to happen) you have to be aware that simply removing your
files is not enough.  You have to clear the snapshot copy of your files too.

I don't know about the scripts to automatically clear the snapshots, but
this is the config that I had, which was very effective (was preconfigured
in the netapp before I ever was hired there)
	A snapshot once every 4 hours for the most recent day
	A snapshot once a day for the most recent week
	A snapshot once a week for the most recent month.
So clearly there was in fact *some* thing in place to automatically clear
snapshots.

It is clear also, that there is necessarily a mechanism to automatically
clear snapshots in some way or another.  If you never clear snapshots, then
your disk will in fact be remembering every change, every deleted file that
ever existed, and it will inevitably fill up.



> -----Original Message-----
> From: bblisa-bounces at bblisa.org [mailto:bblisa-bounces at bblisa.org] On
> Behalf Of Tom Metro
> Sent: Thursday, October 25, 2007 8:05 PM
> To: Theo Van Dinter
> Cc: L-bblisa
> Subject: Re: [BBLISA] ZFS question
> 
> Theo Van Dinter wrote:
> > One thought of how to with this was to simply tack on some generic
> > percentage to the quota, say 20%, which would allow for snapshots,
> > ie: 12G.
> 
> This sounds more like a matter of perspective than a technical problem.
> If the point to using quotas is to limit per-user resource usage, then
> it seems pretty reasonable to include snapshots in the quota. Make the
> user aware of this, and seeing as you are customizing thing per user,
> give them the choice of how much space should be used up by snapshots
> (indirectly by tuning the frequency of the snapshots).
> 
> 
> > However, then there's nothing to stop the user from using all
> > 12G, and we're back in the same situation again.
> 
> Does ZFS support soft quota limits? That would be one way of alerting
> the user that they are going beyond the limits where they will be able
> to maintain both data and snapshots.
> 
>   -Tom
> 
> --
> Tom Metro
> Venture Logic, Newton, MA, USA
> "Enterprise solutions through open source."
> Professional Profile: http://tmetro.venturelogic.com/
> 
> _______________________________________________
> bblisa mailing list
> bblisa at bblisa.org
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