Replacing RAID-1 devices with larger disks

Hello there,

I'm running a btrfs RAID-1 on two 128GB SSDs that were getting kind
of full. I found two 256GB SSDs that I plan to use to replace the 128TB versions.

I've managed to do the actual swap using a series of btrfs replace commands with no special arguments, and the system is now live and booting from the 256GB drives. However, I haven't actually noticed any difference in btrfs fi show output, and usage looks weird. Has anyone seen this before or have a clue as to who?

The relevant partition sizes are now (sdb is identical):

    /dev/sda1   *        2048    83888127    41943040   83  Linux
    /dev/sda3        92276736   427821055   167772160   83  Linux

Here's the show output:

    Label: 'root'  uuid: 670d1132-00dc-4511-a2f6-d28ce08b4d3a
        Total devices 2 FS bytes used 9.33GiB
        devid    1 size 13.97GiB used 11.78GiB path /dev/sda1
        devid    2 size 13.97GiB used 11.78GiB path /dev/sdb1

    Label: 'var'  uuid: 815b3280-e90f-483a-b244-1d2dfe9b6e67
        Total devices 2 FS bytes used 56.14GiB
        devid    1 size 80.00GiB used 80.00GiB path /dev/sda3
        devid    2 size 80.00GiB used 80.00GiB path /dev/sdb3

Those sizes have not changed over the resize; i.e. the original sda1/sdb1 pair was 14GB and the sda3/sdb3 pair was 80GB, and after the replace, they haven't changed.

And usage for / is now weird:

    Overall:
        Device size:          27.94GiB
        Device allocated:         21.56GiB
        Device unallocated:        6.38GiB
        Device missing:          0.00B
        Used:             18.66GiB
        Free (estimated):          3.99GiB  (min: 3.99GiB)
        Data ratio:               2.00
        Metadata ratio:           2.00
        Global reserve:      208.00MiB  (used: 0.00B)

    Data,RAID1: Size:9.00GiB, Used:8.20GiB
       /dev/sda1       9.00GiB
       /dev/sdb1       9.00GiB

    Metadata,RAID1: Size:1.75GiB, Used:1.13GiB
       /dev/sda1       1.75GiB
       /dev/sdb1       1.75GiB

    System,RAID1: Size:32.00MiB, Used:16.00KiB
       /dev/sda1      32.00MiB
       /dev/sdb1      32.00MiB

Usage for /var also looks wrong, but in a different way:

    Overall:
        Device size:         160.00GiB
        Device allocated:        160.00GiB
        Device unallocated:        2.00MiB
        Device missing:          0.00B
        Used:            112.28GiB
        Free (estimated):         21.20GiB  (min: 21.20GiB)
        Data ratio:               2.00
        Metadata ratio:           2.00
        Global reserve:      512.00MiB  (used: 0.00B)

    Data,RAID1: Size:74.97GiB, Used:53.77GiB
       /dev/sda3      74.97GiB
       /dev/sdb3      74.97GiB

    Metadata,RAID1: Size:5.00GiB, Used:2.37GiB
       /dev/sda3       5.00GiB
       /dev/sdb3       5.00GiB

    System,RAID1: Size:32.00MiB, Used:16.00KiB
       /dev/sda3      32.00MiB
       /dev/sdb3      32.00MiB

    Unallocated:
       /dev/sda3       1.00MiB
       /dev/sdb3       1.00MiB

Version information:

    async@riff:~$ uname -a
    Linux riff 4.2.0-30-generic #36~14.04.1-Ubuntu SMP Fri Feb 26 18:49:23
    UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

    async@riff:~$ btrfs --version
    btrfs-progs v4.0

Thanks,

Christian Robottom Reis | [+55 16] 3376 0125   | http://async.com.br/~kiko

Yes, now I do, about 10 minutes after writing that mail. After a btrfs replace, if the device being added is larger than the original device, you need to issue:

btrfs fi resize <devid>:max <path>

to actually use that disk space. So for something like:

You need to do:

btrfs fi resize 1:max /
btrfs fi resize 2:max /

btrfs fi resize 1:max /var
btrfs fi resize 2:max /var

And it looks great now:

    Label: 'root'  uuid: 670d1132-00dc-4511-a2f6-d28ce08b4d3a
        Total devices 2 FS bytes used 9.34GiB
        devid    1 size 40.00GiB used 10.78GiB path /dev/sda1
        devid    2 size 40.00GiB used 10.78GiB path /dev/sdb1

    Label: 'var'  uuid: 815b3280-e90f-483a-b244-1d2dfe9b6e67
        Total devices 2 FS bytes used 56.16GiB
        devid    1 size 160.00GiB used 80.00GiB path /dev/sda3
        devid    2 size 160.00GiB used 80.00GiB path /dev/sdb3

This would be nice to document in the manpage for replace; it would also be a good addition to the best google hit for replace RAID-1:

http://unix.stackexchange.com/questions/227560/how-to-replace-a-device-in-btrfs-raid-1-filesystem

but I don't have enough reputation to do it myself.

Christian Robottom Reis | [+55 16] 3376 0125   | http://async.com.br/~kiko
                        | [+55 16] 991 126 430 | http://launchpad.net/~kiko

Device replace doesn't change the amount of the device that the FS
will use -- Like most FSes, btrfs has its own concept of the amount of a device it should use.

You probably need to run btrfs fi resize on the FS for each device:

# btrfs fi resize 1:max /mountpoint
# btrfs fi resize 2:max /mountpoint

Hugo.

Hugo Mills             | You stay in the theatre because you're afraid of
hugo@... carfax.org.uk | having no money? There's irony...
http://carfax.org.uk/  |
PGP: E2AB1DE4          |                                     Slings and Arrows