Hi Chuck,
Thanks for the reply and confirming the understanding.
Just want to understand any particular reason for not maintaining
alignment for the case other than NFS/RDMA?
Due to this any file system below NFS needs to handle this or suffer
partial write.
Thanks.
Rahul.
On Mon, Jun 4, 2018 at 10:45 PM, Chuck Lever <chuck.lever@xxxxxxxxxx> wrote:
>
>
>> On Jun 4, 2018, at 12:27 PM, Rahul Deshmukh <rahul.deshmukh@xxxxxxxxx> wrote:
>>
>> Hello
>>
>> I was just trying NFS + Lustre i.e. NFS running on Lustre, during this
>> experiment it is observed that the write requests that we get is not page
>> aligned even if the application is sending it correctly. Mostly it is the
>> first and last page which is not aligned.
>>
>> After digging more into code it seems it is because of following code :
>>
>> static int fill_in_write_vector(struct kvec *vec, struct nfsd4_write *write)
>> {
>> int i = 1;
>> int buflen = write->wr_buflen;
>>
>> vec[0].iov_base = write->wr_head.iov_base;
>> vec[0].iov_len = min_t(int, buflen, write->wr_head.iov_len); <======
>> buflen -= vec[0].iov_len;
>>
>> while (buflen) {
>> vec[i].iov_base = page_address(write->wr_pagelist[i - 1]);
>> vec[i].iov_len = min_t(int, PAGE_SIZE, buflen);
>> buflen -= vec[i].iov_len;
>> i++;
>> }
>> return i;
>> }
>>
>> nfsd4_write()
>> {
>> :
>> nvecs = fill_in_write_vector(rqstp->rq_vec, write);
>> :
>> }
>>
>> i.e. 0th vector is filled with min of buflen or wr_head and rest differently
>>
>> Because of this, first and last page is not aligned.
>>
>> The question here is, why 0th vector is separatly filled with
>> different size (as it
>> seems it is causing page un-alinged iovec) ? Or am I missing any
>> thing at my end
>> because of un-alignment is seen ?
>
> The TCP transport fills the sink buffer from page 0 forward, contiguously.
> The first page of that buffer contains the RPC and NFS header information,
> then the first part of the NFS WRITE payload.
>
> The vector is built so that the 0th element points into the first page
> right where the payload starts. Then it goes to the next page of the
> buffer and starts at byte zero, and so on.
>
> NFS/RDMA can transport a payload while retaining its alignment.
>
>
> --
> Chuck Lever
>
>
>
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