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SubjectRe: Integration of SCST in the mainstream Linux kernel
On Tue, 29 Jan 2008 13:31:52 -0800
Roland Dreier <> wrote:

> > . . STGT read SCST read . STGT read SCST read .
> > . . performance performance . performance performance .
> > . . (0.5K, MB/s) (0.5K, MB/s) . (1 MB, MB/s) (1 MB, MB/s) .
> > . iSER (8 Gb/s network) . 250 N/A . 360 N/A .
> > . SRP (8 Gb/s network) . N/A 421 . N/A 683 .
> > On the comparable figures, which only seem to be IPoIB they're showing a
> > 13-18% variance, aren't they? Which isn't an incredible difference.
> Maybe I'm all wet, but I think iSER vs. SRP should be roughly
> comparable. The exact formatting of various messages etc. is
> different but the data path using RDMA is pretty much identical. So
> the big difference between STGT iSER and SCST SRP hints at some big
> difference in the efficiency of the two implementations.

iSER has parameters to limit the maximum size of RDMA (it needs to
repeat RDMA with a poor configuration)?

Anyway, here's the results from Robin Humble:

iSER to 7G ramfs, x86_64, centos4.6, 2.6.22 kernels, git tgtd,
initiator end booted with mem=512M, target with 8G ram

direct i/o dd
write/read 800/751 MB/s
dd if=/dev/zero of=/dev/sdc bs=1M count=5000 oflag=direct
dd of=/dev/null if=/dev/sdc bs=1M count=5000 iflag=direct

I think that STGT is pretty fast with the fast backing storage.

I don't think that there is the notable perfornace difference between
kernel-space and user-space SRP (or ISER) implementations about moving
data between hosts. IB is expected to enable user-space applications
to move data between hosts quickly (if not, what can IB provide us?).

I think that the question is how fast user-space applications can do
I/Os ccompared with I/Os in kernel space. STGT is eager for the advent
of good asynchronous I/O and event notification interfances.

One more possible optimization for STGT is zero-copy data
transfer. STGT uses pre-registered buffers and move data between page
cache and thsse buffers, and then does RDMA transfer. If we implement
own caching mechanism to use pre-registered buffers directly with (AIO
and O_DIRECT), then STGT can move data without data copies.

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