A question regarding "multiple SGL"

Christoph Hellwig hch at lst.de
Wed Oct 26 23:41:15 PDT 2016

Hi Robert,

There is no feature called "Multiple SGL in one NVMe capsule".  The
NVMe over Fabrics specification allows a controller to advertise how
many SGL descriptors it supports using the MSDBD Identify field:

"Maximum SGL Data Block Descriptors (MSDBD): This field indicates the
maximum number of (Keyed) SGL Data Block descriptors that a host is allowed to
place in a capsule. A value of 0h indicates no limit."

Setting this value to 1 is perfectly valid.  Similarly a host is free
to chose any number of SGL descriptors between 0 (only for command that
don't transfer data) to the limit imposed by the controller using the
MSDBD field.

There are no plans to support a MSDBD value larger than 1 in the Linux
NVMe target, and there are no plans to ever submit commands with multiple
SGLs from the host driver either.


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