Prev: fixed resource leak in scripts/mod/modpost.c
Next: ntp: add hardpps implementation
From: Valerie Aurora on 6 Aug 2010 13:20
On Thu, Aug 05, 2010 at 01:13:55PM +0200, Miklos Szeredi wrote:
> On Wed, 4 Aug 2010, Valerie Aurora wrote:
> > > Another idea is to use an internal inode and make all fallthroughs be
> > > hard links to that.
> > >
> > > I think the same would work for whiteouts as well. I don't like the
> > > fact that whiteouts are invisible even when not mounted as part of a
> > > union.
> >
> > I don't know if this helps, but I just wrote support for removing ext2
> > whiteouts and fallthrus using tune2fs and e2fsck. I think this does
> > what people want from a "visible" whiteout feature without adding more
> > complexity to the VFS. It also takes away all consideration of race
> > conditions and dentry conversion that happens with online removal of
> > whiteouts and fallthrus.
> >
> > What are your thoughts on what a visible whiteout/fallthru would look
> > like?
>
> Best would be if it didn't need any modification to filesystems. All
> this having to upgrade util-linux, e2fsprogs, having incompatible
> filesystem features is a pain for users (just been through that).
>
> What we already have in most filesystems:
>
> - extended attributes, e.g. use the system.union.* namespace and
> denote whiteouts and falltroughs with such an attribute
>
> - hard links to make sure a separate inode is not necessary for each
> whiteout/fallthrough entry
>
> - some way for the user to easily identify such files when not
> mounted as part of a union e.g. make it a symlink pointing to
> "(deleted)" or whatever
>
> Later the extended attributes can also be used for other things like
> e.g. chmod()/chown() only copying up metadata, not data, and
> indicating that data is still found on the lower layers.

Just a quick note to say that my explicit design was to do as much as
possible in the VFS, except when adding a little support to the
low-level fs would make it significantly faster, simpler, and more
correct. I think for union mounts to perform moderately well, and to
avoid namespace problems, we can't build it 100% out of existing file
system parts like xattrs. However, I could be wrong and I will
definitely give any other implementation serious consideration.

-VAL
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo(a)vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
From: Miklos Szeredi on 6 Aug 2010 13:50
On Fri, 6 Aug 2010, Valerie Aurora wrote:
> Oh, "mmount -b 8" == "mount -o union".

Right.

> Is this the mmount from mtools
> or something else?

It's primitive utility that basically just wraps the mount(2) syscall
without any fstab/mtab support:

http://www.kernel.org/pub/linux/kernel/people/mszeredi/mmount/

Miklos
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo(a)vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
From: Miklos Szeredi on 13 Aug 2010 10:00
On Sun, 8 Aug 2010, Valerie Aurora wrote:
> Add support for fallthru directory entries to ext2.

This still doesn't work correctly, only now after unmounting/mounting
the top layer the directory appears empty:

uml:~# mount -oloop -r ext3.img /mnt/img/
uml:~# losetup /dev/loop2 ovl.img
uml:~# /host/store/git/e2fsprogs/misc/mke2fs -O whiteout,fallthru /dev/loop2
mke2fs 1.41.12 (17-May-2010)
Filesystem label=
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
Stride=0 blocks, Stripe width=0 blocks
1280 inodes, 10240 blocks
512 blocks (5.00%) reserved for the super user
First data block=1
Maximum filesystem blocks=10485760
2 block groups
8192 blocks per group, 8192 fragments per group
640 inodes per group
Superblock backups stored on blocks:
8193

Allocating group tables: done
Writing inode tables: done
Writing superblocks and filesystem accounting information: done

This filesystem will be automatically checked every 24 mounts or
180 days, whichever comes first. Use tune2fs -c or -i to override.
uml:~# mmount -b 8 -t ext2 /dev/loop2 /mnt/img/
uml:~# "ls" /mnt/img
lost+found union
uml:~# "ls" /mnt/img/union
1 2 3
uml:~# umount /mnt/img
uml:~# mmount -b 8 -t ext2 /dev/loop2 /mnt/img/
uml:~# ls -l /mnt/img/
total 13
drwx------ 2 root root 12288 Aug 13 13:42 lost+found
drwxr-xr-x 2 root root 1024 Aug 13 13:42 union
uml:~# ls -la /mnt/img/union/
total 2
drwxr-xr-x 2 root root 1024 Aug 13 13:42 .
drwxr-xr-x 4 root root 1024 Aug 13 13:42 ..
uml:~# grep /mnt/img /proc/self/mountinfo
21 12 7:0 / /mnt/img ro,relatime - ext3 /dev/loop0 ro,errors=continue,barrier=0,data=writeback
22 21 7:2 / /mnt/img rw,relatime,union - ext2 /dev/loop2 rw,errors=continue

Thanks,
Miklos
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo(a)vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
First  |  Prev  | 
Pages: 1 2
Prev: fixed resource leak in scripts/mod/modpost.c
Next: ntp: add hardpps implementation