linux-kernel - Re: Linux mdadm superblock question.

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Date:	Sat, 13 Feb 2010 13:07:22 -0800
From:	Michael Evans <mjevans1983@...il.com>
To:	david@...g.hm
Cc:	Justin Piszcz <jpiszcz@...idpixels.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-raid@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Linux mdadm superblock question.

On Sat, Feb 13, 2010 at 12:49 PM,  <david@...g.hm> wrote:
> 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"
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>>>>> the body of a message to majordomo@...r.kernel.org
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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
>

The 1.1 and 1.2 formats ALSO play more nicely with stacking partition
contents.  LVM, filesystems, and partition info all begin at the start
of a block device; putting the md labels there too makes it obvious
what order to unpack the structures in.
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