Re: [Qemu-devel] [PATCHv2] block: add native support for NFS

[ronnie sahlberg]

On Wed, Dec 18, 2013 at 2:00 AM, Orit Wasserman <address@hidden> wrote:
> On 12/18/2013 01:03 AM, Peter Lieven wrote:
>>
>>
>>
>>> Am 17.12.2013 um 18:32 schrieb "Daniel P. Berrange"
>>> <address@hidden>:
>>>
>>>> On Tue, Dec 17, 2013 at 10:15:25AM +0100, Peter Lieven wrote:
>>>> This patch adds native support for accessing images on NFS shares
>>>> without
>>>> the requirement to actually mount the entire NFS share on the host.
>>>>
>>>> NFS Images can simply be specified by an url of the form:
>>>> nfs://<host>/<export>/<filename>
>>>>
>>>> For example:
>>>> qemu-img create -f qcow2 nfs://10.0.0.1/qemu-images/test.qcow2
>>>
>>>
>>> Does it support other config tunables, eg specifying which
>>> NFS version to use 2/3/4 ? If so will they be available as
>>> URI parameters in the obvious manner ?
>>
>>
>> currently only v3 is supported by libnfs. what other tunables would you
>> like to see?
>>
>
> For live migration we need the sync option (async ignores O_SYNC and
> O_DIRECT sadly),
> will it be supported? or will it be the default?
>

If you use the high-level API that provides posix like functions, such
as nfs_open() then libnfs does.
nfs_open()/nfs_open_async() takes a mode parameter and libnfs checks
the O_SYNC flag in modes.

By default libnfs will translate any nfs_write*() or nfs_pwrite*() to
NFS/WRITE3+UNSTABLE that allows the server to just write to
cache/memory.

IF you specify O_SYNC in the mode argument to nfds_open/nfs_open_async
then libnfs will flag this handle as sync and any calls to
nfs_write/nfs_pwrite will translate to NFS/WRITE3+FILE_SYNC

Calls to nfs_fsync is translated to NFS/COMMIT3



Of course, as for normal file I/O this is useful but not great since
you can only control the sync vs async per open filehandle.
Libnfs does also allow you to send raw rpc commands to the server and
using this API you can control the sync behaviour for individual
writes.
This means you coould do something like
* emulate SCSI to the guest.
* if guest sends SCSI/WRITE* without any FUA bits set, then for that
I/O you send a NFS3/WRITE+UNSTABLE
* if guest sends SCSI/WRITE* with FUA bits set, then for that I/O you
send NFS3/WRITE+FILE_SYNC
and then the guest kernel can control for each individual write
whether it is sync or not.

But that is probably something that can wait until later and don't
need to be part of the initial patch?
If peter wants to do this in the future I can create a small writeup
on how to mixin the two different APIs using the same context.