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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