Re: Write delegation

[Yudong Gao <stgyd@umich.edu>]

On Wed, Aug 4, 2010 at 5:52 PM, Trond Myklebust
<trond.myklebust@fys.uio.no> wrote:
> On Wed, 2010-08-04 at 14:40 -0700, Yudong Gao wrote:
>> I see. But according to the spec, the write delegation is supposed to
>> break the close-to-open consistency so that the client can avoid
>> flushing dirty page to server when closing the file. Is this part
>> implemented? I can not find it in the source code...
>
> No. We didn't do that. The main reason for continuing to flush the
> writes on close is to avoid slow delegation recalls.
>
This makes sense! Thanks for clarifying!

>> Another question is how the nfs file write interacts with fscache? I
>> was not able to find the code that update the page copy in fscache
>> when the page in memory is modified.
>
> AFAIK fscache is read-only at this point. I think David has plans to
> make it read/write, but I don't know whether he will ever do so for NFS.
>

I think fscache itself support writing. We can find the
fscache_write_page() definition in both the api document and the
source code. I check the source code and it seems fully implemented.

But for the integration with NFS, it is confusing. The
fscache_write_page() is called only in one place:
nfs_readpage_release(). So a NFS page is only written to fscache after
it is read from the server in nfs_readpage. So my question is, if a
page is locally modified, when its data is propagated to fscache? I
check the nfs_write_begin() and nfs_write_end() but cannot find any
relative implementation.

Thanks!

best,

Yudong

> Trond
>
>> Thanks a lot!
>>
>> best,
>>
>> Yudong
>>
>> On Wed, Aug 4, 2010 at 2:22 PM, Trond Myklebust
>> <trond.myklebust@fys.uio.no> wrote:
>> > On Wed, 2010-08-04 at 14:11 -0700, Yudong Gao wrote:
>> >> Thanks for the reply, Andy!
>> >>
>> >> So the write delegation can only reduce the unnecessary open/close and
>> >> lock/locku. But if a client modify the same page for multiple times,
>> >> e.g. editing the file in a editor, is there any optimization to
>> >> prevent the client from sending the half-updated pages? Ideally only
>> >> the final update need to be put on the wire.
>> >>
>> >
>> > Yes. However that optimisation is not linked to whether or not we hold a
>> > write delegation. The NFS client assumes close-to-open cache
>> > consistency, and so will cache writes until either the VM tries to
>> > reclaim memory by writing out dirty page, or the application calls one
>> > of fcntl(F_UNLCK), fsync() or close().
>> >
>> > Trond
>> >
>> >> Thanks!
>> >>
>> >> best,
>> >>
>> >> Yudong
>> >>
>> >> On Wed, Aug 4, 2010 at 1:59 PM, Andy Adamson <andros@netapp.com> wrote:
>> >> >
>> >> > On Aug 4, 2010, at 3:21 PM, Yudong Gao wrote:
>> >> >
>> >> >> Hi,
>> >> >>
>> >> >> I am not able to find the implementation of directory delegation,
>> >> >> either. Similarly, the callback functions CB_NOTIFY is not
>> >> >> implemented, either. I find that in preprocss_nfs41_op() in
>> >> >> callback_xdr.c, whenever a CB_NOTIFY is encountered, an
>> >> >> NFS4ERR_NOTSUPP is returned directly.
>> >> >
>> >> > CB_NOTIFY is not currently supported on the Linux NFS client.
>> >> >
>> >> >>
>> >> >> Am I missing something? Or they are just not supported in the current version?
>> >> >>
>> >> >> Thanks a lot!
>> >> >>
>> >> >> best,
>> >> >>
>> >> >> Yudong
>> >> >>
>> >> >> On Mon, Aug 2, 2010 at 5:17 PM, Yudong Gao <stgyd@umich.edu> wrote:
>> >> >>> Hi,
>> >> >>>
>> >> >>> I am looking at the delegation implementation in the source code of
>> >> >>> NFS 4.1 in kernel 2.6.32.15. I can find the code for read delegation,
>> >> >>> which is working and can serve the read requests locally. But I can
>> >> >>> never find the code about write delegation, which is supposed to cache
>> >> >>> the write update locally. I try to look at the functions including
>> >> >>> nfs_writepage(s), nfs_file_flush() but none of them checks or uses the
>> >> >>> write delegation.
>> >> >>>
>> >> >>> Is write delegation currently implemented in NFS 4.1?
>> >> >
>> >> > Write delegation is supported but I don't think write behavior changes - writes are still cached and flushed as without a write delegation. The write delegation does prevent open/close and lock/locku from being put on the wire.
>> >> >
>> >> > -->Andy
>> >> >
>> >> >>>
>> >> >>> Thanks!
>> >> >>>
>> >> >>> best,
>> >> >>>
>> >> >>> Yudong
>> >> >>>
>> >> >> --
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>> >
>> >
>> >
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