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

[Black_David@emc.com]

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
EMC Corporation, 42 South St., Hopkinton, MA  01748
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black_david@emc.com       Mobile: +1 (978) 394-7754
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