On Tue, Aug 26, 2008 at 07:32:25PM -0400, Theodore Tso wrote:
> On Tue, Aug 26, 2008 at 10:47:37PM +0200, Karel Zak wrote:
> > There is worse scenario (thanks to Milan Broz from DM camp):
> >
> > dmsetup create x --table "0 100 linear /dev/sdb 0"
> > dmsetup create y --table "0 100 linear /dev/mapper/x 0"
> > dmsetup create z --table "0 100 linear /dev/mapper/y 0"
> >
> > # dmsetup ls --tree
> > z (254:3)
> > `-y (254:2)
> > `-x (254:1)
> > `- (8:16)
> >
> > it means all these devices are exactly same, but
> >
> > mount LABEL=foo
> >
> > has to mount /dev/mapper/z (from top of the tree). The sdb, x and
> > y should be invisible for the mount(8).
Well, s/invisible/less important/. Sorry.
> Sure, but consider what happens when you create a snapshot (either
> read-only or read-write) of an existing filesystem? In that case,
That's misunderstanding. I'm talking about LABEL/UUId resolution
where we need *priorities* for duplicate tags. I think dep-tree is
good enough for this purpose.
The snapshot (or arbitrary other device) is mountable when you
explicitly use device "mount /dev/mapper/the_snapshot".
> both the parent and the child filesystem is mountable, and if the
> child filesystem is transient, the praent one may not want to be
> transient.
[...]
> Speaking of which, what is your plan for caching versus non-caching in
> libfsprobe? It seems to me that if you are going to be caching,
Both. I think you remember our (+ Kay Sievers) discussion about it.
We need a library which provides both ways. The smart way (cache,
dependencies, ...) for mount(8) and others standard utils, and the
low-level way for udev (no cache, direct FS probing, ...).
> you'll just be re-inventing blkid. If you don't cache, you'll either
Hehe.. I will directly copy code from blkid and vol_id. It's open
source. I needn't re-inventing ;-)
> (a) have to iterate over all possible devices, which is what we did
> before blkid (it was Ric Wheeler pointed out to me this problem and I
> wrote blkid in response to his request, because it becomes a problem
> if you have hundred of LUN's getting exported by a large EMC storage
> array :-), or (b) do what vol_id does, which is depend on
> /dev/disk/by-label and /dev/disk/by-uuid, which has the charming
> Windows-like attribute of not getting updated until the next reboot
What about fix mkfs tools and send relevant events to udev?
> > It would be nice to have /sys/block/dm-N/name where you can translate
> > the internal dm-N name to the real device name. Alasdair? Milan? :-)
>
> Or maybe the right answer is /proc/partitions should only export
> devicemapper devices that are "supposed" to be visible to mount, and
Yes, I have no clue why the dm-N crap is in /proc/partitions.
Probably any legacy...
> instead of exporting dm-0, dm-1...., we export the real name via
> /proc/partitions? Or do you not want to have the user-visible name
> get pushed into the kernel?
I'm wrong person for these questions. DM guys are in CC: :-)
Karel
--
Karel Zak <kzak@xxxxxxxxxx>
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