9 Nov
2020
9 Nov
'20
5:11 p.m.
All,
I'm not sure if this is relevant here, but I recently tried to use
OverlayFS with an NFS share. It wouldn't work because NFS does not present
to the kernel as a block device. OverlayFS requires a block device
abstraction. If CephFS doesn't present as a block device you won't get it
to work.
Hope this helps.
-Dave
--
Dave Hall
Binghamton University
kdhall(a)binghamton.edu
607-760-2328 (Cell)
607-777-4641 (Office)
On Mon, Nov 9, 2020 at 9:08 AM Frédéric Nass <frederic.nass(a)univ-lorraine.fr>
wrote:
Hi Luis,
Thanks for your help. Sorry I forgot about the kernel details. This is
latest RHEL 7.9.
~/ uname -r
3.10.0-1160.2.2.el7.x86_64
~/ grep CONFIG_TMPFS_XATTR /boot/config-3.10.0-1160.2.2.el7.x86_64
CONFIG_TMPFS_XATTR=y
upper directory /upperdir is using xattrs
~/ ls -l /dev/mapper/vg0-racine
lrwxrwxrwx 1 root root 7 6 mars 2020 /dev/mapper/vg0-racine -> ../dm-0
~/ cat /proc/fs/ext4/dm-0/options | grep xattr
user_xattr
~/ setfattr -n user.name -v upperdir /upperdir
~/ getfattr -n user.name /upperdir
getfattr: Suppression des « / » en tête des chemins absolus
# file: upperdir
user.name="upperdir"
Are you able to modify the content of a snapshot directory using
overlayfs on your side?
Frédéric.
Le 09/11/2020 à 12:39, Luis Henriques a écrit :
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Frédéric Nass
<frederic.nass(a)univ-lorraine.fr> writes:
> Hello,
>
> I would like to use a cephfs snapshot as a read/write volume without
having
to
> clone it first as the cloning operation is -
if I'm not mistaken - still
> inefficient as of now. This is for a data restore use case with Moodle
> application needing a writable data directory to start.
>
> The idea that came to mind was to use overlayFS with cephfs set up as a
> read-only lower layer and a writable local directory set up as an upper
> layer. With this set up, any modifications to the read-only
.snap/testsnap
> directory would normally go to the upper
directory making the snapshot
directory
> somehow writable to the Moodle application.
While this works fine when
a local
> read-only filesystem is set up as the lower
layer, it fails when cephfs
is set
> up as the lower layer. Any modifications to
the .snap/testsnap tree in
the
> /cephfs-snap directory fails with an
"Operation not supported".
>
> $ mkdir /cephfs /upperdir /workdir /cephfs-snap
>
> $ mount -t ceph 100.74.191.129:/volumes/group1/subvolume1/ /cephfs -o
> name=admin,secretfile=/etc/ceph/admin.secret
>
> $ mount -t overlay overlay -o
>
redirect_dir=on,lowerdir=/cephfs/.snap/testsnap,upperdir=/upperdir,workdir=/workdir
> /cephfs-snap
>
> $ ls /cephfs-snap
> usr
>
> $ touch /cephfs-snap/foo.txt <---- writing outside the
lowerdir
> succeeds
>
> $ ls /cephfs-snap
> foo.txt usr
>
> $ ls /usr/etc
>
> $ touch /cephfs-snap/usr/etc/foo <---- writing inside the
lowerdir
> fails
> touch: impossible de faire un touch « /cephfs-snap/usr/etc/foo »:
Opération
non
> supportée
>
> I tried to mount the whole cephfs tree read-only (-o ro), tried to
disable
ACLs
> (-o noacl) as seen here [1] but of no help.
Mounting with ceph-fuse
didn't help
> either. There's been a recent discussion
about this here [2] between
Greg and
Robert
but with no real solution.
I just commented on that bug tracker and, although
I'm not really 100%
sure, I suspect that the tmpfs on that system has been compiled without
xattr support.
Did someone manage to do this?
I
couldn't reproduce you're problem. Is it possible that your upper dir
doesn't support xatttrs either? Also, kernel client details would help.
Cheers,
--
Luis
> Regards,
>
> Frédéric.
>
> [1] https://blog.fai-project.org/posts/overlayfs/
> [2] https://tracker.ceph.com/issues/44821
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