Why AHCI when you can RAID?

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Why AHCI when you can RAID?

Postposted on Sun May 13, 2012 1:10 pm

In modern motherboards, are there any reasons to run in AHCI mode over RAID mode? My current understanding is that RAID mode is a superset of AHCI mode but with the added benefit of creating RAID volumes. So why AHCI when you can RAID? Even for single HDD configurations, if you have a Z68 or newer Intel chipset then running it in RAID mode supposedly doesn't hurt anything and opens up the option of simply throwing in an SSD for SRT or easily adding a RAID volume for redundant storage or whathaveyou.
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Re: Why AHCI when you can RAID?

Postposted on Sun May 13, 2012 1:33 pm

As far as I know, you are correct. There is little or no difference whether running a single drive in AHCI or RAID mode on modern Intel chipsets. I think, however, that unless you are running a RAID array, AHCI mode is still preferable as it probably cuts down on startup time. In RAID mode, at POST, the motherboard will load the BIOS level RAID driver and you will get that extra screen showing status of current raid arrays. This takes a couple of seconds and obviously isn't necessary if you're not running RAID.
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Re: Why AHCI when you can RAID?

Postposted on Sun May 13, 2012 1:55 pm

I have personally run into this situation where I had an Intel-based raid-enabled chip (an ICH-10R if memory serves) that was in AHCI mode, but I wanted to add a raid array.

To do so, I had to manually extract the Intel driver, override Windows' choice of driver, reboot and change to RAID in the bios. Done it a couple times, and always say a little prayer before exiting the bios. :D

The only real reason I can think of besides the longer boot time is that a lot of RAID configurations used to (still do?) require that you download a RAID driver for use during the Windows install.

Good point though, if you have any inkling that you might one day use RAID, may as well save yourself the hassle.
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Re: Why AHCI when you can RAID?

Postposted on Sun May 13, 2012 1:56 pm

There are disadvantages. TRIM does not currently function with the SATA controller in RAID mode, though this may be fixed at some point in the (distant) future ... whenever Intel's 11.5 RST drivers are officially released. I currently can't read SMART information from either of my drives, though I can't remember if that's because I have SRT caching enabled or if that's due to the controller being in RAID mode.
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Re: Why AHCI when you can RAID?

Postposted on Sun May 13, 2012 4:02 pm

Jason181 wrote:The only real reason I can think of besides the longer boot time is that a lot of RAID configurations used to (still do?) require that you download a RAID driver for use during the Windows install.


Yes some recent platforms still require this. The X79 platform (Sandy Bridge-E), which just came out a half-year ago or so, requires you to install an extra RAID driver off of USB or diskette, etc, during Win7 install. You'd think Intel and MS would have figured out a way to avoid this..

TwistedKestrel wrote:There are disadvantages. TRIM does not currently function with the SATA controller in RAID mode.


Really? Unless the bios is set to AHCI, TRIM will not happen? I thought TRIM would work with RAID selected in the BIOS (but obviously not with disk comprising an actually RAID array).
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Re: Why AHCI when you can RAID?

Postposted on Sun May 13, 2012 11:57 pm

cynan wrote:
TwistedKestrel wrote:There are disadvantages. TRIM does not currently function with the SATA controller in RAID mode.


Really? Unless the bios is set to AHCI, TRIM will not happen? I thought TRIM would work with RAID selected in the BIOS (but obviously not with disk comprising an actually RAID array).


Huh ... Apparently you are correct, according to Google.
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Re: Why AHCI when you can RAID?

Postposted on Mon May 14, 2012 5:50 am

If the type of RAID array you want is supported natively by your OS's software RAID capabilities you may be better off leaving the drives in AHCI mode and using the OS's RAID instead. This allows the array to be easily moved intact to a different motherboard/controller if the one you're using fails.
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