Saturday, October 15, 2011

Recovering RAID5 from Multiple Simultaneous Drive Failures

RAID5 is a redundant disk system that protects against a single drive failure. The array can keep functioning allowing you the defective disk and rebuild the raid without any data loss. However if more than one disk fails at a time, RAID5 will not help you (there are other raid levels that can). Sudden multiple disk failure is exactly what happened to my system one night.

Edit (9/15/2012): I added additional information at the bottom of this post which makes the re-assembly process easier. I recommend it rather than the --create procedure detailed below. It is still a good idea to read the entire post though.

Once or twice in the past I have had a single drive fail because of a lose or fault SATA cable. This is easily resolved by powering down the computer and re-securing the cable. I usually notice a drive failure within a week (I should setup and alert system). But recently, I had two drives fail within two hours of each other. I hadn't even noticed the first drive failure before the second had drive failed. Rebooting the computer cleared up the SATA errors that brought the drives down. The drives seemed to be function properly, they hadn't suffered a hardware failure. However, the raid could not rebuild itself because linux had marked both drives as faulty. At this point in time I had 6TB of data at risk, with partial backups several months old. I was mostly worried about photos that I had taken over the past several months that can't be replaced.

So what to do... try not to panic, this is going to get messy.

I began by trying to figure out which drives failed and in which order by issuing mdadm --examine for every device in the array. I focused on the last portion of the output, which contains the status of each device. The data is recorded independently on each device in the raid, so you can compare the output and find differences. In a properly functioning raid the output should be identical for each device. Below is the output for the /dev/sda1 device.

Number Major Minor RaidDevice State
this 3 8 81 3 active sync /dev/sdf1

0 0 8 17 0 active sync /dev/sdb1
1 1 0 0 1 faulty removed
2 2 0 0 2 faulty removed
3 3 8 81 3 active sync /dev/sdf1
4 4 8 97 4 active sync /dev/sdg1


Knowing that I lost 2 drives, I figured that this drive had not failed simply because the failures were recorded on this disk.

Continuing, I eventually found the second drive that failed because it only had a record of one drive failure. Thus drive must have been functioning during the first failure but not the 2nd.
Number Major Minor RaidDevice State
this 1 8 33 1 active sync /dev/sdc1

0 0 8 17 0 active sync /dev/sdb1
1 1 8 33 1 active sync /dev/sdc1
2 2 0 0 2 faulty removed
3 3 8 81 3 active sync /dev/sdf1
4 4 8 97 4 active sync /dev/sdg1


Then I found the first drive that failed because there was no failure recorded on it at all.

Number Major Minor RaidDevice State
this 2 8 49 2 active sync /dev/sdd1

0 0 8 17 0 active sync /dev/sdb1
1 1 8 33 1 active sync /dev/sdc1
2 2 8 49 2 active sync /dev/sdd1
3 3 8 81 3 active sync /dev/sdf1
4 4 8 97 4 active sync /dev/sdg1


Note: I think you can also use the "Update Time" from mdadm --examine to figure this information out. I used it to verify that my logic was correct.

Important Note: My computer likes to move SATA devices around at boot time. So the device names listed in the raid status outputs were not accurate after rebooting. The above mdadm output says /dev/sdd1 had failed, but the device name I queried mdadm for was /dev/sdf1. You MUST match the current device with the array device number. The correct device order is essential for fixing the raid.

Now that I knew the devices and the order in which they failed I could do a little more thinking.

I figured since linux halted the file system and stopped the raid when the 2nd device failed there shouldn't be too much data corruption. Probably only the data that was being written to disk near the time of failure. This data wasn't too important. The last important data had been written a few days earlier so t should have been flushed from the caches to disk. With these assumptions I decided that it should be possible to just tell the array that only the 1st failed drive is broken and that the second is OK. Apparently you can't really do this. The only way to do it is to destroy the array and rebuild it.

Thats right, destroy and then rebuild the array. Pray for all the datas.

I googled around for a while to try to see if my idea was possible, it seemed to be. The best validation for my idea came from this blog.

The rebuilding process...

The first step was to stop the raid device mdadm --stop before I could start destroying and re-creating it. If you don't do this, you get strange errors from mdadm saying it can't write to the devices. It was aggravating to figure out why so just do it.

I decided I needed to protect myself from myself and possibly from mdadm. I wanted to make sure there was no chance that I would accidently rebuild the array using the first failed (most out of sync) drive. I zeroed the device's raid superblock. mdadm --zero-superblock /dev/sdf1. Now it is no longer associated with any raid device.

Next I used the output from the mdadm --examine commands to help me construct the command to rebuild the raid.
mdadm --verbose --create --metadata=0.90 /dev/md0 --chunk=128 --level=5 --raid-devices=5 /dev/sdd1 /dev/sde1 missing /dev/sda1 /dev/sdb1


IMPORTANT: Notice that the device order is not the same order as listed in the mdadm --examine output. This is because my computer moves the SATA devices around. It is CRITICAL that you rebuild array with the devices in the proper order. Use the array device number for "this" device from the output of the mdadm --examine commands to help you order the devices correctly.

I specified the chunk size using the value from mdadm --examine. I found I also had to specify the meta data version. Mdadm by default used a newer meta data version, which altered the amount of space on each device. The space used for the rebuild needed to be exactly the same as the original array setup otherwise the data stripes won't line up (and your data will be munged). You can rebuild the array as many times as you like so long as you don't write data to the broken array setup. I rebuilt my array 3 or 4 times before I got it right.

To check if the array setup was correct I ran e2fsck -B 4096 -n /dev/md0 (linux scan disk utility). I decided it was safer to specify the file system block size to make sure e2fsck got it right. Since I am just testing the array setup, I didn't want e2fsck making any changes to the disk hence the -n. If the array setup is incorrect the striped data won't line up and e2fsck won't find any superblock and will refuse to scan. If e2fsck is able to preform a scan, then the array setup must be OK (at least that's the hope).

The next part is probably the most dangerous part because at this point you are editing the file system data on the disks.

Once I was sure the array setup was correct I ran e2fsck -B 4096 /dev/md0 to fix all the file system errors. There were thousands of group errors, a few dozen inode errors, and a lot of bitmap errors. The wait was nerve racking but eventually it finished. I was able to mount the file system as read only (for safety), and list the files, I was even able to open a picture.

Lastly I added the first failed drive back into the array mdadm -a /dev/md0 /dev/sdf1 and the array began rebuilding the parity bits.

At this point I began dumping all important data to another drive just to have a current backup. Once the parity bits have been rebuilt I will remount the partition as read-write.

That's it, RAID5 recovered from multiple drive failure.

Edit (9/15/2012): I found another page that has helpful advice. http://zackreed.me/articles/50-recovery-from-a-multiple-disk-failure-with-mdadm  Instead of using --create, you can use --assemble --force with the drives that you want mark as clean and use.   This will assemble the array in degraded mode with the devices in the correct order.  You can then zero the super block of the 1st failed drive and then --add it to the array.

2 comments:

  1. Good job. I found mdadm decent when I was using Linux, but then I switched over to ZFS on Solaris and it just blows it out of the water. It's now available natively on Linux too -- http://zfsonlinux.org/faq.html

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  2. Thank you for sharing. Identifying functional failure is an integral part of a thorough reliability centered maintenance analysis.

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