That will be down to the pool the rbd was in, the crush rule for that pool will dictate which osd's store objects. In a standard config that rbd will likely have objects on every osd in your cluster.
On 8 Aug 2016 9:51 a.m., "Georgios Dimitrakakis" <giorgis@xxxxxxxxxxxx> wrote:
Hi,
On 08.08.2016 09:58, Georgios Dimitrakakis wrote:
Dear all,
I would like your help with an emergency issue but first let me describe our environment.
Our environment consists of 2OSD nodes with 10x 2TB HDDs each and 3MON nodes (2 of them are the OSD nodes as well) all with ceph version 0.80.9 (b5a67f0e1d15385bc0d60a6da6e7fc810bde6047)
This environment provides RBD volumes to an OpenStack Icehouse installation.
Although not a state of the art environment is working well and within our expectations.
The issue now is that one of our users accidentally deleted one of the volumes without keeping its data first!
Is there any way (since the data are considered critical and very important) to recover them from CEPH?
Short answer: no
Long answer: no, but....
Consider the way Ceph stores data... each RBD is striped into chunks
(RADOS objects with 4MB size by default); the chunks are distributed
among the OSDs with the configured number of replicates (probably two
in your case since you use 2 OSD hosts). RBD uses thin provisioning,
so chunks are allocated upon first write access.
If an RBD is deleted all of its chunks are deleted on the
corresponding OSDs. If you want to recover a deleted RBD, you need to
recover all individual chunks. Whether this is possible depends on
your filesystem and whether the space of a former chunk is already
assigned to other RADOS objects. The RADOS object names are composed
of the RBD name and the offset position of the chunk, so if an
undelete mechanism exists for the OSDs' filesystem, you have to be
able to recover file by their filename, otherwise you might end up
mixing the content of various deleted RBDs. Due to the thin
provisioning there might be some chunks missing (e.g. never allocated
before).
Given the fact that
- you probably use XFS on the OSDs since it is the preferred
filesystem for OSDs (there is RDR-XFS, but I've never had to use it)
- you would need to stop the complete ceph cluster (recovery tools do
not work on mounted filesystems)
- your cluster has been in use after the RBD was deleted and thus
parts of its former space might already have been overwritten
(replication might help you here, since there are two OSDs to try)
- XFS undelete does not work well on fragmented files (and OSDs tend
to introduce fragmentation...)
the answer is no, since it might not be feasible and the chance of
success are way too low.
If you want to spend time on it I would propose the stop the ceph
cluster as soon as possible, create copies of all involved OSDs, start
the cluster again and attempt the recovery on the copies.
Regards,
Burkhard
Hi! Thanks for the info...I understand that this is a very difficult and probably not feasible task but in case I need to try a recovery what other info should I need? Can I somehow find out on which OSDs the specific data were stored and minimize my search there?
Any ideas on how should I proceed?
Best,
G.
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