Re: Notes on support for multiple devices for a single filesystem

[Christoph Hellwig <hch@infradead.org>]

On Wed, Dec 17, 2008 at 03:50:45PM +0100, Kay Sievers wrote:
> Sounds all sensible. Btrfs already stores the (possibly incomplete)
> device tree state in the kernel, which should make things pretty easy
> for userspace, compared to other already existing subsystems.
> 
> We could have udev maintain a btrfs volume tree:
>   /dev/btrfs/
>   |-- 0cdedd75-2d03-41e6-a1eb-156c0920a021
>   |   |-- 897fac06-569c-4f45-a0b9-a1f91a9564d4 -> ../../sda10
>   |   `-- aac20975-b642-4650-b65b-b92ce22616f2 -> ../../sda9
>   `-- a1ec970a-2463-414e-864c-2eb8ac4e1cf2
>       |-- 4d1f1fff-4c6b-4b87-8486-36f58abc0610 -> ../../sdb2
>       `-- e7fe3065-c39f-4295-a099-a89e839ae350 -> ../../sdb1
> 
> At the same time, by-uuid/ is created:
>   /dev/disk/by-uuid/
>   |-- 0cdedd75-2d03-41e6-a1eb-156c0920a021 -> ../../sda10
>   |-- a1ec970a-2463-414e-864c-2eb8ac4e1cf2 -> ../../sdb2
>   ...

Well, it's not just btrfs, it's also md, lvm and xfs.  I think the right
way is to make the single node for the /dev/disk/by-uuid/ just a legacy
case for potential multiple devices.  E.g. by having

/dev/disk/by-uuid/
	0cdedd75-2d03-41e6-a1eb-156c0920a021	-> ../../sda10
	0cdedd75-2d03-41e6-a1eb-156c0920a021.d
		foo				-> ../../sda10
		bar				-> ../../sda9

where foo nad bar could be uuids if the filesystem / volume manager
supports it, otherwise just the short name for it.


> For recue and recovery cases, it will still be nice to be able to
> trigger "scan all devices" code in btrfsctrl (own code or libbklid),
> but it should be avoided in any normal operation mode.

Again, that's something we should do generically for the whole
/dev/disk/ tree.   For that we need to merge libvolume_id and libblkid
so that it has a few related but separate use cases:

 - a lowlevel probe what fs / volume manager / etc is this for
   the udev callout, mkfs, strip size detection etc
 - a way to rescan everything, either for non-udev static /dev case
   or your above recovery scenario
 - plus potentially some sort of caching for the non-recovery static
   /dev case

I've long planned to put you and Ted into a room and not let you out
until we see white smoke :)