Which 2.5" drives are suitale for either a cache or parity drive? Performance-wise I mean. No SSDs!

You'd want to go for the 7200RPM models. So the WD Blacks, Seagate Momentus 7200.x or other brand's drives.

 

I wouldn't suggest one for a parity, as you'd be limited to that size for your largest array drive.ie, you parity drive has to be equal too, or larger than your largest array HDD. So, if you get a 750GB Parity drive, you can't use 1TB/1.5TB/2TB/3TB/Etc

 

for a cache drive, it would probably be ok to look at one. You can really use any one you want, but there is another thread here where someone has used a WD Blue (5400RPM) and can't see any speed difference with the cache drive, so you probably want a 7200RPM drive.

Yes, you are right about the parity drive. 2.5" capacity is still limited to 1 TB I think. A 500 GB or 1 TB would be possible.

I wouldn't use a 2.5" drive for parity.

1. you are limited by size

2. you are limited by speed

 

For cache, I might choose a seagate momentus 750 7200 rpm. or some other "high density" 7200 rpm drive.  Maybe even a WD VelociRaptor, but then you have the heatsink.

As much as I like the seagate momentus xt's I'm not sure I would spend the extra amount for it unless it was going to double duty as an apps drive.

 

You need a high density drive in order to get the throughput to make it worthwhile for the expenditure.

 

I think the biggest drawback for using 2.5 inch drives is price/GB. Overall they offer excellent performance, but with a higher price and more dense heat load. I use the 600GB 10k drives in RAID6 for my iSCSI storage.