Re: RFC: Immediate data support for SRP

Sagi Grimberg
2015-07-19T16:07:16Z

On 7/16/2015 6:25 PM, Bart Van Assche wrote:
> Hello,

Hi Bart,

I agree it would definitely help as the lack of immediate data
emphasizes the additional latency of doing rdma reads.

>
> As you probably know for write requests "immediate data" means sending
> the data in the same packet as the write command instead of sending it
> as a separate packet. This approach improves performance and reduces
> latency. Although support for immediate data has not been standardized,

Has anyone tried to get it into the standard? It would seem beneficial
to just add it. I wouldn't want this to be a Linux compatible only
feature...

> it is easy to add to the SRP initiator and target drivers.
> Implementations exist in the ib_srp-backport initiator driver and the
> SCST SRP target driver (see also
> https://github.com/bvanassche/ib_srp-backport and
> http://sourceforge.net/p/scst/svn/HEAD/tree/trunk/srpt/). These
> implementations are available since considerable time, work reliably,
> are backwards compatible and support zero-copy. Since using immediate
> data provides a measurable performance improvement I'm wondering whether
> it would be acceptable to add support for immediate data to the SRP
> drivers in the Linux kernel tree (ib_srp and ib_srpt) ?

Was this tested against any other array besides SCST and LIO? I know
people are using SRP against various arrays (Oracle ZFS, RamSan TMS,
DDN Fusion, NIMBUS, NetApp E5400A...). Their not running upstream
kernels, but they will catch up at some point...

Sagi.