[iSCSI] Trying to reach max disk throughput

[Edward Tomasz Napierała]

On 0810T1154, Ben RUBSON wrote:
> Hello,
> 
> I'm facing something strange with iSCSI, I can't manage to reach the expected disk throughput using one (read or write) thread.

[..]

> ### Initiator : iscsi disk throughput :
> 
> ## dd if=/dev/da8 of=/dev/null bs=$((128*1024)) count=81920
> 10737418240 bytes transferred in 34.731815 secs (309152234 bytes/sec) - 295MB/s
> 
> With 2 parallel dd jobs : 345MB/s
> With 4 parallel dd jobs : 502MB/s
> 
> 
> 
> ### Questions :
> 
> Why such a difference ?
> Where are the 167MB/s (462-295) lost ?

Network delays, I suppose.  A single dd(1) would spend some time waiting
for the data to get pushed over the network - due to delays (lag), not
bandwidth.  Having multiple ones makes it possible to compensate, by having
multiple outstanding IO operations.