I have Dell 6800 server with PERC4 Ultra320 SCSI RAID controller with 256 MB cache.
I did some tests and I have found, that this cache is probably divided to cca 240 MB read cache and only 7 MB write cache. I have used xdd test tool to bypass filesystem cache and I was able to read 240 MB files with very high transfer rate while with 245 MB files transfer rate dropped to about 30%. Write test showed this transfer rate drop when file size changed from 7 MB to 8 MB.
I'd like to increase write portion of the cache. I'm using command line utility named megarc to set controller parameters and I can't see any parameter, that could affect cache division.
Does anybody know, whether it is possible to change the size of write cache on PERC4 controller ?