Re: Replacing RAID-1 devices with larger disks

[Christian Robottom Reis <kiko@async.com.br>]

On Sun, Feb 28, 2016 at 05:15:32PM -0300, Christian Robottom Reis wrote:
> 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?

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:

>     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

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