Re: [PATCH v3 0/5] target: Fix several problems related to T10-PI support

["Martin K. Petersen" <martin.petersen@oracle.com>]

>>>>> "Sagi" == Sagi Grimberg <sagig@dev.mellanox.co.il> writes:

Sagi> The problem is that the HBA does not have the write_same
Sagi> functionality you introduce here, i.e. generate multiple same
Sagi> protection fields for a single data block.

Adding support to DIX would be problematic since it would essentially
turn a WRITE SAME into a WRITE. You'd only do one block of DMA but you'd
get N blocks going over the wire.

In target mode it is conceivable to set up a prot sgl after parsing the
CDB and let the HBA do the work. But I'm not aware of any hardware that
allows that.

Sagi> In this case, for WRITE_SAME, have the fabrics generate/verify a
Sagi> single data block (one integrity field) like they do today, and
Sagi> then the core will expand it to the correct number of sectors
Sagi> using some form of sbc_dif_expand_same()

Yeah. In a simple world you'd just keep overriding the ref tag in the
received PI tuple. But for performance reasons you'll obviously want to
do I/O in units bigger than a single block. Blindly preallocating PI to
fit the entire I/O is also be problematic, however, since a block count
of 0 unfortunately means "the whole disk".

-- 
Martin K. Petersen	Oracle Linux Engineering