postgres is extrem slow

[dea]

I'm really surprised that enabling WRITE hd cache, your bugger READS are improved.
Also, if is really HD (and not controller's) write cache you have enabled, it's not protected by raid controller bbu so you will loose data in case of black out

I think the problem is in the terminology used by HP.
Controller P400 does not come natively with the battery, it is an optional.
If the battery is not present the cache (on controller) is disabled by default in writing, if it is present can be activated.
Obviously enable write caching is dangerous if the system is not on UPS, but I am sure that the cache named in the bios is not the HDD cache but identifies the on board cache behavior.

IMHO

Luca