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I am trying to decide on a filesystem and would like to know if it is possible to replace a failed drive in btrfs RAID without downtime.
Suppose I create a new btrfs filesystem using the command
mkfs.btrfs -d raid1 /dev/sdb /dev/sdc
Now suppose one day
/dev/sdc
fails. There are two possibilities: it can fail gradually, showing S.M.A.R.T. errors - in this situation I can add a new device withbtrfs device add /dev/sde /mnt; btrfs filesystem balance /mnt
and then remove the old one withbtrfs device delete /dev/sdc /mnt
.But if it suddenly fails, becoming unreadable... A web search says in this situation I must first unmount the filesystem, mount in degraded mode, add a new device, then remove the missing device.
umount /mnt mount -o degraded /dev/sdb /mnt btrfs device add /dev/sdf /mnt btrfs device delete missing /mnt
An unmount is obviously a disruptive operation so there would be downtime - any application using the filesystem would get an I/O error. But these kind of "tutorials" on btrfs look outdated, considering btrfs is under heavy development.
Question is: considering current state of btrfs, is it possible to do this online, i.e. without unmounting?
If not, there is a software-only solution that can fulfill this need?
1If one drive catches fire, the rest of your system is probably on fire too – Journeyman Geek – 2013-12-21T14:21:04.090
1@JourneymanGeek Funny you.. :) I just wanted to make it very clear that I meant a catastrophic, sudden and unpredictable failure - the drive simply stops working. This is rather uncommon, hard disk usually fail gradually and with effective monitoring I can replace them before that happens, but what if... – NothingsImpossible – 2013-12-21T15:23:15.720
2In Linux 3.8
btrfs replace mountpoint old_disk new_disk
was added. – Brian – 2013-12-28T14:29:42.333@Brian woow... That is the answer. I googled for "btrfs replace" and this showed up http://lwn.net/Articles/524589/ . It is exactly what I was looking for. Please post it as an answer so I can accept it.
– NothingsImpossible – 2013-12-29T14:38:15.457