Re: Why FCP doesn't need RDMA? It has a better way.

[Matt Wakeley <matt_wakeley@agilent.com>]

Yahoo! for once I agree with David!
Nice summary!

-Matt

Black_David@emc.com wrote:

> With my WG co-chair hat off:
>
> > Actually, RDMA is not needed in FCP because all protocol chips
> > implemented perform a real peer-to-peer DMA straight to the
> > data areas specified by the user's interaction with the operating
> > systems allocation algorithms.  The combination of the FCP/SCSI
> > pointer structure, task tagging, and the FC relative offset perform the
> > function you would otherwise have to use RDMA to accomplish.
>
> And this illuminates the design tradeoff that may motivate RDMA.  If
> one only wants to accelerate one protocol (SCSI/FCP in the above
> example) then having hardware understand its headers and doing
> the DMA on that basis is a fairly obvious way to go - HBAs for both
> parallel SCSI and Fibre Channel (SCSI/FCP) do this.  RDMA may be
> interesting if there are multiple protocols involved, and there are
> engineering
> concerns that lead to not wanting to implement hardware support for
> all of them.
>
> From an iSCSI viewpoint, I don't see iSCSI by itself as being sufficient
> to motivate a protocol-independent RDMA - an iSCSI HBA could understand
> the iSCSI headers and interact with DMA in the same fashion as existing
> HBAs.  The task before those interested in RDMA is to identify a set
> of protocols for which a common RDMA mechanism makes sense from
> an engineering standpoint.  I tend to agree with the previous emails
> that iSCSI could make optional use of a common RDMA mechanism
> if available, but must not REQUIRE its use.
>
> --David
>
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> David L. Black, Senior Technologist
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