[CentOS] more software raid questions

Nataraj incoming-centos at rjl.com
Tue Oct 19 22:34:19 EDT 2010


fred smith wrote:helppain/backups/disks/
> hi all!
>
> back in Aug several of you assisted me in solving a problem where one
> of my drives had dropped out of (or been kicked out of) the raid1 array.
>
> something vaguely similar appears to have happened just a few mins ago,
> upon rebooting after a small update. I received four emails like this,
> one for /dev/md0, one for /dev/md1, one for /dev/md125 and one for
> /dev/md126:
>
> 	Subject: DegradedArray event on /dev/md125:fcshome.stoneham.ma.us
> 	X-Spambayes-Classification: unsure; 0.24
> 	Status: RO
> 	Content-Length: 564
> 	Lines: 23
>
> 	This is an automatically generated mail message from mdadm
> 	running on fcshome.stoneham.ma.us
>
> 	A DegradedArray event had been detected on md device /dev/md125.
>
> 	Faithfully yours, etc.resources/
>
> 	P.S. The /proc/mdstat file currently contains the following:
>
> 	Personalities : [raid1] 
> 	md0 : active raid1 sda1[0]
> 	      104320 blocks [2/1] [U_]
> 	      
> 	md126 : active raid1 sdb1[1]
> 	      104320 blocks [2/1] [_U]
> 	      
> 	md125 : active raid1 sdb2[1]
> 	      312464128 blocks [2/1] [_U]
> 	      
> 	md1 : active raid1 sda2[0]
> 	      312464128 blocks [2/1] [U_]
> 	      
> 	unused devices: <none>
>
> firstly, what the heck are md125 and md126? previously there was
> only md0 and md1.... ????
>
> secondly, I'm not sure what it's trying to tell me. it says there was a 
> "degradedarray event" but at the bottom it says there are no unused devices.
>
> there are also some messages in /var/log/messages from the time of the
> boot earlier today, but they do NOT say anything about "kicking out"
> any of the md member devices (as they did in the event back in August):
>
> 	Oct 19 18:29:41 fcshome kernel: device-mapper: dm-raid45: initialized v0.2594l
> 	Oct 19 18:29:41 fcshome kernel: md: Autodetecting RAID arrays.
> 	Oct 19 18:29:41 fcshome kernel: md: autorun ...
> 	Oct 19 18:29:41 fcshome kernel: md: considering sdb2 ...
> 	Oct 19 18:29:41 fcshome kernel: md:  adding sdb2 ...
> 	Oct 19 18:29:41 fcshome kernel: md: sdb1 has different UUID to sdb2
> 	Oct 19 18:29:41 fcshome kernel: md: sda2 has same UUID but different superblock 
> 	to sdb2
> 	Oct 19 18:29:41 fcshome kernel: md: sda1 has different UUID to sdb2
> 	Oct 19 18:29:41 fcshome kernel: md: created md125
> 	Oct 19 18:29:41 fcshome kernel: md: bind<sdb2>
> 	Oct 19 18:29:41 fcshome kernel: md: running: <sdb2>
> 	Oct 19 18:29:41 fcshome kernel: raid1: raid set md125 active with 1 out of 2 mir
> 	rors
> 	Oct 19 18:29:41 fcshome kernel: md: considering sdb1 ...
> 	Oct 19 18:29:41 fcshome kernel: md:  adding sdb1 ...
> 	Oct 19 18:29:41 fcshome kernel: md: sda2 has different UUID to sdb1
> 	Oct 19 18:29:41 fcshome kernel: md: sda1 has same UUID but different superblock 
> 	to sdb1
> 	Oct 19 18:29:41 fcshome kernel: md: created md126
> 	Oct 19 18:29:41 fcshome kernel: md: bind<sdb1>
> 	Oct 19 18:29:41 fcshome kernel: md: running: <sdb1>
> 	Oct 19 18:29:41 fcshome kernel: raid1: raid set md126 active with 1 out of 2 mirrors
> 	Oct 19 18:29:41 fcshome kernel: md: considering sda2 ...
> 	Oct 19 18:29:41 fcshome kernel: md:  adding sda2 ...
> 	Oct 19 18:29:41 fcshome kernel: md: sda1 has different UUID to sda2
> 	Oct 19 18:29:41 fcshome kernel: md: created md1
> 	Oct 19 18:29:41 fcshome kernel: md: bind<sda2>
> 	Oct 19 18:29:41 fcshome kernel: md: running: <sda2>
> 	Oct 19 18:29:41 fcshome kernel: raid1: raid set md1 active with 1 out of 2 mirrors
> 	Oct 19 18:29:41 fcshome kernel: md: considering sda1 ...
> 	Oct 19 18:29:41 fcshome kernel: md:  adding sda1 ...
> 	Oct 19 18:29:41 fcshome kernel: md: created md0
> 	Oct 19 18:29:41 fcshome kernel: md: bind<sda1>
> 	Oct 19 18:29:41 fcshome kernel: md: running: <sda1>
> 	Oct 19 18:29:41 fcshome kernel: raid1: raid set md0 active with 1 out of 2 mirrors
> 	Oct 19 18:29:41 fcshome kernel: md: ... autorun DONE.
>
> and here's /etc/mdadm.conf:
>
> 	# cat /etc/mdadm.conf
>
> 	# mdadm.conf written out by anaconda
> 	DEVICE partitions
> 	MAILADDR fredex
> 	ARRAY /dev/md0 level=raid1 num-devices=2 uuid=4eb13e45:b5228982:f03cd503:f935bd69
> 	ARRAY /dev/md1 level=raid1 num-devices=2 uuid=5c79b138:e36d4286:df9cf6f6:62ae1f12
>
> which doesn't say anything about md125 or md126,... might they be some kind of detritus
> or fragments left over from whatever kind of failure caused the array to become degraded?
>
> do ya suppose a boot from power-off might somehow give it a whack upside the head so
> it'll reassemble itself according to mdadm.conf?
>
> I'm not sure which devices need to be failed and re-added to make it clean again (which
> is all I had to do when I had the aforementioned earlier problem.)
>
> Thanks in advance for any advice!
>
> Fred
>
>   
I've seen this kind of thing happen when the autodetection stuff 
misbehaves. I'm not sure why it does this or how to prevent it. Anyway, 
to recover, I would use something like:

mdadm --stop /dev/md125
mdadm --stop /dev/md126

If for some reason the above commands fail, check and make sure it has 
not automounted the file systems from md125 and md126. Hopefully this 
won't happen.

Then use:
mdadm /dev/md0 -a /dev/sdXX
To add back the drive which belongs in md0, and similar for md1. In 
general, it won't let you add the wrong drive, but if you want to check use:
mdadm --examine /dev/sda1 | grep UUID
and so forth for all your drives and find the ones with the same UUID.

When I create my Raid arrays, I always use the option --bitmap=internal. 
With this option set, a bitmap is used to keep track of which pages on 
the drive are out of date and then you only resync pages which need 
updating instead of recopying the whole drive when this happens. In the 
past I once added a bitmap to an existing raid1 array using something 
like this. This may not be the exact command, but I know it can be done:
mdadm /dev/mdN --bitmap=internal

Adding the bitmap is very worthwhile and saves time and risk of data 
loss by not having to recopy the whole partition.

Nataraj


More information about the CentOS mailing list