It's almost worth filing a bugreport for that.
Surely in the case of multiple backing devices, if you don't specify which one, the default should be *all*, not the *first*.
I bet every single person who ever resizes a multiple-backing-device btrfs will have hit this issue.

On Sat, 19 Sep 2015 at 18:43 James Harper <james@ejbdigital.com.au> wrote:
> > On Wed, 16 Sep 2015 08:56:04 PM James Harper wrote:
> > > I've tried running a btrfs resize but it hasn't changed anything.
> >
> > What was the exact resize command you used?
> >
>
> The docs say:
>
> # btrfs filesystem resize amount /mount-point
>
> So I said:
>
> btrfs filesystem resize max /
>
> and Linux says:
>
> Resize '/' of 'max'
>
> Which doesn't really make any sense...
>

Sorted. The special sauce was to specify the device id, so:

# btrfs filesystem show /
Label: none  uuid: 3826b465-0224-41df-9e1e-182d5eb68904
        Total devices 4 FS bytes used 5.15TiB
        devid    1 size 2.73TiB used 2.73TiB path /dev/sda3
        devid    2 size 2.73TiB used 2.73TiB path /dev/sdb3
        devid    3 size 2.73TiB used 2.73TiB path /dev/sdc3
        devid    4 size 2.73TiB used 2.73TiB path /dev/sde3

# btrfs filesystem resize 4:max /
Resize '/' of '4:max'

# btrfs filesystem show /
Label: none  uuid: 3826b465-0224-41df-9e1e-182d5eb68904
        Total devices 4 FS bytes used 5.15TiB
        devid    1 size 2.73TiB used 2.73TiB path /dev/sda3
        devid    2 size 2.73TiB used 2.73TiB path /dev/sdb3
        devid    3 size 2.73TiB used 2.73TiB path /dev/sdc3
        devid    4 size 5.46TiB used 2.73TiB path /dev/sde3

w00t!

James
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