lkml.org 
[lkml]   [2011]   [Jun]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH 0/7] overlay filesystem: request for inclusion
Date
Michal Suchanek <hramrach@centrum.cz> writes:

> On 15 June 2011 18:14, J. R. Okajima <hooanon05@yahoo.co.jp> wrote:
>> For example, in rename(2) dir where the target dir already existed, aufs
>> renames the target dir to a temporary unique whiteouted name before the
>
> This is generally not possible in solutions that don't reserve any filenames.
>
> However, it should be possible to create whiteout of a non-existent
> entry in a directory while it is locked without affecting userspace.

Yes, creation of whiteout and renaming it to target or vice versa works
if target is non-directory.

Cases where this trick could make operations atomic:

- create/mknod/symlink/link over whiteout
- rename non-directory to whiteout
- remove of non-directory with whiteout creation
- copy up

Cases where atomicity is not possible with this:

- mkdir over whiteout
- rename directory to whiteout
- rename where source needs whiteout
- rmdir with whiteout creation


>> actual rename on a branch and then handles other actions (make it opaque,
>> update the attributes, etc). If an error happens in these actions, aufs
>> simply renames the whiteouted name back and returns an error. If all are
>> succeeded, aufs registers a function to remove the whiteouted unique
>> temporary name completely and asynchronously to the system global
>> workqueue.
>
> Removing the whiteout asynchronously does not seem like a good idea.
> It should be gone before the directory containing the whiteout is
> unlocked. Otherwise there might be an entry created which conflicts
> with this whiteout that did not exist when the operation started. Also
> if you unlock the directory while the artifical whiteout exists an
> asynchronous process might replace the whiteout and the rollback would
> fail.
>
> As an alternative way to perform atomic renames I would suggest
> "fallthrough symlinks". If you want to rename an entry which is
> "fallthrough" (ie pointing to the entry with the same name in the
> lower layer in the same directory) you can replace it with a
> "fallthrough symlink" which points to the lower layer and does not
> just implicitly say "here" but specifies a path relative to the
> mountpoint instead. This can then be moved like any other entry. it is
> in no way special anymore.

This is a nice idea, but doesn't have a lot to do with atomicity. It
allows rename of non-pure upper directory (they return EXDEV currently).

> Moving a directory tree which is partially
> in the upper layer is still time-consuming but can be performed with
> reasonable semantics imho.

Shouldn't be time consuming, really. The upper, mixed directory is
renamed and given a "trusted.overlay.redirect" attribute to show where
its lower directory resides.

Thanks,
Miklos


\
 
 \ /
  Last update: 2011-06-15 20:15    [W:0.083 / U:6.208 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site