On 06/08/2015 09:23 PM, Alexandre DERUMIER wrote:
In the short-term, you can remove the "rbd cache" setting from your ceph.conf
That's not true, you need to remove the ceph.conf file.
Removing rbd_cache is not enough or default rbd_cache=false will apply.
I have done tests, here the result matrix
host ceph.conf : no rbd_cache : guest cache=writeback : result : nocache (wrong)
host ceph.conf : rbd_cache=false : guest cache=writeback : result : nocache (wrong)
host ceph.conf : rbd_cache=true : guest cache=writeback : result : cache
host ceph.conf : no rbd_cache : guest cache=none : result : nocache
host ceph.conf : rbd_cache=false : guest cache=none : result : no cache
host ceph.conf : rbd_cache=true : guest cache=none : result : cache (wrong)
QEMU patch 3/4 fixes this:
http://comments.gmane.org/gmane.comp.emulators.qemu.block/2500
Josh
----- Mail original -----
De: "Jason Dillaman" <dillaman@xxxxxxxxxx>
À: "Andrey Korolyov" <andrey@xxxxxxx>
Cc: "Josh Durgin" <jdurgin@xxxxxxxxxx>, "aderumier" <aderumier@xxxxxxxxx>, "ceph-users" <ceph-users@xxxxxxxxxxxxxx>
Envoyé: Lundi 8 Juin 2015 22:29:10
Objet: Re: rbd cache + libvirt
On Mon, Jun 8, 2015 at 10:43 PM, Josh Durgin <jdurgin@xxxxxxxxxx> wrote:
On 06/08/2015 11:19 AM, Alexandre DERUMIER wrote:
Hi,
looking at the latest version of QEMU,
It's seem that it's was already this behaviour since the add of rbd_cache
parsing in rbd.c by josh in 2012
http://git.qemu.org/?p=qemu.git;a=blobdiff;f=block/rbd.c;h=eebc3344620058322bb53ba8376af4a82388d277;hp=1280d66d3ca73e552642d7a60743a0e2ce05f664;hb=b11f38fcdf837c6ba1d4287b1c685eb3ae5351a8;hpb=166acf546f476d3594a1c1746dc265f1984c5c85
I'll do tests on my side tomorrow to be sure.
It seems like we should switch the order so ceph.conf is overridden by
qemu's cache settings. I don't remember a good reason to have it the
other way around.
Josh
Erm, doesn`t this code *already* represent the right priorities?
Cache=none setting should set a BDRV_O_NOCACHE which is effectively
disabling cache in a mentioned snippet.
Yes, the override is applied (correctly) based upon your QEMU cache settings. However, it then reads your configuration file and re-applies the "rbd_cache" setting based upon what is in the file (if it exists). So in the case where a configuration file has "rbd cache = true", the override of "rbd cache = false" derived from your QEMU cache setting would get wiped out. The long term solution would be to, as Josh noted, switch the order (so long as there wasn't a use-case for applying values in this order). In the short-term, you can remove the "rbd cache" setting from your ceph.conf so that QEMU controls it (i.e. it cannot get overridden when reading the configuration file) or use a different ceph.conf for a drive which requires different cache settings from the default configuration's settings.
Jason
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