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

[Vu Pham]

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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