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.
Did someone manage to do this?
Regards,
Frédéric.
[1] https://blog.fai-project.org/posts/overlayfs/
[2] https://tracker.ceph.com/issues/44821
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