linear raid space utilization

linear raid space utilization

[Andy Gajetzki]

Hi there,  I recently had a disk go bad in a linear RAID built with
mdadm. The particular disk that failed was the last  device of the
RAID. I am curious about how  devices are utilized in a linear RAID.
Would the md be filled sequentially from device 1 upto 5? In other
words, what are the odds that the last disk was "empty" when it went
south?

Thanks for your time,

Andy

Re: linear raid space utilization

[David Greaves]

Andy Gajetzki wrote:

>Hi there,  I recently had a disk go bad in a linear RAID built with
>mdadm. The particular disk that failed was the last  device of the
>RAID. I am curious about how  devices are utilized in a linear RAID.
>Would the md be filled sequentially from device 1 upto 5? In other
>words, what are the odds that the last disk was "empty" when it went
>south?
>
>Thanks for your time,
>  
>
Linear raid does provide sequential blocks on disks 1...n

So as long as the filesystem you used was something like 'dd
of=/dev/md0' then you'd be OK <grin>

However if you made a 'proper' filesystem on it then the disk usage
would vary depending on whether it was ext2,ext3,xfs,jfs etc etc

If the bad disk is totally gone then you're at the mercy of fs deign and
fsck - otherwise try and use ddrescue to get an image of it (and the
other disks). Make a 'safe' image and a 'working' image of the bad disk
before attempting repair on 'working' images.
(Of course you need lots of disk space so you may need new disks -
depends how valuable your data is)

HTH

David

-- 

Re: linear raid space utilization

[Andy Gajetzki]

Thanks for the help. I used ddrescue to grab /dev/hdb and am now using
e2retrieve. I am somewhat optimistic that there will be at least
partial recovery ;). Have a great remainder of the weekend,
Andy Gajetzki

On 1/21/06, David Greaves <david@dgreaves.com> wrote:
> Andy Gajetzki wrote:
>
> >Hi there,  I recently had a disk go bad in a linear RAID built with
> >mdadm. The particular disk that failed was the last  device of the
> >RAID. I am curious about how  devices are utilized in a linear RAID.
> >Would the md be filled sequentially from device 1 upto 5? In other
> >words, what are the odds that the last disk was "empty" when it went
> >south?
> >
> >Thanks for your time,
> >
> >
> Linear raid does provide sequential blocks on disks 1...n
>
> So as long as the filesystem you used was something like 'dd
> of=/dev/md0' then you'd be OK <grin>
>
> However if you made a 'proper' filesystem on it then the disk usage
> would vary depending on whether it was ext2,ext3,xfs,jfs etc etc
>
> If the bad disk is totally gone then you're at the mercy of fs deign and
> fsck - otherwise try and use ddrescue to get an image of it (and the
> other disks). Make a 'safe' image and a 'working' image of the bad disk
> before attempting repair on 'working' images.
> (Of course you need lots of disk space so you may need new disks -
> depends how valuable your data is)
>
> HTH
>
> David
>
> --
>
>


--
Andrew Gajetzki  |  (403) 330-6552 | Fax: (780) 675-9402 | www.gajetzki.com
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Re: linear raid space utilization

[Neil Brown]

On Saturday January 21, andy.gajetzki@gmail.com wrote:
> Hi there,  I recently had a disk go bad in a linear RAID built with
> mdadm. The particular disk that failed was the last  device of the
> RAID. I am curious about how  devices are utilized in a linear RAID.
> Would the md be filled sequentially from device 1 upto 5? In other
> words, what are the odds that the last disk was "empty" when it went
> south?

How the space is used depends on the filesystem you had created on the
device.

Many filesystems tend to spread data out so there is plenty of room to
insert new data 'close' to related data.
So if your filesystem was more than about 10% full, there if a very
good chance that a significant fraction was in that last device, and
even if it was less full than that,  their is at least a fair chance
that you have lost a reasonable amount of data.

NeilBrown