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Re: raid5 resync aborted under heavy XFS use

To: Chris Bednar <cjb@xxxxxxxxxxxxxxxxxxxxxxxxx>
Subject: Re: raid5 resync aborted under heavy XFS use
From: Seth Mos <knuffie@xxxxxxxxx>
Date: Mon, 30 Jul 2001 00:12:27 +0200 (CEST)
Cc: Linux XFS Mailing List <linux-xfs@xxxxxxxxxxx>
In-reply-to: <Pine.LNX.4.10.10107291519140.11670-100000@linux1.production.mnd.com>
Sender: owner-linux-xfs@xxxxxxxxxxx
On Sun, 29 Jul 2001, Chris Bednar wrote:

> > there are some slight problems with xfs over software raid5 that have just 
> > been fixed in the CVS tree. There were also IO stall problems with this 
> > setup when you have a internal log. Basically you also want to make the log 
> > an external log which has a large performance boost.
> > 
> > Search the archive for discussion about this. 1 week ago it was discussed 
> > and it has some bencmarks to back it up.
>     I've seen them. For some reason, I'm suspicious that this
> is a different problem.
>     An external log is scary to me;  I can afford lackluster
> write performance easier than I can afford:
>   ``You know that $16k RAID setup you bought? it's gone to Hell
>     because the one disk I was using for the log croaked.''
> or:
>   ``You know that big RAID system? Well, I moved it from one 
>     machine to another, and now it won't work.''

Then make sure it's on a raid1 disk. And mark it with bright red labels.
Otherwise you have a $16k raid setup that won't perform.
Btw, if you do a clean umount and then move it you might have
lost the log but you won't have lost the files.

> In my opinion, an internal log has to work reasonably well for
> XFS to be viable. It's fine, of course, if an external log works
> better, and I don't mind doing that on my own systems.

It's called flexibility ;)
There must be a reason that it was originally thought off.

>     I'll take a shot at CVS... by the way, is an internal log
> also a performance issue on IRIX systems?

The issue with a log on raid5 is only valid for software raid5. I have
seen no such issues with a hardware raid5 IIRC.
It's just that the md raid code got in the way of the log.


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