I have 4 HDDs (Seagate Constellation, SATA 6Gb, 1st generation, model ST1000NM0011) organized in 2 RAID groups and 4 RAID volumes (one RAID 0 and one RAID 1 on each group) maintained by Intel's on-board RAID of my Asus H87M-Plus desktop motherboard. When I tested the disk performance with CrystalDiskMark 3.0 in my Windows 7 64-bit, I noticed that the 4KB random write performance was much better than the 4KB random read performance (on a RAID 0 volume and on a RAID 1 volume), which made me suspect that there is some kind of write caching (that I want to avoid because of the risk of data loss in a case of sudden power loss, as far I do not have an UPS and I'm not going to set one).
In fact, I found that the disk drive write caches had been enabled (I'm talking about the 64MB internal cache on each of the disk drives), I was able to check that in Intel RST management application (the latest version 184.108.40.206 from Asus support site) and in Windows Device Manager (Device Manager -> Disk "RAID XXXX Volume" -> Policy -> Write Cache). I disabled disk caches of both RAID groups and set RAID volume caching to "Off" in Intel RST management application and then I performed the benchmark test again. (Disabling disk caches on the RAID groups marked as disabled the disk caches of all the underlying disk drives). To my surprise I got the same benchmark results. I tried then several other combinations of disk cache and RAID volume cache settings, including computer restarts, but each time the benchmark results were marginally the same.
Then I inspected the drive details of my disk drives in order to see what their internal cache settings were. I used the "Intel SSD Toolbox" application for this purpose, which is able to retrieve and show internal disk drive feature settings. The problem is that no matter what cache settings I set in Intel RST and Windows Device Manager, the internal details of my disk drives remain "Word 85, Bit 5 - Volatile Write Cache Enabled = 1", i.e. the internal disk caches remain enabled. In contrast, when I disabled disk cache on my laptop drive (which is not in any RAID), its drive setting became "Word 85, Bit 5 - Volatile Write Cache Enabled = 0".
Does anybody know why I am not able to switch disk drive cache off effectively? Is it possible that Seagate Constellation drives are configured by Seagate to have their drive cache always on? If it is so, are these drives power-loss safe (e.g. do they employ some secret approach for avoiding data loss in a case of power loss, which makes Seagate able to keep the disk cache always enabled)? Is it possible that Intel RST application does not submit properly the requested cache settings to drives when the drives are in RAID but are not standalone ones?
Unfortunately, I am not able to check these disk drives as standalone drives (e.g. not in RAID) because I should destroy my RAID volumes (in order to perform a "standalone" test) and re-build and reload them again after the "standalone" test.
P.S. Even more interesting is the fact that after a Windows reboot the disk cache setting of my bootable RAID group becomes "Enabled" in Intel RST and Windows Device Manager, but the disk cache setting of the other RAID group remains "Disabled" (however, all the internal drive caches are enabled in fact).