asking help for performance issue

[tiger2000]

Dear all,

after using zimbra 5.0.13 (FOSS) for 3 years in this > 40000 accounts environment, everything looks great except the performance.
we don't have such kind of performance-tuning experts in our team thus i would like to seek assistance here in this forum.

we have read the Wiki article : Performance Tuning Guidelines for Large Deployments - Zimbra :: Wiki several times, however, this article doesn't help a lot when we saw the performance graph/info, no idea where to start. The daily stat chart also doesn't have some notes to explain where's the bottleneck.

we have collected several messages according to :
King0770-Notes - Zimbra :: Wiki

1. the basic system info
2. zmmailboxd.out (collected during *high* load)
3. pstack <jetty> result

other hardware info:
Dell PowerEdge 1950 (2 Quad-core CPU)
8GB RAM
Using HDS 2100 SAN Storage (but with SATA hard drives)

the performance is even worse especially when someone send email to distribution list (such as: ALL@our.domain). during this time, user's webmail is almost hanging, every click will consume about several seconds to display the email contents.

i fully understand this is tough but any message or hint to us will be highly appreciated.

we have plans to do the following changes in the near future:
1. upgrade to latest zcs 6.x
2. during upgrade, we will upgrade the RAM to 16GB and change CentOS 4.8 to CentOS 5.x (64-bit)
3. change to multi-server environment, such as 15000-user in first server and 20000-user in another server. (if it's still same after making adjustment)

Thanks.