[stgt] iSer performance tuning

[Chris Worley]

Running OFED 1.4, and discovering targets with "iscsi_discovery <IP>
-t iser -f -l" seems to perform great w/ one target (>1GB/s), but
"iostat" on the target doesn't seem to show as much activity as the
initiator claims, and testing more than one target drive
simultaneously, the tgtd hangs.

Using the default transport, "-t tcp" seems reliable with multiple
targets simultaneously active, but the performance is slow (reported
as <400MB/s on the initiators, and the targets show only 10% that
performance... even though these devices together easily scale to many
GB/s).

The last time I gave iSer a try I noticed that the block sizes on the
target were very small (compared to the user apps block sizes on the
initiator).  I'm seeing this again; so I tried the fix I found before
(add O_DIRECT to the flags in the backed_file_open call in
bs_rdwr_open), and that reduced the cached memory usage, but
performance went down.

I've got disks and IB that should be getting many GB/s performance,
and I only see a fraction of that.

Any hints on how to configure this to get decent performance?

Thanks,

Chris
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