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

[Orit Wasserman <owasserm@redhat.com>]

On 02/07/2014 03:44 PM, Marcin Gibuła wrote:
> 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?
>

I think is should be enough.