On Thu, Apr 7, 2016, 5:01 PM Jeffrey Ross <jeff@xxxxxxxxxx> wrote: > > I had an raid1 partition with ext4 on it which was empty. > > it was /dev/md124 which was made up of /dev/sda1 and /dev/sdb1, I decided > to change this to be a btrfs partition. > > Initially I figured I'd simply unmount /dev/md124 and simply do a > "mkfs.btrfs -f -L home2 /dev/md124" and remount the partition, however > after doing some reading I believe btrfs supports raid1 directly without > using the software raid driver md (?). Correct. > > so I then tried "mkfs.btrfs -m raid1 -d raid1 -L home2 -f /dev/sdb1 > /dev/sda1" and something was successfully created but this is where I am > confused, is this truly a raid1 partition? Yes. It is not block level raid1, however. It's done at the Btrfs chunk level, where each chunk type will have two stripes (copies). The btrfs chunk and stripe are reused terms that don't match prior mdadmn usage. The distinction isn't important day to day, but if you want to understand how btrfs does things differently is worth noting. > secondly mounting this > partition I simply specify something like "mount /dev/sda1 /mntpoint". Correct. Or /dev/sdb1 also works. Each device superblock contains a reference to all other devices used in a multiple device Btrfs volume, and mount will fail if any are missing. Strictly speaking the member devices aren't mounted, rather the volume is what's mounted. Be aware of the duplicate volume UUID gotcha (using dd or lvm snapshots): https://btrfs.wiki.kernel.org/index.php/Gotchas > > I can see - > # btrfs filesystem show > Label: 'home2' uuid: 635be1e8-31d2-4b5c-b81c-1ec2cd8d9101 > Total devices 2 FS bytes used 664.00KiB > devid 1 size 376.46GiB used 2.01GiB path /dev/sdb1 > devid 2 size 376.46GiB used 2.01GiB path /dev/sda1 > > To me this looks like two different partitions but I maybe wrong, so > assuming it is one raid partition how would I go around having this auto > mounted in /etc/fstab? I would assume this entry - > > UUID=635be1e8-31d2-4b5c-b81c-1ec2cd8d9101 /home2 btrfs defaults 0 0 Correct. > > but I'm looking for some confirmation first that I actually have a raid > partition and I've done everything correctly first. See 'btrfs filesystem df' and 'btrfs filesystem usage' The 'usage' subcommand is probably more useful, it's newer and was meant to resolve the confusion with show and df, as well as combined their information. show and df are almost considered legacy, but will continue to be around for a while. # btrfs fi df /mnt/1 Data, RAID1: total=1.00GiB, used=457.50MiB System, RAID1: total=32.00MiB, used=16.00KiB Metadata, RAID1: total=1.00GiB, used=592.00KiB GlobalReserve, single: total=16.00MiB, used=0.00B # btrfs fi us /mnt/1 Overall: Device size: 100.00GiB Device allocated: 4.06GiB Device unallocated: 95.94GiB Device missing: 0.00B Used: 916.19MiB Free (estimated): 48.52GiB (min: 48.52GiB) Data ratio: 2.00 Metadata ratio: 2.00 Global reserve: 16.00MiB (used: 0.00B) Data,RAID1: Size:1.00GiB, Used:457.50MiB /dev/dm-7 1.00GiB /dev/mapper/VG-testbtr1 1.00GiB Metadata,RAID1: Size:1.00GiB, Used:592.00KiB /dev/dm-7 1.00GiB /dev/mapper/VG-testbtr1 1.00GiB System,RAID1: Size:32.00MiB, Used:16.00KiB /dev/dm-7 32.00MiB /dev/mapper/VG-testbtr1 32.00MiB Unallocated: /dev/dm-7 47.97GiB /dev/mapper/VG-testbtr1 47.97GiB > > Lastly if I'm reading correctly the system will NOT automatically mount a > degraded array, how can I force it to automatically mount on a reload even > if the array is degraded. Device failure notification and recovery is distinctly weak and manual on Btrfs right now. There are some patches available upstream that are being tested, but I don't expect them to get into kernel 4.6. You could use the 'degraded' mount option in fstab, but then that shows as a mount option and kernel messages, the same as if you're really degraded. So you have no idea if you're really degraded or not unless you check 'btrfs fi show' or 'btrfs fi us'. You really need to know if you've mounted degraded because there's no automatic rebuilds either. So if devid1 mounts degraded silently and devid2 appears later, well it's not going to get added back in automatically. Even more neat, is if you're making changes to devid1 and not devid2, then later they get mounted together, devid2 is not automatically caught up with devid1. You have to scrub for it to catch up. If you don't scrub and then later let's say devid1 is late and doesn't get mounted, but devid2 mounts and you write to it, now you have different writes to devid1 and devid2 and if they're later combined again, the file system will totally face plant, it's generally complete corruption. So anyway, I don't recommend using degraded in fstab. Instead use nofail, so the startup isn't left hanging while waiting for it to mount. And then you'll figure out something is wrong since your stuff isn't where you expect it at /home2, and you can investigate the problem and make sure if you have to mount degraded that if you later get the missing device reattached or whatever the problem was, you can then scrub to make sure they're both on the same page. Chris Murphy -- users mailing list users@xxxxxxxxxxxxxxxxxxxxxxx To unsubscribe or change subscription options: http://lists.fedoraproject.org/admin/lists/users@xxxxxxxxxxxxxxxxxxxxxxx Fedora Code of Conduct: http://fedoraproject.org/code-of-conduct Guidelines: http://fedoraproject.org/wiki/Mailing_list_guidelines Have a question? Ask away: http://ask.fedoraproject.org
