best practice to watch TCP parms of established sockets

[Harald Schmalzbauer]

Am 17.02.2010 19:56, schrieb Chuck Swiger:
> Hi--
> 
> On Feb 17, 2010, at 8:03 AM, Harald Schmalzbauer wrote:
>> while doing some ZFS tests with RELENG_8 I recognized a mysterious performace drop after an hour uptime.
>> Now my first idea is to compare MSS and windows sizes before and after the performance drop.
>> How do I best capture them? tdpcump? It's GbE linkspeed...
> 
> It seems more likely that ZFS is running into slowdowns from resource contention, memory fragmentation, etc than your network would suddenly drop out, but tcpdump -w outfile.pcap is a good method of looking....

Thanks, but fisrt tests showed that ZFS is not causing the slowdown.
I noticed that disabling window scaling (rfc1323) speeds up the transfer
rate by factor 1.5 (rsync transfers start at 75MB/s, settling down at
55MB/s, while rfc1323 enabled leads to half the windows size (8k) and
rsync transfers start with 50MB/s and go down to 38MB/s).
Now I'm going through "internet core protocols" becaus it's been a long
time ago I had such a deep look into TCP/IP and I don't have all the TCP
sequences and options in memory.

Not falsified yet, but as soon as I open a second high data rate
transfer, simultanious to the rsync, (a smb transfer of a large file for
example) the shared speed will stay at that level, even if the second
transfer has finbished. Meaning: I can rsync with 50MB/s. If I
simultaniously transfer another file via samba, I have two 25MB/s
transfers. From that moment on I can never get more that 25MB/s per
transfer until I reboot the machine.

Like mentioned eralier, I first have to refresh some networking basics,
but expect some more results soon.

Thanks,

-Harry

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