Re: fsck in -current

From: Michael Hamburg <hamburg_at_fas.harvard.edu>
Date: Sun, 16 May 2004 01:43:37 -0400
On May 15, 2004, at 10:41 PM, Marc G. Fournier wrote:

> On Sat, 15 May 2004, Michael Hamburg wrote:
>
>> 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.
>
> 'k ... fsck just started spewing messages to the screen, which is nice
> cause now I know its actually doing something:
>
> ZERO LENGTH DIR I=5159669  OWNER=root MODE=40755
> SIZE=0 MTIME=May 10 17:32 2004
> CLEAR? yes
>
> now, the ZERO LENGTH DIR is a result of using unionfs ... but would 
> that
> cause any issues with the snapshotting?  like, how much 'free space' 
> would
> you need, and what happens ifyou don't have enough? :(
>
I don't know about ZERO LENGTH DIRs... I've never had to deal with 
anything terribly interesting in an interactive fsck.

You don't need much space unless you have high turnover -- the default 
extra space in FFS that you can't allocate anyway should be plenty, but 
as far as I can tell, if you do run out of space, Very Bad Things 
Happen (tm).  Just what those Very Bad Things are probably depends on 
the release.

Mike
Received on Sat May 15 2004 - 20:43:42 UTC

This archive was generated by hypermail 2.4.0 : Wed May 19 2021 - 11:37:54 UTC