thanks that was the answer I was looking for. regarding the backup issue, lets just say all my data is in external HDDs (to share easily among all my computers) and the important stuff is backed up, I will use it purely for the performance (I might use the space but as redundant etc) anyway I have another questions:
you said:
"if you are just buying drives it is better to buy all the same drives."
does that mean I can actually buy different (I mean, i thought that was a requirement no matter what, now I am wondering if I could do it with different HDD)
Yes, you can. It's not recommended IMO!!! You're basically asking for the worst preforming drive to slow everything up. And I have experienced cases in the past where it actually wouldn't work at all because of an odd drive.
what is the "optimum" number of HDD for an array (this is the issue, I was going to buy 3-4 1TB HDD for the array, but I could as well get 5-6 for the same price (640-750GB) the total storage will not be the same though, but if it offers better performance I'd rather do that, so will more drives is always better or after certain number it does not matter how many more you add the performance will remain the same.
Speed tops out on almost all controllers for SOHO (small office/home office) use at 5 drives in a level 0 RAID set - That would probably include Mac Pro's software RAID 0 too but there's only 4 connectors.

Beyond 5 drives you get zero (or nearly zero) R/W speed increase but with some controllers you can get better bandwidth with more than 5. Some very good cards scale linearly all the way up to 8 drives. So for a busy file-server serving large data more than 5 might make sense but otherwise not.
Also a lot depends on the drives you buy! What is the "fastest" HDD used as a single drive may or may not even be in the top 10 when working in a RAID. So try to find some RAID-0 tests on-line to see which ones are fast for RAID.
You probably already know that the outer sectors of a drive are faster than the inner sectors. The same applies to a RAID array as well so it's beneficial in some cases to run a media test that scans across the entire platter and tells you how your drives profile and then to partition the RAID for those areas. Typical R/W speeds (averaged) for a 3 drive RAID-0 are like 250 MB/s. But for example, the three I have can achieve 400 MB/s (±20 MB/s) in the outer plater areas. So if I need to be assured of obtaining that kind of speed I can partition off the 1st 15% or so and have it.

The inner-most portions of my platters can get as low 180 MB/s. And the middle 70% or so gets between 240 MB/s and 340 MB/s. That's with the drive's caches turned off and as a matter of note; any test software should never allow system cache to be used - as it's completely meaningless and absurd.

I guess it can be fun though - I just wouldn't trust any test tool that allowed it.
Also there's two kinds of tests. One os Random R/W and the other in Continuous R/W. Which is the most meaningful to you depends on what you use the RAID for. If it's video and sound editing then it's continuous. If you're a developer or doing DB stuff, etc. then Random. I'm a video and sound guy.
Look around on Tom's Hardware site. There's 4 good (exhaustive) articles I know of on there about this very thing. They include benchmarks and explanation deeper than really possible here.
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