Hi! There's a bug in Kernel 2.6.17 and / or mdadm which prevents (re)adding a disk to a degraded RAID5-Array. The mail I'm replying to was sent to linux-raid only. A summary of my problem is in the quoted part, and everything you need to reproduce it is below. There's more information (kernel log, output of mdadm -E, ...) in the original mail ( Subject: RAID5 degraded after mdadm -S, mdadm --assemble (everytime) Message-ID: <20060624104745.GA6352@xxxxxxxxxxxxx> It can be found here for example: http://www.spinics.net/lists/raid/msg12859.html ) More about this problem below the quoted pard. On Sat Jun 24 12:47:45 2006, I wrote: > I set up a RAID5 array of 4 disks. I initially created a degraded array > and added the fourth disk (sda1) later. > > The array is "clean", but when I do > mdadm -S /dev/md0 > mdadm --assemble /dev/md0 /dev/sd[abcd]1 > it won't start. It always says sda1 is "failed". > > When I remove sda1 and add it again everything seems to be fine until I > stop the array. CPU: AMD-K6(tm) 3D processor Kernel: Linux version 2.6.17 (root@ganges) (gcc version 4.0.3 (Debian 4.0.3-1)) #2 Tue Jun 20 17:48:32 CEST 2006 The problem is: The superblocks get inconsistent, but I couldn't find where this actually happens. Here are some simple steps to reproduce it (don't forget to adjust the device-names if you're allready using /dev/md1 or /dev/loop[0-3]): The behaviour changes when you execute --zero-superblock in the example below (it looks even more broken). It also changes when you fail some other disk instead of loop2. When loop3 is failed (without executing --zero-superblock) it can successfully be re-added. ############################################ cd /tmp; mkdir raiddtest; cd raidtest dd bs=1M count=1 if=/dev/zero of=disk0 dd bs=1M count=1 if=/dev/zero of=disk1 dd bs=1M count=1 if=/dev/zero of=disk2 dd bs=1M count=1 if=/dev/zero of=disk3 losetup /dev/loop0 disk0 losetup /dev/loop1 disk1 losetup /dev/loop2 disk2 losetup /dev/loop3 disk3 mdadm --create /dev/md1 --level=5 --raid-devices=4 /dev/loop0 /dev/loop1 /dev/loop2 /dev/loop3 mdadm /dev/md1 --fail /dev/loop2 mdadm /dev/md1 --remove /dev/loop2 #mdadm --zero-superblock /dev/loop2 # here something goes wrong mdadm /dev/md1 --add /dev/loop2 mdadm --stop /dev/md1 # can't reassemble mdadm --assemble /dev/md1 /dev/loop0 /dev/loop1 /dev/loop2 /dev/loop3 ############################################ To cleanup everything :) ############################################ mdadm --stop /dev/md1 losetup -d /dev/loop0 losetup -d /dev/loop1 losetup -d /dev/loop2 losetup -d /dev/loop3 rm disk0 disk1 disk2 disk3 ########################################### After mdadm --create the superblocks are ok, but look a little bit strange (the failed device): dev_roles[i]: 0000 0001 0002 fffe 0003 the disks have dev_num: 0,1,2,4 But after --fail --remove --add dev_roles[i]: 0000 0001 fffe fffe 0003 0002 but the disks still have dev_num: 0,1,2,4. Either loop2 must have disk_num=5 or dev_roles needs to be 0,1,2,0xfffe,4. Greetings, Roni
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