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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.

  1. Suppose I create a new btrfs filesystem using the command

    mkfs.btrfs -d raid1 /dev/sdb /dev/sdc

  2. 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 with btrfs device add /dev/sde /mnt; btrfs filesystem balance /mnt and then remove the old one with btrfs device delete /dev/sdc /mnt.

  3. 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 But these kind of "tutorials" on btrfs look outdated, considering btrfs is under heavy development.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?

NothingsImpossible
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    If one drive catches fire, the rest of your system is probably on fire too – Journeyman Geek Dec 21 '13 at 14:21
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    @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 Dec 21 '13 at 15:23
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    In Linux 3.8 `btrfs replace mountpoint old_disk new_disk` was added. – Brian Dec 28 '13 at 14:29
  • @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 Dec 29 '13 at 14:38

2 Answers2

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In Linux 3.8, btrfs replace mountpoint old_disk new_disk was added. If you're running a recent kernel, it will provide the functionality you are looking for.

G-Man Says 'Reinstate Monica'
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Brian
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    This would now be `btrfs replace start /dev/old /dev/new /mountpoint` (**start** has been added). Also see `man btrfs-replace`. – basic6 Nov 12 '15 at 12:33
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small correction, current syntax is:

btrfs replace start OLDDEV NEWDEV MOUNTPOINT

which then backgrounds.

You can check the status with

btrfs replace status MOUNTPOINT

which will show you a continuously updated status of the replace operation.

Volker
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    This is **not** an answer to the original question. To critique or request clarification from an author, leave a comment below their post - you can always comment on your own posts, and once you have sufficient [reputation](http://superuser.com/help/whats-reputation) you will be able to [comment on any post](http://superuser.com/help/privileges/comment). – DavidPostill Nov 05 '14 at 08:10