linux-kernel - Re: limits on raid

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Message-ID: <Pine.LNX.4.64.0706171019490.24938@twinlark.arctic.org>
Date:	Sun, 17 Jun 2007 10:28:02 -0700 (PDT)
From:	dean gaudet <dean@...tic.org>
To:	Wakko Warner <wakko@...mx.eu.org>
cc:	Neil Brown <neilb@...e.de>, david@...g.hm,
	linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org
Subject: Re: limits on raid

On Sun, 17 Jun 2007, Wakko Warner wrote:

> dean gaudet wrote:
> > On Sat, 16 Jun 2007, Wakko Warner wrote:
> > 
> > > When I've had an unclean shutdown on one of my systems (10x 50gb raid5) it's
> > > always slowed the system down when booting up.  Quite significantly I must
> > > say.  I wait until I can login and change the rebuild max speed to slow it
> > > down while I'm using it.   But that is another thing.
> > 
> > i use an external write-intent bitmap on a raid1 to avoid this... you 
> > could use internal bitmap but that slows down i/o too much for my tastes.  
> > i also use an external xfs journal for the same reason.  2 disk raid1 for 
> > root/journal/bitmap, N disk raid5 for bulk storage.  no spindles in 
> > common.
> 
> I must remember this if I have to rebuild the array.  Although I'm
> considering moving to a hardware raid solution when I upgrade my storage.

you can do it without a rebuild -- that's in fact how i did it the first 
time.

to add an external bitmap:

mdadm --grow --bitmap /bitmapfile /dev/mdX

plus add "bitmap=/bitmapfile" to mdadm.conf... as in:

ARRAY /dev/md4 bitmap=/bitmap.md4 UUID=dbc3be0b:b5853930:a02e038c:13ba8cdc

you can also easily move an ext3 journal to an external journal with 
tune2fs (see man page).

if you use XFS it's a bit more of a challenge to convert from internal to 
external, but see this thread:

http://marc.theaimsgroup.com/?l=linux-xfs&m=106929781232520&w=2

i found that i had to do "sb 1", "sb 2", ..., "sb N" for all sb rather 
than just the "sb 0" that email instructed me to do.

-dean
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