[Stgt-devel] iSER patches, third release

Pete Wyckoff pw
Mon Dec 10 16:03:25 CET 2007

This is the third major release of support for iSCSI Extensions
for RDMA (iSER) to the existing TGT user space SCSI target.  It
uses OpenFabrics libraries and kernel drivers to act as a SCSI
target over RDMA-capable devices.  The code has been tested
against the existing Linux iSER initiator over InfiniBand cards,
but should be specification compliant and work generally.

A bit of documentation is included, and a short technical report is
available at http://www.osc.edu/~pw/papers/iser-snapi07.pdf with
slides from a presentation at
http://www.osc.edu/~pw/papers/wyckoff-iser-snapi07-talk.pdf .

The iSER patches can be downloaded from:


or browsed at:


Changes since previous series are as follows.

Tomo redid substantial parts of tgt, including data buffer handling,
and merged many of the easier patches from the previous iSER
patches.  This reduces the load from 20 patches down to 6 this time.
(Not counting two little task->data ones recently posted.)

There is not a lot of change in the remaining patches from the
previous release, just some minor alterations to accommodate core
tgt changes and to follow some suggestions from Tomo.  Here's the

1 iser docs - just doc/

2 iser task transport data - need private data in struct task,
    could merge as is, or let iscsi transport do allocation as
    with connection struct.

3 iser rounding - removed "if (conn->tp->rdma)" approach, now uses
    "conn->tp->data_padding".  Also deleted some roundup()s that
    were unnecessary in both cases.

4 iser params - new parameters for iSER straight from spec docs.  No
    ifdef on ISCSI_RDMA, we let initiator say if it wants RDMA or

5 iser iscsid changes - five little "if (conn->tp->rdma)" changes,
    all necessary due to protocol differences and different event

6 iser core - add iscsi/iscsi_rdma.c and hooks to use it

Performance has changed a bit for the worse in tgt core since the
last release, unfortunately.  Brief results from an unscientfic
study, using the same kernel (2.6.23-rc1) but different userspace
libraries (f8 vs f7).  All ramdisk.  Same tuning in all cases.

GigE: I'm seeing about 5 MB/s less for both read and write.  IPoIB:
writes are unchanged but reads go up(!) from 140 to 180 MB/s.  Must
be libibverbs.so improvements.  iSER: writes are a bit worse this
time around, by maybe 50 MB/s or so, and reads lose about 100 MB/s
for some transfer sizes.  Still 400 MB/s-ish for writes but now only
500 MB/s-ish for reads.  Hopefully none of this shows up when using
real disks.

I'll look at what causes these performance changes and try out some
more recent kernels.  But I wanted to get the patches to people so
as not to hold up any other work.  Would love to see some more
scientific numbers from people, and help figuring out where the time
is going.

		-- Pete

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