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Re: [linux-lvm] LVM label lost on MD RAID5 reshape?





On 9/23/08, Peter A. Castro <doctor fruitbat org> wrote:
On Tue, 23 Sep 2008, Larry Dickson wrote:

Hi Bob,

Notes interpolated.

On 9/23/08, Peter A. Castro <doctor fruitbat org> wrote:

On Tue, 23 Sep 2008, Bob Bell wrote:

Greetings, Bob,

I'm up a creek, and hoping someone out there can help rescue my data, or at
least tell me what went wrong so that I don't have a repeat event.  I'm
starting with this question on linux-lvm, though let me know if you think
the discussion needs to be on linux-raid as well.

I'm setting up a new server running Ubuntu's Hardy Heron release.  `lvm
version` reports:
LVM version:     2.02.26 (2007-06-15)
Library version: 1.02.20 (2007-06-15)
Driver version:  4.12.0
`uname -a` reports:
Linux sherwood 2.6.24-16-server #1 SMP Thu Apr 10 13:58:00 UTC 2008 i686
GNU/Linux

I initially created an md RAID5 device with only two components (matching
320 GB SATA HDDs).


This is puzzling, since by the usual definition, there is no such thing as a
2-disk RAID5. Your description of its virtual size indicates it is a RAID1,
or equivalent.

While it's unusual to have only 2 disks in a "RAID5" (usally you want 3
drives min), it is possible, if unreliable (can you say
"half-of-a-mirror" ?)
 
If you are meaning a degraded 3-disk RAID5, then he should have had a virtual size of 640GB. He reported 320GB which would be the value expected of a RAID1. You can apply the RAID5 algorithm to 2 disks, but then you just get the equivalent of a RAID1, since a parity sector made so the XOR sum of it and ++ one ++ data sector is zero must equal that data sector.
 
Larry

 I created a single Physical Volume using the entirety of that md device
(320 GB), and then created several Logical Volumes for different filesystems
(all ext3).  This was done using the Ubuntu installer.  After installing I
used lvresize to increase the size of a few of the Logical Volumes, as I was
conservative regarding the size during installation.  These filesystems hold
data that is not a critical part of the system (mail, music, video, etc.).


The LVs live atop the virtual md device so this should not have done
anything essentially disturbing.

Yep, I have several VG's with LV's on top a several MDs without any trouble.



(I also have a similar setup with a couple of IDE drives that I use for the
system (/home, /var, /boot, /), but I haven't touched those since the
install and they continue to work fine.)

I then copied data from a third matching 320 GB SATA HDD to one of the
Logical Volume filesystems on the md device.  After freeing up that drive
(by relocating its contents to the LVM/MD setup), I added the drive to the
md device, which brought the total to 2 active devices and 1 spare device.
 I then grew the number of devices to 3 and waited for  the reshape to
finish (increasing the capacity to 640 GB).


If this is a reshape in md, then LVM should have seen nothing (since the 320
GB it knew about should still be there).

 I bumped the values in /proc/sys/dev/raid/ so that I wouldn't have to
wait as long.


My understanding of RAID5 is that once you configure the set of disks to
the array, you can't just add an additional disk to it.  Attempting to do
so does more than just "reshape" the array, it changes the logical layout
of the sectors completely.  It's no wonder that LVM can't find anything,
since it's data (if it survived the reorg) is no longer where it should
be.  And, once you've done this, there's really no going back to recover.
This is assuming my understanding of RAID5 is correct.

I disagree, because assuming the md reshape operates intuitively, the old
320 GB of data should still be visible as the first 320 GB of data on the
new VIRTUAL 640GB device. And LVM should know nothing about the underlying
RAIDing, so it should be happy to look there. However, I have no idea what
"bumped the values in /proc/sys/dev/raid/" means, or if it causes LVM to
look in the wrong places for things.

I don't believe the MD/RAID5 operates in this way.  He started with a
2-disk RAID5 (effectively having the data logically interleaved between
the two drives), then he added an additional disk.  The way RAID works,
it doesn't just add the disk space to the end of the array (that would
not create a recoverable scenario should it or one of the others fail),
it re-integrates and effectively re-interleaves the logical sectors to
re-spread the data across all three drives.  The trouble is, I don't
think you can actually do this with current MD tech (I could be wrong).

Perhaps a better question would be to ask how, exactly, Bob create his
array initially (what commands) and what steps he took to add the new
drive (again, what commands did he issue).  That would give us a better
idea of how the array looked before and after.

All of this is orthogonal to LVM, however.  I suspect a RAID/MD email
list would be a better forum to discuss this.


Larry Dickson
Cutting Edge Networked Storage

Now that the reshaping is completed, LVM can't find the physical volume on
that device anymore.  I tried rebooting the system, but the problem
remained. Checking /proc/mdstat shows that the md device is up and healthy.
 The pvdisplay command only shows my other Physical Volume (for the IDE
drives). I found the pvck command and ran that on the md device, and it
states that there is no LVM label on the device.

It is my understanding that the steps I outlined should have worked.  I
planned to follow them with pvresize, then lvresize, then umount, resize2fs,
and mount again.  I've seen this procedure outlined a few different places,
including at http://gentoo-wiki.com/Resize_LVM2_on_RAID5.

Did I do something wrong?  Is there anyway to rescue my data?  I saved the
contents of /etc/lvm/backup/ when I noticed the problem -- perhaps that
might help?  If there's no way of saving the data, I'd at least like to
figure out what happened in the first place.

Thank you.  Your thoughtfulness and help is appreciated.


--
Peter A. Castro <doctor fruitbat org> or <Peter Castro oracle com>
      "Cats are just autistic Dogs" -- Dr. Tony Attwood

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read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/



--
Peter A. Castro <doctor fruitbat org> or <Peter Castro oracle com>
       "Cats are just autistic Dogs" -- Dr. Tony Attwood

_______________________________________________
linux-lvm mailing list
linux-lvm redhat com
https://www.redhat.com/mailman/listinfo/linux-lvm
read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
_______________________________________________
linux-lvm mailing list
linux-lvm redhat com
https://www.redhat.com/mailman/listinfo/linux-lvm
read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/


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