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I have a PERC 5/i RAID controller currently hosting two drives in RAID 1. I am about to add a third drive in "direct" mode, just a normal physical volume.

Would performance be better if the drive is attached to the motherboard's controller or to the RAID controller? A co-worker suggested that the RAID controller's caching might provide better perf even with a single drive.

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up vote 2 down vote accepted

In terms of performance I think the difference between the direct attachment to the motherboard and the PERC 5/i is in favor of the RAID card. How much in favor would be pretty difficult to guess, but I don't see any obvious performance related penalties thus far.

I'd say that the features of the PERC 5/i itself like the Intel processor (IOP333), XScale, 256 MB of DDR2, etc. etc. would be the reasons to use it over the direct interface to the motherboard. Plus, what if later on you want to use that single drive into a RAID 0,1 array?

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This is a tough question and is dependent on some hardware capabilities of the raid card itself. Check out the performance characteristics of the card and determine how the addiional drive will be handled by the hardware. Most of the time these choices make themselves when you do this deep dive on the capabilities. Generally speaking, disk cache will indeed speed I/O but also has other benefits like being able to cache unwritten data during a power failure (This is assuming your raid card has a battery to keep the cache powered up).

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