linux-kernel - Re: Linux mdadm superblock question.

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Message-ID: <alpine.DEB.2.00.1002131248390.4721@asgard.lang.hm>
Date:	Sat, 13 Feb 2010 12:49:13 -0800 (PST)
From:	david@...g.hm
To:	Justin Piszcz <jpiszcz@...idpixels.com>
cc:	"H. Peter Anvin" <hpa@...or.com>,
	Michael Evans <mjevans1983@...il.com>,
	linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Linux mdadm superblock question.

On Sat, 13 Feb 2010, Justin Piszcz wrote:

> On Sat, 13 Feb 2010, H. Peter Anvin wrote:
>
>> On 02/11/2010 05:52 PM, Michael Evans wrote:
>>> On Thu, Feb 11, 2010 at 3:00 PM, 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?
>>>> 
>>>> Justin.
>>>> --
>>>> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
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>>>> 
>>> 
>>> You need the superblock at the end of the partition:  If you read the
>>> manual that is clearly either version 0.90 OR 1.0 (NOT 1.1 and also
>>> NOT 1.2; those use the same superblock layout but different
>>> locations).
>> 
>> 0.9 has the *serious* problem that it is hard to distinguish a whole-volume
>> 
>> However, apparently mdadm recently switched to a 1.1 default.  I
>> strongly urge Neil to change that to either 1.0 and 1.2, as I have
>> started to get complaints from users that they have made RAID volumes
>> with newer mdadm which apparently default to 1.1, and then want to boot
>> from them (without playing MBR games like Grub does.)  I have to tell
>> them that they have to regenerate their disks -- the superblock occupies
>> the boot sector and there is nothing I can do about it.  It's the same
>> pathology XFS has.
>>
>> 	-hpa
>> 
>
> My original question was does the newer superblock do anything special or 
> offer new features *BESIDES* the quicker resync?

the older superblocks have limits on the number of devices that can be 
part of the raid set.

David Lang
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