[pve-devel] default cache option=none vs writeback

[Alexandre DERUMIER]

qemu rbd driver :( /block/rbd.c)

    if (flags & BDRV_O_NOCACHE) {
        rados_conf_set(s->cluster, "rbd_cache", "false");
    } else {
        rados_conf_set(s->cluster, "rbd_cache", "true");
    }


and since qemu 1.3

default is BDRV_O_CACHE_WB 
(http://git.qemu.org/?p=qemu.git;a=blobdiff;f=blockdev.c;h=7c83baa353ca11e10af5d6b241aec219d94ff11d;hp=8669142704b9d71c459c9740559c26d91ae772a2;hb=1f212b9d3edd8679bafd3bcf0301795206438724;hpb=9315cbfd8d7074eca44fbc5f93902e97b27d5240)


so, rbd cache should work with default qemu 1.3 cache.


----- Mail original ----- 

De: "Alexandre DERUMIER" <aderumier at odiso.com> 
À: "Stefan Priebe - Profihost AG" <s.priebe at profihost.ag> 
Cc: "Dietmar Maurer" <dietmar at proxmox.com>, pve-devel at pve.proxmox.com 
Envoyé: Jeudi 14 Février 2013 09:12:47 
Objet: Re: [pve-devel] default cache option=none vs writeback 

>>see for example here: 
>>http://www.spinics.net/lists/ceph-devel/msg06920.html 

This seem an "old" post from June, when default cache mode was not writeback. (default is writeback since qemu 1.3) 

Maybe can we check the rbd qemu code to be sure ? 



----- Mail original ----- 
. 
De: "Stefan Priebe - Profihost AG" <s.priebe at profihost.ag> 
À: "Alexandre DERUMIER" <aderumier at odiso.com> 
Cc: "Dietmar Maurer" <dietmar at proxmox.com>, pve-devel at pve.proxmox.com 
Envoyé: Jeudi 14 Février 2013 08:50:34 
Objet: Re: [pve-devel] default cache option=none vs writeback 

Hi, 

but i think it's still crazy for the users - for me too. An example is 
RBD. Default is writeback but rbd still behaves differently if you 
select default or if you select writeback. 

see for example here: 
http://www.spinics.net/lists/ceph-devel/msg06920.html 

Stefan 
Am 14.02.2013 07:49, schrieb Alexandre DERUMIER: 
>>> Maybe we should just display 'default' without '= no cache'? 
> 
> I think we should display default(writeback) or default(nocache), but keep displaying the information. 
> Because nobody rtfm the known what is default ;) 
> 
> maybe the best way is to follow qemu: 
> 
> default(writeback) = no qemu cache option for all storages. (with no specific .raw, cache=none, trick) 
> 
> That shouldn't change in the futur I think. 
> 
> 
>>> AFAIK you get very bad performance with qcow2 files and cache=none 
> 
> Not sure about it, it's true with writethrough/directsync because of the fsync and qcow2 metadatas. 
> But it really depends of the physicals storages behind, and their cache implementation. 
> 
> My tests my netapp and nexenta seem to show that cache=writeback have a little overhead vs cache=none, 
> mainly when the host need to flush datas.(tested with iscsi, raw, qcow2). 
> 
> But some storage like rbd have a good writeback cache feature. 
> 
> So it's not easy to auto detect what is the best cache option value. 
> 
> 
> 
> So, maybe keep default qemu writeback is a good compromise ? 
> 
> ----- Mail original ----- 
> 
> De: "Dietmar Maurer" <dietmar at proxmox.com> 
> À: "Alexandre DERUMIER" <aderumier at odiso.com>, pve-devel at pve.proxmox.com 
> Envoyé: Mercredi 13 Février 2013 16:31:27 
> Objet: RE: [pve-devel] default cache option=none vs writeback 
> 
>> Currently pve-manager display default = no cache, and currently only raw 
>> files are forced with cache=none. 
> 
> Maybe we should just display 'default' without '= no cache'? 
> 
>> 
>> but since qemu 1.3, default is writeback. It's very confusing for the users. 
>> 
>> 
>> So maybe settings cache=none to disk options for all storages as default 
>> could be great ? 
> 
> AFAIK you get very bad performance with qcow2 files and cache=none 
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