Re: fsck, raid reconstruction & bad bad 2.4.3

[Ookhoi <ookhoi@dds.nl>]

Hi Linas Vepstas,

(nice name ;-)

> First problem:  In kernel-2.4.2 and earlier, if the machine is not cleanly
> shut down, then upon reboot, RAID reconstruction is automatically started.
> (For RAID-1, this more-or-less ammounts to copying the entire contents
> of one disk partition on one disk to another).   The reconstruction
> code seems to be clever: it will try to use the full bandwidth when
> the system is idle, and it will throttle back when busy.  It will
> only throttle back so far: it tries to maintain at least a minimum amount
> of work going, in order to gaurentee forward progress even on a busy system.
> 
> The problem:  this dramatically slows fsck after an unclean shut-down.
> You can hear the drives machine-gunning.  I haven't stop-watch timed it,
> but its on the order of 5x slower to fsck a raid partition when there's
> reconstruction going on, then when the raid thinks its clean.  This
> makes unclean reboots quite painful.
> 
> (There is no config file to disable/alter this .. no work-around that I
> know of ..)

One possible 'work-around' is to use a journaling filesystem (like
reiserfs) which eliminates the fsck after a unclean shutdown. It's very
nice to have a crashed system back online fast. The raid sync makes the
system a bit slow, but as you said, it syncs at full speed when idle,
and is nice when less idle. :-)

	Ookhoi