IO Wait very high and real IO speed is very low

[Tang Jianwei]

Hi All,

     recently I get a very weird problem from my Ubuntu Server 1204 LTS 
64bit:
     1. the write speed can reach 500MB/s when there is no other 
processes running
     2. the write speed down to around 10MB/s when there are many other 
processes and meanwhile:
             a. only very high usage from io wait
             b. sys, user and all the other cpu usages are very low
             c. the system load average can reach to 15 and even more higher

     can any one give me some advice what is wrong with my system, and 
what I should to adjust in hardware/software?

     the platform is:
     MB: Intel S3240GPLC
     CPU: I3-540
     RAM: Kingston 4G ECC
     Storage: LSI MegaRaid 9270-8i, Raid 5 with 5 drives + 1 spare, 
ForcedWB with no BBU
     Drive: WD 3T


     thank you very much.


Best Regards
Tomas