stgt - Poor Performance - seeking suggestions

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With GREAT Hope once again i am trying to get a reply from the experts... (with few more test results in other info section)


Techies,

I am a newbie to iscsi and just 
started to test different scenarios for one of our requirement. I had 
different testing on CentOS 6, but in both i didnt get the expected 
performance. In the first test it was just 40 MB/s and in the second it 
was just 50 MB/s. The details are as follows;

Test 1: 


Initiator:

OS: CentOS 6 64 bit with tgt
initiator package: iscsi-initiator-utils-6.2.0.872-10.el6.x86_64

Server: Gateway GR 180 F1
CPU: Intel(R) Xeon(R) CPU           E5506  @ 2.13GHz ( Quad Core )

RAM: 6 GB
Disks: WD 1 TB 7200 RPM 
NIC: Gigabit



Target: 

OS: CentOS 6 64 bit with tgt
target package: scsi-target-utils-1.0.4-3.el6_0.1.x86_64

Server: Gateway GR 180 F1
CPU: Intel(R) Xeon(R) CPU           E5506  @ 2.13GHz ( Quad Core )

RAM: 6 GB
Disks: WD 1 TB 7200 RPM 
NIC: Gigabit


other infos:

a. from target, 6 disks is exported through the tgt iscsi (raw disks, i mean there is no any filesystem) as direct store.

b. from initiator 6 disks is connected by iscsiadm
c. in initiator these 6 disks is configured with ZFS as a raidz (raid 5) volume
d. ran the "iozone" and "dd" tests almost for 10 times. in both cases i 
got
 the performance in between 37 MB/s to 44 MB/s (i didnt find any diff in
 both commands, sometimes it goes as low to 37 MB/s and the maximum 
to 44 MB/s)
e. while monitoring the network speed (using iftop), i can see the transfer speed is almost steady around 300 - 350 Mbps


network speed related info:
a. i tested with "iperf" and "ttcp" and got a steady speed of 920 Mbps.

Test 2:

Same Target and Initiator h/w and software configuration 


other infos: 

a.
 in target i configured a raid 5 device (/dev/md0) with 3 disks and then
 exported /dev/md0 as backing-store through tgt iscsi (without any 
filesystem)

b. from initiator /dev/md0 (riad disk @ target) disk is connected by iscsiadm
c. in initiator, i created a small partition of 25 GB in the RAID5 
device and then formatted with ext3 and then mounted to a directory

d. ran the "iozone" and "dd" tests almost for 10 times. in both 
cases i got the performance in between 48 MB/s to 52 MB/s (i didnt find 
any diff in both commands, sometimes it goes as low to 48 MB/s and the 
maximum to 52 MB/s)
e. while monitoring the network speed (using iftop), i can see the transfer speed varies highly in between 300 Mbps to 700 Mbps.
f. while tested by connecting to the loopback interface (127.0.0.1), i got a small hike in performance which is 60 MB/s
g. when i tried just with 3 disks in the zfs pool (instead of 6 disk set in zfs raidz), the performance again slower, which is only 35 MB/s
h. when i transfered to a local zfs pool (no iscsi), the performance was really good, which was up to 170 MB/s.
i. when i checked with 3 disks set + zfs raidz with scst iscsi (took almost 3 days to configure scst), got 50% hike in the speed and reached up to 52.2 MB/s (when compared to 35 MB/s in stgt with 3 disk set), which is still low performance in a Gigabit environment (which i think so).


My questions are;

1. What is the regular speed of stgt iscsi in such and hardware and software environment.
2. why i am getting this much slow performance
3. how can i increase my iscsi performance.
4. Which is the best distro and its version, in which tgt iscsi will 
perform very well (i am going to test with Debian and Ubuntu too)
5. As i am trying to build a storage solution for our production 
environment, is this way of proceeding is good. i mean the combination 
of centos+tgt iscsi is a dependable solution?
6. Do anybody had any 
advices/suggestions to make our effort succesful as a home made storage 
solution for our production environment.

Expecting all of your kind support and co-operation
Thanks in Advance :)
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