WG: AW: AW: AW: WG: Re: read performance much slower than write

Hi Peter!

Hope you are well again!

Meanwhile I was playing with different settings of options on both server
and client side.

I discovered that changing options on the client side had no real effect:
options tried out additional to -b 4096:
-n 1 -n 2 -n 3 ... to 8
-m
-a (both client and server)
-e

options on the server side additional to -b 4096:
-a (also on client)
-K
-h 0 to 10000

and finally -n

The option -n made a huge difference! The read performance with raid5 using
4 devices changed from 26MB/s up to 43MB/s. Unfortunately the write
performance is almost cut into half the speed (from 65MB/S to 36MB/s which
is not bad) with this option.


Another test was also interesting to me. When I used 2 different
client-machines with the same setup using raid5 with 4 devices (2 partitions
exported on each server side) I got always half the read speed compared to
using 1 client-machine.
I expected that using 2 clients with the same setup would result in doubling
the performance (43MB/s twice), since the limit is not the network card.

test scenario:
- 1 enbd-client using raid5 with 4 enbd-devices without the option -n on the
4 servers:
  26MB/s measured with time dd 

- 2 enbd-clients both using raid5 with 4 enbd-devices without the option -n
on the 4 servers:
  13MB/s measured with time dd on both clients

- 1 enbd-client using raid5 with 4 enbd-devices using the option -n on the 4
servers:
  43MB/s measured with time dd

- 1 enbd-client using raid5 with 4 enbd-devices using the option -n on the 4
servers:
  27MB/s measured with time dd on first client
  15MB/s measured with time dd on second client


I do not understand the test results! The problem seems to be on the
server-side of enbd. The servers only deliver the maximum data rate of
43MB/s. But if I reduce the servers used (from 4 to 3 servers) the speed
increases which seems to be a problem on the client side.
1 backend=65MB/s
2 backends=79MB/s (65MB/s with raid1)
3 backends=51MB/s raid5
4 backends=43MB/s raid5


I am more confused than ever before but maybe you can use the test results!


best whishes for your health,


Christian