On Sun, 6 May 2012 10:17:52 +0000 Garðar Arnarsson <gardar@xxxxxxxxxxx> wrote: > My raid5 array has gotten a bit big, it's containing total 10 drives > right now (I started out with 3 drives). So I am going to convert it > to raid6 before it gets any bigger. > > I am doing a test-run on a virtual machine with virtual drives to see > that everything works flawlessly. > > When I tried to convert the array to raid6 I got a error message about > a missing backup-file > > mdadm --grow /dev/md0 --raid-devices=5 --level=6 > > mdadm level of /dev/md0 changed to raid6 > mdadm: /dev/md0: Cannot grow - need backup-file > mdadm: aborting level change > > I added the backup file and was able to convert the array successfully > after that. > > My question is, how big is this backup file going to be? My real raid > array consists of 2tb drives, will the backup file be as big as one > drive in the array, or will it just be few megabytes or gigabytes? > I'm asking because I'm wondering if I need to buy an extra hdd for the > backup file or if the backup file can just be on my OS hdd that has > around 100gb free. The backup file is a few megabytes. Around 16MB I think. However if you are likely to add another device in the not too distant future you can save yourself a bit of time. If you mdadm --grow /dev/md0 --level=6 --layout=preserve It will just make the new few a 'Q-block' device, containing the extra RAID6 'parity' block for each stripe. This doesn't require any reshape or or any backup file and is a lot faster. All it requires is a normal recovery operation. Then when you later add another device you can mdadm --grow /dev/md0 --raid-devices=N+1 --layout=normalise This will convert from the Q-on-the-last-device layout to a more normal rotated-P-and-Q layout at the same time as adding extra space. NeilBrown
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