[pve-devel] Default cache mode for VM hard drives

Eneko Lacunza elacunza at binovo.es
Thu May 28 13:20:49 CEST 2015


Hi Stanislav,

On 28/05/15 13:10, Stanislav German-Evtushenko wrote:
> Alexandre,
>
> The important point is whether O_DIRECT is used with Ceph or not. 
> Don't you know?
>
> > qemu rbd access is only userland, so host don't have any cache or 
> buffer.
> If RBD device does not use host cache then it is very likely that RBD 
> utilizes O_DIRECT. I am not sure if there are other ways to avoid host 
> cache.
>
> > When data is written to ceph, it's written to the journal of each 
> osd and replicas, before ack to the client.
> It can't be written to all destination right at the same time. If 
> buffer changed meanwhile then data that reach different nodes data can 
> differ.
Qemu-img is using a library, not a syscall to write to RBD. I don't see 
any O_DIRECT option in the API:
https://github.com/ceph/ceph/blob/master/src/include/rbd/librbd.h

Cheers
Eneko

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