[Lustre-devel] Query to understand the Lustre request/reply message

[Vilobh Meshram]

Hi Alexey/Nicholas/All,

I had a look at the 2.0 side of the code and seems like there have been some
significant modifications at the MDS side e.g. seems like the request
processing part at MDS has been redefined.

I need to stick to 1.8.1.1 since my most of the modifications are at the MDS
side and the project also demands the same.So I will need to play around
with the low-level message packing and unpacking stuff which is pretty
complicated from my previous experience.

Here is my understanding of the way the request are processed .Please
correct me if I am wrong.
1) Seems like in Lustre code base for each RPC you have a static structure
"size" and count defined which defines the way the message would be layed
out (after doing all the rounding off operations etc) i.e the offset at
which the buffers will be packed and so on.
2) At MDS side the swabber() function + some associated functions extract
the buffer information.

*What I need is :-*

1) Is it possible that without writing a new RPC in Lustre 1.8.1.1 I can
append some string such as "Hello" to the exsisting message sent by the
Client (with the buffer size set at client side by the count,size fields).I
tried modifying the "size" of the request for one of the RPC in-built in
Lustre

         __u32 size[2] = { [MSG_PTLRPC_BODY_OFF] = sizeof(struct
ptlrpc_body),
                                    [DLM_LOCKREQ_OFF]     = sizeof(struct
ldlm_request) };

---->>
     __u32 size[3] = { [MSG_PTLRPC_BODY_OFF] = sizeof(struct ptlrpc_body),
                                  [DLM_LOCKREQ_OFF]     = sizeof(struct
ldlm_request) ,
                                  //how to add "char *str=Hello" ofcourse we
will have sizeof(str) but how to choose the MACRO like DLM_LOCKREQ_OFF bcz
for a specific kind of RPC there are limited number of such MACROS
                                            };

The thing I want to know is how can I send a buffer from the client side by
modifying the static structure "size" mentioned above.What all main places
do I need to consider to make this work.

If the above step i.e appending a buffer in the "size" array is not possible
then I can move to write a new RPC.


Thanks,
Vilobh
*Graduate Research Associate*
*Department of Computer Science*
*The Ohio State University Columbus Ohio*


On Wed, Oct 13, 2010 at 3:51 AM, Alexey Lyashkov <
alexey.lyashkov at clusterstor.com> wrote:

>
> On Oct 13, 2010, at 10:43, Nicolas Williams wrote:
>
> > On Wed, Oct 13, 2010 at 10:27:55AM +0300, Alexey Lyashkov wrote:
> >> eh.. Nicolas,
> >>
> >> Format for messages which want to reconstructed after resend and don't
> >> want recontructed - is different.
> >>
> >> As quick example it is OPEN request (via MDS_REINT command), that type
> >> message need a have extra buffer to store LOV EA, which to be send to
> >> MDS in replay case (with additional flag in header).  (client have a
> >> copy data from a mds reply after ptlrpc finish processing request).
> >> That is why i say about "Reconstruct/replay case"
> >
> > Sure, but this buffer needs to be declared a priori.  If you won't know
> > whether you'll need a buffer until later, that's OK: you declare it
> > anyways and you set its size to zero if you don't need it.
> >
> > You can't change a capsule's format to add buffers; you can only set the
> > size of unnecessary buffers to zero.
> but you can reassing format for a message :)
> if you look to MDT code, you can see - for MDS_REINT you have first format
> for operation inside REINT lustre use second format
>
> static int mdt_reint(struct mdt_thread_info *info)
> {
>        long opc;
>        int  rc;
>
>        static const struct req_format *reint_fmts[REINT_MAX] = {
>                [REINT_SETATTR]  = &RQF_MDS_REINT_SETATTR,
>                [REINT_CREATE]   = &RQF_MDS_REINT_CREATE,
>                [REINT_LINK]     = &RQF_MDS_REINT_LINK,
>                [REINT_UNLINK]   = &RQF_MDS_REINT_UNLINK,
>                [REINT_RENAME]   = &RQF_MDS_REINT_RENAME,
>                [REINT_OPEN]     = &RQF_MDS_REINT_OPEN,
>                [REINT_SETXATTR] = &RQF_MDS_REINT_SETXATTR
>        };
>
> ..
>
> understand me ?
>
>
> --------------------------------------
> Alexey Lyashkov
> alexey.lyashkov at clusterstor.com
>
>
>
>
>
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