Yes Wido, you are correct. There is a RBD pool in the cluster, but is not currently running with a cache attached. The Pool I’m trying to manage here is only used by Librados to write objects directly to the pool as opposed to any of the other niceties that ceph provides.
Specifically I ran:
`ceph osd tier cache-mode <hot-storage> forward`
which returned `Error EPERM: 'forward' is not a well-supported cache mode and may corrupt your data. pass --yes-i-really-mean-it to force.`
Currently we are running 10.2.5. I suspect that it’s fine in our use case, however given the sparsity of the documentation I didn’t like to assume anything.
Regards
Stuart
On 10 Jan 2017, at 09:52, Wido den Hollander < wido@xxxxxxxx> wrote:
Op 10 januari 2017 om 9:52 schreef Nick Fisk <nick@xxxxxxxxxx>:
-----Original Message-----
From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx] On Behalf Of Wido den Hollander
Sent: 10 January 2017 07:54
To: ceph new <ceph-users@xxxxxxxxxxxxxx>; Stuart Harland <s.harland@xxxxxxxxxxxxxxxxxxxxxx>
Subject: Re: Write back cache removal
Op 9 januari 2017 om 13:02 schreef Stuart Harland <s.harland@xxxxxxxxxxxxxxxxxxxxxx>:
Hi,
We’ve been operating a ceph storage system storing files using librados (using a replicated pool on rust disks). We implemented a
cache over the top of this with SSDs, however we now want to turn this off.
The documentation suggests setting the cache mode to forward before draining the pool, however the ceph management
controller spits out an error about this saying that it is unsupported and hence dangerous.
What version of Ceph are you running?
And can you paste the exact command and the output?
Wido
Hi Wido,
I think this has been discussed before and looks like it might be a current limitation. Not sure if it's on anybody's radar to fix.
https://www.mail-archive.com/ceph-users@xxxxxxxxxxxxxx/msg24472.html
Might be, but afaik they are using their own application which writes to RADOS using librados, not RBD.Is that correct Stuart?WidoNick
The thing is I cannot really locate any documentation as to why it’s considered unsupported and under what conditions it is expected
to fail: I have read a passing comment about EC pools having data corruption, but we are using replicated pools.
Is this something that is safe to do?
Otherwise I have noted the read proxy mode of cache tiers which is documented as a mechanism to transition from write back to
disabled, however the documentation is even sparser on this than forward mode. Would this be a better approach if there is some
unsupported behaviour in the forward mode cache option?
Any thoughts would be appreciated - we really cannot afford to corrupt the data, and I really do not want to have to do some
manual software based eviction on this data.
regards
Stuart
− Stuart Harland:
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