Hello,

Bart Van Assche, on 04/12/2009 10:09 PM wrote:
> Hello,
> 
> While running iSCSI performance tests I noticed that the performance
> for certain block sizes deviated significantly (more than ten times)
> from the performance for other block sizes, both larger and smaller.
> This surprised me.
> 
> The test I ran was as follows:
> * A file of 1 GB residing on a tmpfs filesystem was exported via iSCSI
> target software. The test has been repeated with both SCST and STGT.
> * On the initiator system open-iscsi version 2.0.870 was used for
> performing reads and writes with dd via direct I/O. Read-ahead was set
> to zero.
> * Both systems were running kernel 2.6.29.1 in run level 3 (no X
> server) and the 1 GbE interfaces in the two systems were connected via
> a crossed cable. The MTU has been left to its default value, 1500
> bytes. Netperf reported a throughput of 600 Mbit/s = 75 MB/s for the
> TCP/IP stream test on this setup.
> * 128 MB of data has been transferred during each test.
> * Each measurement has been repeated three times.
> * All caches were flushed before each test.
> * The ratio of standard deviation to average was 2% or lower for all
> measurements.
> * The measurement result are as follows (transfer speeds in MB/s):
> 
> Block   SCST    STGT    SCST    STGT
>  size  writing writing reading reading
> ------ ------- ------- ------- -------
>  64 MB  71.7    63.3    62.1    58.4
>  32 MB  71.9    63.4    61.7    58.1
>  16 MB  72.4    63.0    61.7    57.1
>   8 MB  72.7    63.3    61.7    56.9
>   4 MB  72.9    63.5    61.3    57.0
>   2 MB  72.8    59.5    60.3    56.9
>   1 MB  72.1    38.7    59.4    56.0
> 512 KB  67.3    21.4    58.0    54.4
> 256 KB  67.4    22.8    55.5    53.4
> 128 KB  60.9    22.6    53.3    51.7
>  64 KB  53.2    22.2    53.0    45.7
>  32 KB  48.9    21.6    40.0    40.0
>  16 KB  40.0    20.8     0.6     1.3
>   8 KB  20.0    19.9    19.9    20.0
>   4 KB   0.6     1.6    18.9    10.3
> 
> All results look normal to me, except the write throughput for a block
> size of 4 KB and the read throughput for a block size of 16 KB.
> 
> Regarding CPU load: during the 4 KB write test, the CPU load was 0.9
> on the initiator system and 0.1 on the target.

I would suggest you to make sure you have any hardware coalescing 
disabled on both hosts. You can do that by using "ethtool -c"

> Has anyone observed similar behavior before ?
> 
> Bart.
> 
> > 
> 


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