On Wed, 05 May 2010 13:23:03 +0100, Baz Walter wrote:
>>> so
>>> if several filesystems are mounted in the same parent directory, there is
>>> no way to tell which of them is the "right" one.
>>
>> The only case which would cause a problem here is if you mount the same
>> device on two different subdirectories of a common directory. But in that
>> case, it doesn't really matter which answer you get, as they're both
>> equivalent in any sense that matters.
>
> nope! just to be clear:
>
> here's what i get on my system, where '/dev/sda1' and '/dev/sda6' are
> mounted at '/boot' and '/home' respectively:
>
> >>> os.stat('/').st_ino
> 2L
> >>> os.stat('/usr').st_ino
> 212993L
> >>> os.stat('/boot').st_ino
> 2L
> >>> os.stat('/home').st_ino
> 2L
> >>>
Right. /, /boot and /home are separate filesystems; ext2/ext3 filesystems
normally use inode #2 as the root directory. If you look at the st_dev
field, all three will be different (for /boot and /home, they'll
correspond to sda1 and sda6 respectively).
> if the algorithm is climbing up from '/home/baz/tmp/xxx', what does it
> do when it searches os.listdir('../../../..')? how can it tell whether
> 'boot' or 'home' is the correct next parent if it only checks the inode
> number?
It doesn't. Inode numbers are only unique within a single device; you need
to use both st_dev and st_ino to uniquely identify an inode within a
complete filesystem.
> i think the algorithm would at least need to take account of
> changes in the current device id. not sure whether that would be enough
> to cover all cases, though.
st_dev+st_ino is sufficient.
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