Re: [RFC PATCH 22/22] nfs: add support for read_iter, write_iter

[Dave Kleikamp]

On 02/27/2012 04:08 PM, Myklebust, Trond wrote:
> On Mon, 2012-02-27 at 15:19 -0600, Dave Kleikamp wrote:
>> This patch implements the read_iter and write_iter file operations which
>> allow kernel code to initiate directIO. This allows the loop device to
>> read and write directly to the server, bypassing the page cache.
>>
>> Signed-off-by: Dave Kleikamp <dave.kleikamp@xxxxxxxxxx>
>> Cc: Zach Brown <zab@xxxxxxxxx>
>> Cc: Trond Myklebust <Trond.Myklebust@xxxxxxxxxx>
>> Cc: linux-nfs@xxxxxxxxxxxxxxx
> 
> Performance is going to be absolutely terrible for O_DIRECT bvecs if you
> send just one page per RPC call. We are working on merging the O_DIRECT
> and page cache code in order to give O_DIRECT the ability to coalesce
> requests and do pNFS, and I'm hoping that code will be available soon.
> 
> In the meantime, wouldn't it be possible to add basic coalescing to
> nfs_direct_read_schedule_bvec/nfs_direct_write_schedule_bvec more or
> less in the same way that we do for multi-page iovec segments?
> i.e. if the next bvec is contiguous with the previous, and the resulting
> RPC read length < rsize / write length < wsize, then add it to the same
> RPC call.

I basically followed the example of what the block layer was doing, but
coalescing makes more sense for nfs. I'll rework it to do that.

Thanks,
Shaggy
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