[DRBD-user] Re: Poor network performance on 0.7.22

Oliver Hookins oliver.hookins at anchor.com.au
Wed Jun 18 07:39:01 CEST 2008

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

Another snippet of information that might twig someone's memory... I took a
tcpdump of DRBD traffic when doing a large file write and although the MTU
is set to 9000 over the direct 1Gbps connection, both systems have their TCP
windows set to very small values, such as around 800.

During a 10 second packet capture I'm also seeing 25 TCP out-of-order
segments and 1427 TCP Window updates, which seems to be very high. I've
already had a go at raising TCP buffers in /proc/sys/net/core and
/proc/sys/net/ipv4 but without any noticeable change in connected speed...

On Wed Jun 18, 2008 at 12:39:14 +1000, Oliver Hookins wrote:
>Anybody have any tips at all for this issue? I'm running out of ideas...
>On Thu Jun 12, 2008 at 16:04:10 +1000, Oliver Hookins wrote:
>>Hi again,
>>I've been doing a lot of testing and I'm fairly certain I've narrowed down
>>my performance issues to the network connection. Previously I was getting
>>fairly abysmal performance in even DRBD-disconnected mode but I realise now
>>this was mainly due to my test file size far exceeding the al-extents
>>I am performing dd tests (bs=1G, count=8) with syncs on the connected DRBD
>>resources and getting about 10MB/s only. The disks are 10krpm 300GB SCSI and
>>can easily get sustained speeds of 60-70MB/s when DRBD is disconnected or
>>not used. There is a direct cable between the machines giving them full
>>gigabit connectivity via their Intel 80003ES2LAN adaptors (running the e1000
>>driver version 7.3.20-k2-NAPI that is standard with RHEL4 x86_64). I have tested
>>this connection with Netpipe and get up to 940Mbps.
>>However DRBD still crawls along at 10MB/s. I have attempted to increase the
>>/proc/sys/net/core/{r,w}mem_{default,max} settings which were previously all
>>at 132KB, to 1MB for defaults and 2MB for max without any increase in
>>performance. MTU on the link is set to 9000 bytes.
>>In drbd.conf I have sndbuf-size 2M; max-buffers 8192; max-epoch-size 8192.
>>I've also played a little with the unplug watermark setting it to very low
>>and very high values without any apparent change.
>>Taking a look at a tcpdump of the traffic the only weird things I could see
>>are a lot of TCP window size change notifications and some strange packet
>>"clumping", but it's not really offering me any insights I can immediately
>>Is there anything else I could tune to solve this problem?

Oliver Hookins

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