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AHCI or Raid 0

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  • SSD
  • NAS / RAID
  • Windows 7
  • Storage
Last response: in Storage
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May 20, 2010 2:47:02 PM

I've been researching all night with no definite conclusions. I just got two 100gig OCZ Vertex Limited Edition SSDs. Currently I have Windows 7 running and the SSDs in AHCI mode. I was originally going to set them up for Raid 0 until I learned about the Windows 7 Trim feature.....which seems much needed for long term performance. Obviously the drives have to stay in AHCI for Trim to work. Can anyone else elaborate on this topic and provide some good input as to whether I should leave it in AHCI or switch over to Raid? Thanks.

More about : ahci raid

a c 127 G Storage
May 20, 2010 2:57:31 PM

So either no RAID but two SSDs in AHCI and you have TRIM.

OR, you let go of TRIM and make sure you do not need TRIM.

Two 100GB in RAID0 makes 200GB. If you create a 150GB partition and leave the rest unused, you will reserved an additional 25% capacity to the SSD internally.

So you have one 150GB partition and 50GB of unpartitioned free space. Never use this free space. If you've written a single byte on the SSDs before; secure erase them before doing this or it may not work. The 50GB unpartitioned space may never be written to by the OS; if you do the reserved space will not be used neither by you neither by the SSD, which is not what we want.

With this 25% capacity reserved, you do not need TRIM.
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May 20, 2010 3:14:40 PM

How unlikely is it that over time all of the writes won't fill up both the 150gig partition and the 50 gig unpartitioned space? And is the performance gain of having them in a striped array worth sacrificing Trim?
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a c 127 G Storage
May 20, 2010 3:18:40 PM

How big your performance scales depends on your RAID engine (drivers) and on your CPU/memory speed.

Eventually, the 50GB unused space will be used by the SSD internally; as that's the only spot you won't have written to after some time. So the SSD would use this as internal space to keep write amplification low.

If you do not have TRIM and also do not reserve space, you would considerably drop in both lifetime and performance.

Whether the performance increase is worth it, is something you will need to benchmark yourself. I suggest CrystalDiskMark and AS SSD. Watch the random I/O scores both single queue and multiqueue (qd=32/64). Those are the real performance specs of SSDs. Sequential read/write is not that important, really.
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May 20, 2010 4:16:55 PM

Sounds good. I think I might just go ahead and leave them in AHCI. They're performing really good now.....much better then my striped Raptors. Is there anything you recommend doing or checking in Windows? I know Windows is supposed to automatically detect SSDs and enable Trim and disable other things like Superfetch and Defrag. I saw somewhere that the motherboard needs to support Trim also for it to work. I tried doing a search but couldn't find anything. My mobo is an Asus M4A79T Deluxe with the AMD 790FX chipset. Thanks again for all of your help.
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a c 127 G Storage
May 20, 2010 7:59:12 PM

No your motherboard does not need TRIM support; just your SSD, OS (windows 7, linux, freebsd) and a compatible driver (recent intel, microsoft ahci).
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