Technical > Computer Hardware Help > ATA/ACHI/RAID confusion

ATA/ACHI/RAID confusion

ATA mode = controller operates in ATA-7 mode (a.k.a. UDMA 6 mode), usually, via compatibility shims[1]. You will get good read/write speeds using this mode, but will lose out on reliable hot-swapping and some features such as NCQ (latter is due to OS driver manufacturers not implementing it as part of their stock ATA drives).

AHCI mode = enables use of the AHCI option ROM and AHCI features. This includes hot-swapping, NCQ, and a general more robust + slimmer communication layer between the disk, the controller, and your OS (i.e. less overhead). Windows Vista and Windows 7 natively have AHCI support (particularly for Intel controllers), while Windows XP lacks it (Intel does provide drivers however).

RAID mode = enables use of AHCI option ROM and AHCI features, but also makes use of RAID functionality if you want it. Most controllers will let you enable RAID mode and then choose to leave a disk attached as JBOD or "single", meaning not part of an actual RAID array. However, you still need to make sure your driver set for your OS supports this mode.

I should note that the above is BIOS-level RAID. There is very little hardware offloading support for I/O tasks in this mode; 95% of all the I/O is still CPU-bound (driver-level, not silicon-level). As such, it falls under the "software RAID" category. All the controller does in this mode is hide metadata

One downside to using RAID mode with SSDs, even if the SSD is JBOD/single and not part of an array, is that you lose native TRIM capability.

Another downside to using RAID mode (with SSDs or MHDDs) that more often than not it's difficult to get SMART stats out of them for monitoring. For folks like me, this makes it very difficult to provide a prognosis for individuals having issues which they think are disk-related but could be elsewhere. Some tools like smartmontools have begun to support disks behind Intel chipsets in RAID mode, but it's still finicky. Most other SMART monitoring software out there lacks the accuracy of smartmontools.

Otherwise see the AHCI link I provided, section "Operation modes" for some additional details.

As far as your drivers go, re: "Intel(R) Desktop/Workstation/Server Express Chipset SATA AHCI Controller" -- this would provide compatibility for AHCI mode, but your BIOS does not offer that as a standalone mode. All it offers is ATA and RAID. Please direct all complaints to your motherboard manufacturer; they should be able to provide all 3 (ATA, AHCI, and RAID).

BTW, the data you provided for your controller is not accurate enough. There are multiple sub-models (maybe 8 or 9, if not more by now) of that southbridge chipset, and each one offers different capabilities, port capacity, and other things. If you want me to link you to the PDF that mentions them all, I can do so, but you will probably be very overwhelmed with low-level technical details all the way down to the electrical and silicon characteristics.

Hope this helps.

P.S. -- Always use the Intel RST drivers, and not the older Intel MatrixRAID drivers. The term MatrixRAID sticks (such as on Wikipedia) because it was the term Intel used originally; they keep renaming it, messing about for marketing reasons.

[1]: This is not the same thing as "IDE emulation mode" or "PATA emulation mode", also sometimes just called "IDE", which some other controllers offer -- this mode truly makes your drive appear and operate like a classic IDE ATA/133 drive, literally Primary vs. Secondary Channel, Master vs. Slave, etc.. You definitely do not want that, as it does limit your I/O speed. That mode is only intended to be used when you're using an extremely old OS (we're talking original MS-DOS days).
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Making life hard for others since 1977.
I speak for myself and not my employer/affiliates of my employer.

According to the official ICH10 data sheet (see Section 1), the ICH10R only comes in one model: the 82801JIR.

Section 1.3 Table 1-3 states quite clearly the ICH10R supports both AHCI mode as well as RAID mode -- meaning, AHCI mode is an independent setting from RAID (but RAID uses AHCI).

So, given that piece of information, your motherboard manufacturer -- for reasons unknown -- has chosen to not add AHCI mode in their system BIOS. There's no justification for this that I can think of. You will need to contact the vendor or motherboard manufacturer. Feel free to point them to this thread for evidence. They really should offer all 3 (ATA, AHCI, and RAID).

As for TRIM -- nice to see that semi-recent RST drivers at least provide native TRIM under RAID when an SSD is JBOD / not part of an array. There is no way to guarantee this functionality is working without a hardware ATA protocol analyser, so you'll just have to trust what Intel says here.

Good luck, and hope this answers your question.
--
Making life hard for others since 1977.
I speak for myself and not my employer/affiliates of my employer.

reply to Anony Mous
My understanding is there is still not TRIM support in the Intel RST regardless of what many websites say and what Intel promised. I haven't looked in a couple of months however.