Re: Shutdown filesystem when a thin pool become full

[Brian Foster <bfoster@redhat.com>]

On Tue, Jun 20, 2017 at 05:03:42PM +0200, Gionatan Danti wrote:
> Il 20-06-2017 13:05 Carlos Maiolino ha scritto:
> > 
...
> > Surely this can be improved, but at the end, the application will always
> > need to
> > check for its own data.
> 
> I think the key improvement would be to let the filesystem know about the
> full thin pool - ie: returing ENOSPC at some convenient time (a wild guess:
> can we return ENOSPC during delayed block allocation?)
> 

FWIW, I played with something like this a while ago. See the following
(and its predecessor for a more detailed cover letter):

  http://oss.sgi.com/pipermail/xfs/2016-April/048166.html

You lose some allocation efficiency with this approach because XFS
relies on a worst case allocation reservation in dm-thin, but IIRC that
only really manifested when the volume was near ENOSPC. If one finds
that tradeoff acceptable, I think it's otherwise possible to forward
ENOSPC from the the block device earlier than is done currently.

Brian

> > 
> > I am not really a device-mapper developer and I don't know much about
> > its code
> > in depth. But, I know it will issue warnings when there isn't more space
> > left,
> > and you can configure a watermark too, to warn the admin when the space
> > used
> > reaches that watermark.
> > 
> > By now, I believe the best solution is to have a reasonable watermark
> > set on the
> > thin device, and the Admin take the appropriate action whenever this
> > watermark
> > is achieved.
> 
> Yeah, lvmthin *will* return appropriate warnings during pool filling.
> However, this require active monitoring which, albeit a great idea and "the
> right thing to do (tm)", it adds complexity and can itself fail. In recent
> enought (experimental) versions, lvmthin can be instructed to execute
> specific actions when data allocation is higher than some threshold, which
> somewhat addresses my concerns at the block layer.
> 
> Thank you for your patience and sharing, Carlos.
> 
> -- 
> Danti Gionatan
> Supporto Tecnico
> Assyoma S.r.l. - www.assyoma.it
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