Re: SUSE 9.2: SoftRAID infinte sync!
From: Michael Heiming (michael+USENET_at_www.heiming.de)
Date: 11/21/04
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Date: Sun, 21 Nov 2004 16:32:31 +0100
In comp.os.linux.setup Peter T. Breuer <ptb@lab.it.uc3m.es>:
> Michael Heiming <michael+USENET@www.heiming.de> wrote:
>> In comp.os.linux.setup Peter T. Breuer <ptb@lab.it.uc3m.es>:
>> > Harald Schneider <h_schneider@marketmix.com> wrote:
[..]
>> IIRC, mirroring the complete system via sw raid doesn't work on
>> suse, at least on sles 8 it doesn't. Setup sw raid on an already
>> installed (unmirrored) system, which worked fine, but it couldn't
>> reboot. For the fun of it, tried installing from scratch, but
>> alas the installer said it couldn't setup the complete system
>> with sw raid. Strange, this setup works on redhat quite nice, the
>> raid can be setup via kickstart auto-magically.
> It is purely a kernel thing. If they have the kernel module, and their
> patches have not messed it up (which is possible!), then it will work.
> There are no ifs and buts - nothing else is involved apart from the
> kernel, which you can change to your taste :).
>> It can still boot with a single disk in case of failure.
> Hmm.
IIRC, both disks are marked boot-able and lilo directly uses
/dev/md?. There was only a single disk failure, those aren't
hot-pluggable, so the box had to be taken down to put in a new IDE
drive. Came up fine with the degraded array, just needing a few
commands to start rebuilding.
[..]
> No - it does not work fine because it can't. Journalled file systems
> cannot work (in the sense of doing what they are supposed to :) unless
> the order of write requests hitting the disk matches the order of write
> requests emitted. And reads must hit disk after the write request that
> preceded them! You can understand why.
> Under ordinary conditions the ide driver just sends and receives in the
> order that it gets the requests. But even the ide driver has an
> internal queue - internally it could reorder writes, and that would
> invalidate the consistency on disk of the file system, as it is
> purported to be "guaranteed" by the Jfs. This happens more often the
> more parallelism and asynchronicity there is in the system, and the
> more heavily it is used. Put two disks in there on two different
> channels and a large internal queue on each channel and all bets are
> off as to the ordering. And THEN there is the problem that nothing that
> passes through the raid driver gets any guarantee of order preservatoon
> at all, even if there is only one disk in the raid, since the driver
> does not attempt to check or maintain ordering. It's all down to luck.
In theory this sounds good, in reality it works for me, unsure
why the installer makes it possible?
There's a "Boot + Root + Raid + Lilo : Software Raid mini-HOWTO"
describing the process if some installer can't do it, or one
would like to setup sw raid while running from a single disk.
You could mount with "data=journal,noatime" thinking about
/var/spool.
In theory 'parted' should have been able to resize this
partition:
/dev/uba5 120G 8.9G 111G 8% /mnt/backup_extra
In reality it didn't, so I backup-ed 8.9G and will delete/recreate
the partition.
[..]
> As in softraid - provided you can hotplug on the bus in question at all.
> If it is scsi, umm, probably you can. If it is IDE, pray.
Those are IDE and not hotplug, have another box with scsi
(hot-plug) and the same sw raid setup, but you seldom see scsi
disks breaking without lot of warnings long before.
AFAIK it should work with some per-caution, probably not just rip
of a running disk like hw raid controller do perfectly. [..]
[..]
> A local lady representative from redhat emailed me to say that she had
> been trying to contact me, but I never had a phone call from her (nor
> had she phoned me when I was out, to my knowledge). I emailed back to
> say that email would be fine and there was no need to phone. I'll
> remind her.
Good idea, the discussion pops up from time to time and it would
be great to get some answer.
Regards
-- Michael Heiming (X-PGP-Sig > GPG-Key ID: EDD27B94) mail: echo zvpunry@urvzvat.qr | perl -pe 'y/a-z/n-za-m/' #bofh excuse 124: user to computer ration too low.
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