Re: [Scst-devel] ISCSI-SCST performance (with also IET and STGT data)

[Vladislav Bolkhovitin <vst@vlnb.net>]

Bart Van Assche, on 04/02/2009 12:14 AM wrote:
> On Mon, Mar 30, 2009 at 7:33 PM, Vladislav Bolkhovitin <vst@vlnb.net> wrote:
> ==================================================================
>> I. SEQUENTIAL ACCESS OVER SINGLE LINE
>>
>> 1. # dd if=/dev/sdX of=/dev/null bs=512K count=2000
>>
>>                        ISCSI-SCST      IET             STGT
>> NULLIO:                 106             105             103
>> FILEIO/CFQ:             82              57              55
>> FILEIO/deadline         69              69              67
>> BLOCKIO/CFQ             81              28              -
>> BLOCKIO/deadline        80              66              -
> 
> I have repeated some of these performance tests for iSCSI over IPoIB
> (two DDR PCIe 1.0 ConnectX HCA's connected back to back). The results
> for the buffered I/O test with a block size of 512K (initiator)
> against a file of 1GB residing on a tmpfs filesystem on the target are
> as follows:
> 
> write-test: iSCSI-SCST 243 MB/s; IET 192 MB/s.
> read-test: iSCSI-SCST 291 MB/s; IET 223 MB/s.
> 
> And for a block size of 4 KB:
> 
> write-test: iSCSI-SCST 43 MB/s; IET 42 MB/s.
> read-test: iSCSI-SCST 288 MB/s; IET 221 MB/s.

Do you have any thoughts why writes are so bad? It shouldn't be so..

> Or: depending on the test scenario, SCST transfers data between 2% and
> 30% faster via the iSCSI protocol over this network.
> 
> Something that is not relevant for this comparison, but interesting to
> know: with the SRP implementation in SCST the maximal read throughput
> is 1290 MB/s on the same setup.

This can be well explained. The limiting factor for iSCSI is that 
iSCSI/TCP processing overloads a single CPU core. You can prove that on 
vmstat output during the test. Sum of user and sys time should be about 
100/(number of CPUs) or higher. SRP has a lot more CPU effective, hence 
better has throughput.

If you try to test with 2 or more parallel IO streams, you should have 
the correspondingly increased aggregate throughput up to the moment you 
hit your memory copy bandwidth.

Thanks,
Vlad


> Bart.
> 
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