[stgt] [PATCH 14/15] Fix iSCSI Data-Out solicitation

Pete Wyckoff pw at padd.com
Tue Jun 16 12:24:45 CEST 2009

arne.redlich at googlemail.com wrote on Tue, 09 Jun 2009 18:22 +0200:
> Currently the iSCSI driver only solicits MaxXmitDSL(!) bytes with each R2T,
> although it could solicit MaxBurstLength bytes.

This fix makes sense.  R2T should ask for a whole burst.

> This was introduced with the iSER driver and a RDMA_TRANSFER_SIZE constant to
> limit the size of RDMA transfers. However, MaxBurstLength can and should be
> used for that.

But RDMA should not necessarily use MAX_BURST for its data transfer
sizes.  This size is completely controlled by the target and not
even visible to the initiator.  The initiator should not be involved
in the negotiation of the size.

There was a longish discussion of this value in the archive
somewhere around the time iser was getting added to tgt.

Unfortunately I don't have gear to test this anymore.  One risk is
that iser initiators will negotiate a small MaxBurstLength and get
bad performance.  Another is they negotiate something bigger than
the mempool_size constant and break the iser code, if someone tries
to set target's MaxBurstLength bigger hoping for faster transfers.

If you want to be safe, you can leave data_inout_max_length alone.
Just change iscsi_r2t_build to "if (rdma)" to select max_burst or

If I know Fujita code style, he won't like the "if (rdma)" you added
here or the one I'm suggesting.  In which case you might add a
second conn parameter, data_r2t_max_length, that gets set appropriately.
And rename data_inout_max_length to remove the "in" part now that
you've figured out the R2T situation.

		-- Pete
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