On 20.01.2012 11:55, Goswin von Brederlow wrote:
[]
>>> And yes it is kinda trivial to add a call to realpath(3) to
>>> busybox (or equivalent).
>>
>> Or add readlink into initramfs-tools, for that matter.
>>
>> But it is still not clear if it is a bug or not :)
>
> As a side note: With LVM you get entries like:
>
> /dev/mapper/r-usr 7224824 6392404 465460 94% /usr
>
> I would much rather have:
>
> /dev/r/usr 7224824 6392404 465460 94% /usr
> But I guess the solution for this would be to have udev make /dev/r/usr
> the real device and /dev/mapper/r-usr a symlink.
Exactly. And note that actually, these are dm devices, which
usually appear as /dev/dm-N (cf. multipath devices created by
multipathd - they symlink from /dev/mapper/name to /dev/dm-N).
And this is an interesting place.
The "problem" is that kernel does not know what /dev/mapper
is, or what does /dev/r/usr thing mean. It knows these by
their canonical (and meaningless) dm-N names, like this:
[ 6.887981] EXT4-fs (dm-0): mounted filesystem with ordered data mode. Opts:
(null)
[ 6.917555] EXT4-fs (dm-1): mounted filesystem with ordered data mode. Opts:
usrquota
Go figure which is dm-0 and which is dm-1 (lvm is here)!
There are references to these dm-N names in /proc/partitions
and in /sys and elsewhere, too.
I consider this a bug, a quite serious one at it: there's
no device node which corresponds to kernel names! So it is
impossible to get, eg, device usage statistics for a
particular volume, or map i/o error to particular dvice and
so on. This is wrong.
But this is a different problem entirely.
My point is that actually, _both_ /dev/mapper/r-usr and
/dev/r/usr are wrong! But at least they - hopefully -
lead to the actual device, unlike kernel messages I
mentioned above which leads to nothing due to /dev/dm-0
non-existing.
> What result does "LABEL=..." get?
It is exactly the same as with UUID=. For both of these,
util-linux mount will canonicalize the name to final
device (like /dev/sda1 or /dev/dm-0 or /dev/mapper/r-usr
as in your case), be it LABEL= or /dev/disk/by-label/LABEL.
While neither busybox nor klibc will do that. Besides,
klibc does not support LABEL or UUID, in busybox it is
optional (compile-time), and it will do its own resolution
of LABEL/UUID, without using /dev/disk/by-*/.
In initramfs, both are handled the same way
(omiting some details for brevity):
initramfs:init:
case $ROOT in
LABEL=*)
ROOT="${ROOT#LABEL=}"
ROOT="/dev/disk/by-label/${ROOT}"
;;
UUID=*)
ROOT="/dev/disk/by-uuid/${ROOT#UUID=}"
;;
So this gives /dev/disk/... arg to mount, be it from
busybox or klibc, and neither calls realpath(3).
Thanks,
/mjt
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