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

[Stefan Priebe - Profihost AG]

OK that this was maybe just during qemu 1.2 sorry for the confusion.

Stefan

Am 14.02.2013 09:26, schrieb 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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>>