fsck of big disk

Josh Paetzel josh at tcbug.org
Tue Dec 11 07:54:43 PST 2007

On Tuesday 11 December 2007 09:19:46 am Valerio Daelli wrote:
> Hi,
> thanks a lot for your answer.
> > > we have a freshly installed FreeBSD 6.2 machine with a gstriped
> > > external disk, of 5.3Tb. The disk is composed of two slices of 2.6 Tb.
> > > We are trying to have a (background) fsck of it but few hours later
> > > since the start of the check
> > > the host get unresponsive: it responds to ping but it doesn't let
> > > login anyone, nor by ssh
> > > nor by console.
> >
> > What do you get if you press Control-T on the console when it is
> > unresponsive?
> We are not able to login via console nor via ssh.
> Pheraps you suggest me to press Control-T on the console even if I am not
> logged on. I did not try, sorry.
> > When you do a background fsck there is a point at which a snapshot of
> > the filesystem is taken, although I'd say that happens at the beginning
> > of the check. The problem is that with 5.3 Tb, it may take quite a while
> > to take the snapshot, and during that time the system blocks any process
> > that tries to write to the filesystem. Maybe that's what you are
> > experiencing.
> What you explained us sounds very interesting. But anyway we simply
> solved by disabling background fsck on such a big partition. We had
> panics and reboot
> and we did not want to risk on a production host.
> Bye
> Valerio Daelli

What you are probably running in to is memory starvation.  In my experience 
fsck on a moderately filled disk uses something on the order of 1 gig of RAM 
per TB of filesystem.  If you have this in an array with less than 4 gigs of 
RAM it's possible that it was just buried in swap and spending all it's time 
moving pages in and out as opposed to doing anything useful.


Josh Paetzel

PGP: 8A48 EF36 5E9F 4EDA 5A8C 11B4 26F9 01F1 27AF AECB
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