Re: fsck in -current

From: Michael Hamburg <hamburg_at_fas.harvard.edu>
Date: Sat, 15 May 2004 21:39:09 -0400
On May 15, 2004, at 9:08 PM, Marc G. Fournier wrote:

>
> I'm seriously considering putting 5.x onto my next server, to take
> advantage of, if nothing else, the reduction in the GIANT LOCK reliance
> ... one "concern" I have is how fsck works in 5.x ...
>
> Right now, on 4.x, I have an fsck running that has been going for ~3hrs
> now:
>
> # date; ps aux | grep fsck
> Sat May 15 22:04:00 ADT 2004
> root    40 99.0  4.5 185756 185796  p0  R+    6:55PM 164:01.60 fsck -y 
> /dev/da0s1h
>
> and is in Phase 4 ...
>
> In 5.x, if I'm not mistaken, fsck's are backgrounded on reboot, so that
> the system comes up faster ... but:
>
> a. wouldn't that slow down the fsck itself, since all the processes on 
> the
>    machine would be using CPU/memory?

Yes.  You can probably renice it or something, though, and it wouldn't 
take that much longer.

Furthermore, if I recall correctly, it doesn't check as much after a 
crash when soft-updates are enabled.  See below.

> b. how long could fsck run in the background safely ... like, if I
>    rebooted a machine, fsck backgrounded and then all the processes
>    started up, is there a risk involved?
>

No, fsck should be able to run in the background indefinitely with 
basically no risk, so long as you have free space on your disk.  The 
way that the latest fsck works is that it snapshots the drive, and then 
checks the snapshot; the only type of error that it's expecting to find 
is files which have been deleted but whose space has not been 
reclaimed.  This space can be recovered without confusing running 
processes.

I've read that the snapshot code is safe for fsck and dump, but can 
still deadlock if you try to mount a snapshot or shut down with a 
snapshot present, but the document dates from two years ago.  Is this 
still true?

> ----
> Marc G. Fournier           Hub.Org Networking Services 
> (http://www.hub.org)
> Email: scrappy_at_hub.org           Yahoo!: yscrappy              ICQ: 
> 7615664

Mike Hamburg
Received on Sat May 15 2004 - 16:39:15 UTC

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