Re: [Qemu-devel] migration question: disk images on nfs server

["Marcin Gibuła" <m.gibula@beyond.pl>]

On 07.02.2014 14:36, Orit Wasserman wrote:
>> Do you know if is applies to linux O_DIRECT writes as well?
>>
>
>  From the man of open:
>
>         The behaviour of O_DIRECT with NFS will differ from local
>         filesystems.  Older kernels, or kernels configured in certain ways,
>         may not support this combination.  The NFS protocol does not
> support
>         passing the flag to the server, so O_DIRECT I/O will bypass the
> page
>         cache only on the client; the server may still cache the I/O.  The
>         client asks the server to make the I/O synchronous to preserve the
>         synchronous semantics of O_DIRECT.  Some servers will perform
> poorly
>         under these circumstances, especially if the I/O size is small.
> Some
>         servers may also be configured to lie to clients about the I/O
> having
>         reached stable storage; this will avoid the performance penalty at
>         some risk to data integrity in the event of server power failure.
>         The Linux NFS client places no alignment restrictions on O_DIRECT
>         I/O.
>
> To summaries it depends on your kernel (NFS client).

So, assuming new kernel (where nfs O_DIRECT translates to no cache at 
client side) and cache coherent server, is it enough or is 'sync' mount 
(or O_SYNC flag) still required for some reason?

-- 
mg