linux-kernel - Re: Linux mdadm superblock question.

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Message-ID: <20100218101154.5074ecdc@notabene.brown>
Date:	Thu, 18 Feb 2010 10:11:54 +1100
From:	Neil Brown <neilb@...e.de>
To:	Justin Piszcz <jpiszcz@...idpixels.com>
Cc:	linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Linux mdadm superblock question.

On Tue, 16 Feb 2010 08:14:21 -0500 (EST)
Justin Piszcz <jpiszcz@...idpixels.com> wrote:

> 
> 
> On Tue, 16 Feb 2010, Neil Brown wrote:
> 
> > On Thu, 11 Feb 2010 18:00:23 -0500 (EST)
> > Justin Piszcz <jpiszcz@...idpixels.com> wrote:
> >
> >> Hi,
> >>
> >> I may be converting a host to ext4 and was curious, is 0.90 still the only
> >> superblock version for mdadm/raid-1 that you can boot from without having
> >> to create an initrd/etc?
> >>
> >> Are there any benefits to using a superblock > 0.90 for a raid-1 boot
> >> volume < 2TB?
> >
> > The only noticeable differences that I can think of are:
> > 1/ If you reboot during recovery of a spare, then 0.90 will restart the
> >    recovery at the start, while 1.x will restart from where it was up to.
> > 2/ The /sys/class/block/mdXX/md/dev-YYY/errors counter is reset on each
> >    re-assembly with 0.90, but is preserved across stop/start with 1.x
> > 3/ If your partition starts on a multiple of 64K from the start of the
> >    device and is the last partition and contains 0.90 metadata, then
> >    mdadm can get confused by it.
> > 4/ If you move the devices to a host with a different arch and different
> >    byte-ordering, then extra effort will be needed to see the array for
> >    0.90, but not for 1.x
> >
> > I suspect none of these is a big issue.
> >
> > It is likely that future extensions will only be supported on 1.x metadata.
> > For example I hope to add support for storing a bad-block list, so that a
> > read error during recovery will only be fatal for that block, not the whole
> > recovery process.  This is unlikely ever to be supported on 0.90.  However
> > it may not be possible to hot-enable it on 1.x either, depending on how much
> > space has been reserved for extra metadata, so there is no guarantee that
> > using 1.x now makes you future-proof.
> >
> > And yes, 0.90 is still the only superblock version that supports in-kernel
> > autodetect, and I have no intention of adding in-kernel autodetect for any
> > other version.
> >
> > NeilBrown
> >
> 
> Hi Neil,
> 
> Thanks for the response, this is exactly what I was looking for and 
> probably should be put in a FAQ.
>
I believe the linux-raid wiki is open for anyone to update.  Feel free :-)

NeilBrown
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