Re: [Scst-devel] Integration of SCST in the mainstream Linux kernel

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FUJITA Tomonori wrote:
On Tue, 29 Jan 2008 13:31:52 -0800
Roland Dreier <rdreier@xxxxxxxxx> 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


Both Robin (iser/stgt) and Bart (scst/srp) using ramfs

Robin's numbers come from DDR IB HCAs

Bart's numbers come from SDR IB HCAs:
Results with /dev/ram0 configured as backing store on the target (buffered I/O): Read Write Read Write performance performance performance performance (0.5K, MB/s) (0.5K, MB/s) (1 MB, MB/s) (1 MB, MB/s) STGT + iSER 250 48 349 781 SCST + SRP 411 66 659 746

Results with /dev/ram0 configured as backing store on the target (direct I/O): Read Write Read Write performance performance performance performance (0.5K, MB/s) (0.5K, MB/s) (1 MB, MB/s) (1 MB, MB/s) STGT + iSER 7.9 9.8 589 647 SCST + SRP 12.3 9.7 811 794

http://www.mail-archive.com/linux-scsi@xxxxxxxxxxxxxxx/msg13514.html

Here are my numbers with DDR IB HCAs, SCST/SRP 5G /dev/ram0 block_io mode, RHEL5 2.6.18-8.el5

direct i/o dd
   write/read  1100/895 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

buffered i/o dd
   write/read  950/770 MB/s
     dd if=/dev/zero of=/dev/sdc bs=1M count=5000
     dd of=/dev/null if=/dev/sdc bs=1M count=5000

So when using DDR IB hcas:

              stgt/iser   scst/srp
direct I/O     800/751     1100/895
buffered I/O   1109/350    950/770


-vu
http://www.mail-archive.com/linux-scsi@xxxxxxxxxxxxxxx/msg13502.html

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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