On 4.01.19 г. 12:26 ч., David Disseldorp wrote:
> On Fri, 4 Jan 2019 09:05:00 +0200, Nikolay Borisov wrote:
>
>> btrfs inspect-internal dump-super already supports showing seeding, why
>> do we need btrfstune support for that as well?
>
> Ah, I wasn't aware of that. IMO the logical place to look is next to
> where the user enables seeding (btrfstune -S1).
>
>> Also the way you've
>> phrased is a bit misleading, since checking the superflag doesn't mean
>> seeding for a particular _device_ is enabled but that the filesystem
>> itself is set as seeding.
>
> Not quite sure I follow. For a multi-dev FS (one seeding), I see:
the seeding flag is a feature of the filesystem not any particular
device in it. It's set per-superblock, the superblock on every device is
(or should be) identical to the superblock on any other device.
>
> rapido1:/# btrfstune -s /dev/zram0
> Seeding flag is currently unset
> rapido1:/# btrfstune -S 1 /dev/zram0
> rapido1:/# btrfstune -s /dev/zram0
> Seeding flag is currently set
> rapido1:/# mount /dev/zram0 /mnt/test/
> mount: /dev/zram0 is write-protected, mounting read-only
> ...
> rapido1:/# btrfs device add -f /dev/zram1 /mnt/test/
> [ 77.805374] BTRFS info (device zram0): disk added /dev/zram1
> rapido1:/# umount /mnt/test/
> rapido1:/# btrfstune -s /dev/zram1
> Seeding flag is currently unset
> rapido1:/# btrfstune -s /dev/zram0
> Seeding flag is currently set
Right, i think this is a side effect rather than a deliberate behavior.
What I mean by this is if you have a raid0 filesystem and you set it to
seed then the flag will be reflected on both devices, but not because
the code said "mark device 1 and device 2 as seed devices" but because
the logic goes "mark this superblock as being a seed and write it to all
devices".
So when you actually add the new device to an existing filesystem which
is seed what btrfs does is initialize a new filesystem on the newly
added device. For reference you can check the code around
btrfs_init_new_device and btrfs_prepare_sprout. IMO seed has really been
hacked on "just because" and is lacking coherent design, I'd prefer it
didn't proliferate and rely on what we have currently.
>
> Cheers, David
>
