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IDE to AHCI - doubled performance?

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April 7, 2011 10:14:20 PM


Some time ago I discovered that the BIOS setting for my disks were IDE and I read that it could be changed to AHCI, which would be better when using SATA disks. However, I also read that there might not be that big difference, so it took me a while to actually change (through REGEDIT and then BIOS.)

Out of curiosity I did some tests before and after, and I was very surprised. For sequential access in lower IO sizes the performance was almost doubled! Both in terms of IOPS and throughput.

The disk is a Samsung Spinpoint F3, 1TB (with 300 MB partition in the beginning of the disk).

This is using IOMETER with 512 bytes blocks, all read and all sequential, before the change to AHCI. Around 28000 IOPS, 13 MB/s throughput and average latency at 0.14 ms.



Here is after AHCI is enabled:



What I can extract from this is that the IOPS rise from 28000 to amazing 50000, while the latency was cut in half and the MB throughput was dubbled.


Here is a test with ATTO Disk Benchmark. I found that already on IDE mode there was excellent throughput on larger IO sizes, around 145 MB/second on 16 KB (and up) IO sizes, while on 4 KB I got around 80 MB/s.



Here is finally after AHCI enabled, the 4 KB io size increased the throughput with 40 MB/s up to almost 120 MB/s.



I was actually feeling quite a bit :o  around this. Is it actually excepted to get this large differences (in benchmarks) with the shift from IDE to AHCI mode?
a c 179 G Storage
April 7, 2011 10:59:42 PM

AHCI enables Ordered seek queueing, the ability to reorder seeks to minimize access time. In a server environment with many outstanding i/o operations, this can be a benefit. I think what you are seeing is the results of an artificial benchmark that generates large queue lengths. That is not what a typical desktop user does, so in actual use, you should see no difference.

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a b G Storage
April 8, 2011 3:11:45 AM

I still use IDE mode myself. If I ever get around to making the switch I'd use HD tune (?) to check transfer rates, a stopwatch to measure game/level load times, and probably some form of large zip file to test "real world" disk usage. (I know, also CPU dependent.) Game load times are really what matter for me.
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April 8, 2011 4:55:31 AM

geofelt said:
AHCI enables Ordered seek queueing, the ability to reorder seeks to minimize access time. In a server environment with many outstanding i/o operations, this can be a benefit. I think what you are seeing is the results of an artificial benchmark that generates large queue lengths.


Well of course it is kind of artificial since the benchmark tools generate extreme amount of small IO requests, but as to the ability to reorder requests I am unsure if this goes into play here when the tests does only sequential read access?

The queue depth were in both IOmeter and Atto set to 4, but my different tests earlier have shown that this does not matter much for local storage. When doing benchmark tests on servers attached to Fibre Channel or iSCSI san it matters a lot.


geofelt said:
That is not what a typical desktop user does, so in actual use, you should see no difference.


That might be possible too, but I mainly use my computer to run many Virtual Machines through VMware Workstation, and when running multiple server operating systems it could be many small IOs.
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a c 179 G Storage
April 8, 2011 3:04:40 PM

It is difficult to change from IDE to AHCI without a windows install. Possible, but tricky.

Regardless, stick with AHCI, it provides some functions like hot swap, and trim support for ssd's.

If you can feel any significant performance difference when doing your real work, investigate, since there may be something else going on.
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April 8, 2011 7:36:47 PM

It usually will give you a large warning in the BIOS about switching to AHCI , something along the lines that your system may not boot. It's been a while. I always run mine in AHCI for the above mentioned functions, especially now that I'm going with SSDs for the OS. Not all SSDs use trim from what I've been reading.
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a c 179 G Storage
April 8, 2011 7:41:39 PM

brianb23 said:
It usually will give you a large warning in the BIOS about switching to AHCI , something along the lines that your system may not boot. It's been a while. I always run mine in AHCI for the above mentioned functions, especially now that I'm going with SSDs for the OS. Not all SSDs use trim from what I've been reading.

The reason is that to boot, you need the proper drivers for the drive to be already installed. At initial installation using IDE, the AHCI drivers are not installed.
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April 8, 2011 7:46:41 PM

geofelt said:
The reason is that to boot, you need the proper drivers for the drive to be already installed. At initial installation using IDE, the AHCI drivers are not installed.


Yes, thank you geo.. i think i knew that..
It's been a long day.. felt like looking through the forums :wahoo: 

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April 8, 2011 11:14:01 PM

geofelt said:
It is difficult to change from IDE to AHCI without a windows install. Possible, but tricky.


I would say it actually was easy. Doing a Windows re-install would take me days to install the OS, all patches, all applications and customizations, but the change from IDE to AHCI involves only changing a single registry value. Took me less than a minute, including backing up the registry key.

The whole process was done in perhaps ten minutes:

1. rebooted the system to make sure everything worked normally
2. backup of the registry key, changed the value
3. rebooted the system to make sure it still worked
4. rebooted again and got into BIOS and made the change
5. windows automaticly installed the new drivers, reboot final time


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April 9, 2011 9:46:36 PM


Anyhow, does anyone has any clue of why the IOPS number on small IO sizes was raised a lot?
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