On Thu, Jan 13, 2011 at 16:59, Karel Zak <kzak@xxxxxxxxxx> wrote:
> On Thu, Jan 13, 2011 at 03:43:38PM +0100, Kay Sievers wrote:
>> On Thu, Jan 13, 2011 at 15:30, Tejun Heo <tj@xxxxxxxxxx> wrote:
>> > On Thu, Jan 13, 2011 at 3:25 PM, Milan Broz <mbroz@xxxxxxxxxx> wrote:
>> >> Maybe, but this was not invented in DM/MD camp:-)
>> >> Probably Kay or Greg can answer why it was done this way?
>>
>> It's not from Greg or Kay. It just appeared some day in the context of dm. :)
>>
>> And yes, symlinks *look* nice and simple for the outside, but they are
>> not, and have all sorts of problems like non-atomic updates, make it
>
> ÂSounds like sysfs implementation problem, right?
It's a normal multi-file problem. It can by-definition not be atomic
without doing really weird locking things.
> ÂIf there is noway to fix sysfs then we can add a generic ioctl or
> Â/sys/block/<device>/{slave,holder}_list files with list of
> Âholders/slaves.
Yeah, we've been there with the btrfs problem. For btrfs it woud
probably need to be something statfs()-like.
> ÂBut please, don't force userspace to use *claimer-specific*
> Âmethods to answer *generic questions* like slave/holder dependencies
> Âbetween devices.
The links exist only for dm and md so far, I think. It's the classical
multiple-parents-in-a-tree problem. We have that for bonded network
devices and some IO buses too. There is no nice representation for
these reversed-trees-in-the-tree so far.
Kay
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