linux-kernel - Re: Linux mdadm superblock question.

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Date:	Tue, 16 Feb 2010 12:24:20 -0500
From:	Bill Davidsen <davidsen@....com>
To:	Neil Brown <neilb@...e.de>
CC:	Justin Piszcz <jpiszcz@...idpixels.com>,
	linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Linux mdadm superblock question.

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.
>   

Given that 4k sector drives make that a lot more likely that it used to 
be, I suspect some effort will be needed to address this sooner or later.

>  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.
>   


-- 
Bill Davidsen <davidsen@....com>
  "We can't solve today's problems by using the same thinking we
   used in creating them." - Einstein

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