Re: 64-btrfs.rules and degraded boot

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2016-07-06 13:19, Chris Murphy wrote:
> On Wed, Jul 6, 2016 at 3:51 AM, Andrei Borzenkov <arvidjaar@gmail.com> wrote:
>> 3) can we query btrfs whether it is mountable in degraded mode?
>> according to documentation, "btrfs device ready" (which udev builtin
>> follows) checks "if it has ALL of it’s devices in cache for mounting".
>> This is required for proper systemd ordering of services.
>
> Where does udev builtin use btrfs itself? I see "btrfs ready $device"
> which is not a valid btrfs user space command.
>
> I never get any errors from "btrfs device ready" even when too many
> devices are missing. I don't know what it even does or if it's broken.
>
> This is a three device raid1 where I removed 2 devices and "btrfs
> device ready" does not complain, it always returns silent for me no
> matter what. It's been this way for years as far as I know.
>
> [root@f24s ~]# lvs
>   LV         VG Attr       LSize  Pool       Origin Data%  Meta%  Move
> Log Cpy%Sync Convert
>   1          VG Vwi-a-tz-- 50.00g thintastic        2.55
>   2          VG Vwi-a-tz-- 50.00g thintastic        4.00
>   3          VG Vwi-a-tz-- 50.00g thintastic        2.54
>   thintastic VG twi-aotz-- 90.00g                   5.05   2.92
> [root@f24s ~]# btrfs fi show
> Label: none  uuid: 96240fd9-ea76-47e7-8cf4-05d3570ccfd7
>     Total devices 3 FS bytes used 2.26GiB
>     devid    1 size 50.00GiB used 3.00GiB path /dev/mapper/VG-1
>     devid    2 size 50.00GiB used 2.01GiB path /dev/mapper/VG-2
>     devid    3 size 50.00GiB used 3.01GiB path /dev/mapper/VG-3
>
> [root@f24s ~]# btrfs device ready /dev/mapper/VG-1
> [root@f24s ~]#
> [root@f24s ~]# lvchange -an VG/1
> [root@f24s ~]# lvchange -an VG/2
> [root@f24s ~]# btrfs dev scan
> Scanning for Btrfs filesystems
> [root@f24s ~]# lvs
>   LV         VG Attr       LSize  Pool       Origin Data%  Meta%  Move
> Log Cpy%Sync Convert
>   1          VG Vwi---tz-- 50.00g thintastic
>   2          VG Vwi---tz-- 50.00g thintastic
>   3          VG Vwi-a-tz-- 50.00g thintastic        2.54
>   thintastic VG twi-aotz-- 90.00g                   5.05   2.92
> [root@f24s ~]# btrfs fi show
> warning, device 2 is missing
> Label: none  uuid: 96240fd9-ea76-47e7-8cf4-05d3570ccfd7
>     Total devices 3 FS bytes used 2.26GiB
>     devid    3 size 50.00GiB used 3.01GiB path /dev/mapper/VG-3
>     *** Some devices missing
>
> [root@f24s ~]# btrfs device ready /dev/mapper/VG-3
> [root@f24s ~]#
You won't get any output from it regardless, you have to check the 
return code as it's intended to be a tool for scripts and such.