Server Fault is a question and answer site for system and network administrators. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I have two virtual servers under VMware Server 2, both running CentOS Linux. Server A is under fairly constant load of about 4, server B is under load anywhere between 1 and 4. Both are on the host and thus connected to the same network, same router, etc. I am noting that the network perfoamce on server B is atrocious compared to that of A. I created a 10 MB file on both servers and when transferring the file via FTP, SFTP or HTTP, I get over double the transfer speed when coping from A to my workstation than from B to my workstation.

I assume something in sysctl or /proc/sys/net is misconfigured on server B but am unsure of where to look...

EDIT: I compared everything in /proc/sys/net on both machines, they're almost identical. I also noticed that download times are much worse on server B. Any advice to troubleshoot possible network problems would be greatly appreciated!

EDIT2: Here are some results from iperf for server A:

[ ID] Interval       Transfer     Bandwidth
[  4]  0.0-11.2 sec    632 KBytes    463 Kbits/sec
[ ID] Interval       Transfer     Bandwidth
[  4]  0.0-10.7 sec    624 KBytes    477 Kbits/sec
[ ID] Interval       Transfer     Bandwidth
[  4]  0.0-20.2 sec  1.16 MBytes    483 Kbits/sec
[ ID] Interval       Transfer     Bandwidth
[  4]  0.0-22.2 sec  1.23 MBytes    463 Kbits/sec
[ ID] Interval       Transfer     Bandwidth
[  4]  0.0-61.1 sec  3.35 MBytes    460 Kbits/sec
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0-10.1 sec    688 KBytes    560 Kbits/sec

And from server B:

[ ID] Interval       Transfer     Bandwidth
[  5]  0.0- 9.7 sec    400 KBytes    339 Kbits/sec
[ ID] Interval       Transfer     Bandwidth
[  4]  0.0- 8.0 sec    400 KBytes    409 Kbits/sec
[ ID] Interval       Transfer     Bandwidth
[  4]  0.0- 9.9 sec    408 KBytes    338 Kbits/sec
[ ID] Interval       Transfer     Bandwidth
[  4]  0.0-20.3 sec    776 KBytes    313 Kbits/sec
[ ID] Interval       Transfer     Bandwidth
[  4]  0.0-18.7 sec    744 KBytes    325 Kbits/sec
[ ID] Interval       Transfer     Bandwidth
[  4]  0.0-53.0 sec   2.16 MBytes    343 Kbits/sec

So, there's a difference there, and I'm not sure why. But it's not nearly as large as a difference as I see when using FTP/SFTP/HTTP. Any tips on troubleshooting further would be greatly appreciated...

share|improve this question
up vote 1 down vote accepted

It turned out to be that a number of the ext3 partitions on server B had errors. Server B eventually kernel paniced (I believe due to bad RAM) and upon reboot, fsck corrected a number of problems. Since that time, the issue has gone away. This leads me to believe that the filesystem errors somehow caused the behavior I was seeing...

share|improve this answer

try to run a tcpdump, see why you're getting bad network performance. does the virtual NIC in both servers use PV?

share|improve this answer
I've used tcpdump before to watch traffic over the wire, but how would I use it to troubleshoot performance? Also, forgive my ignorance, but what is PV? – Josh Oct 9 '09 at 15:27
to troubleshoot performance you can see whether there are dropped packets and excess repeating during transmission. PV stands for paravirtualized, i.e. not completely software driven NICs – dyasny Oct 10 '09 at 14:24
@dyasny: thanks. I'm not seeing much packet loss, both servers loov very similar. Neither one is paravirtualized. – Josh Oct 12 '09 at 12:46
Is the physical NIC on the host by any chance Broadcom? – dyasny Oct 13 '09 at 10:51

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.