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Well its known that HDDs including the velociraptor prefer sequential I/O over random I/O - when you install an OS cleanly it will boot very fast on a HDD, but after you've been working with it for months and installed and removed all kind of applications and files, fragmentation will cause non-sequential access to the storage medium, and SSDs can handle those incredibly fast but HDDs have to reposition the head or "seek" alot of times, which is very slow. That's where you'll start noticing the real performance difference.
While alot of mechanisms are built into Windows and storage drivers and HDD firmware/controller to negate the bad non-sequential performance of mechanical harddrives, their ultimate weakness cannot be overcome: they have to physically move stuff in order to access data, that can never perform on the same level as other parts of your computer system, which are fully electronic aside from optical drive. Optical drive will die too, since it has the same problem and is not reliable at all. Good SSD's however, offer both fast and extremely reliable storage which excells mechanical disks in multiple orders of magnitude. The waiting game is for the intelligent controllers to make use of all the potential. Don't be surprised to see 2000MB/s and 100.000 IOps+ SSD solutions coming soon, using PCIe as interface because SATA is way too slow. A PCIe adapter offering ~1000MB/s is already being sold, i can lookup the source if you would like me to.
True, though with native command queuing, the Velociraptors are not slow at random IO either. With a queue depth of only 2, I can get 80MB/s throughput on 2kb random reads from my velociraptor array as measured with ATTO. For comparison, my data drive (a WD 1TB Caviar Black, hardly a slow drive in its own right) only manages 17MB/s on the same test. This shows just how superior a Velociraptor array is to a standard drive at random IO, the area at which SSDs excel. Again, it isn't the same speed as an SSD, but they certainly are not slow. They are also quite reliable - for all the claims that hard drives are the weak point in a system, I have not found that to be the case. From my experience, an average hard drive will easily outlast an average high performance video card for example. In fact, hard drive reliability isn't an issue (other than a few bad models, like the 7200.11 series from Seagate) from my experience - I almost always replace them out of obsolescence before they crash. I have an old 6GB Quantum Fireball that still works just fine for example, but I have absolutely no use for it now.
As for PCI-E SSDs, they are a gimmick at best right now, as booting from them or installing a main OS to them is not supported. SATA 6GB/s is more likely as a good interface with 600MB/s capability.
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Oh, and I would argue that for system responsiveness once Windows has already started (as opposed to boot time), you're better off investing in more RAM. It will give a noticeable improvement for far less money than an SSD.
More memory than what applications use (remember many are still 32-bit so use maximum of 2GB per process) is only used as file cache, remembering previous data that came from the HDD in the internal memory. While this is useful, i don't think a difference between 4GB or 12GB is really that significant, unless you use applications that use alot of memory themselves. Your memory can't and won't read-ahead future accesses, and you'd need more memory than your storage space to pre-cache everything, which is not efficient at all. Its far better to invest in an SSD IMO.
Do you have any sources for your claims, like benchmarks or tests?
I'm not referring to the difference between 4 and 12 gigs here - my system is far from typical. I was saying that there is a fairly significant difference from 2 to 4 gigs for example, and a smaller difference from 4 to 8. I have 12 gigs because of some of the applications I run, not because of system responsiveness (and it is not uncommon for me to be using 7+ gigs at a time). Also, 32 bit apps can use 4 gigs per application when in a 64 bit OS, not 2 gigs. I'm not claiming that the difference between 4 and 8 gigs for example is more significant than installing an SSD (it isn't). However, it is a hell of a lot cheaper, and gives some of the same gains, depending on your usage pattern. Basically, it's a better choice because it's cheaper, not because it is faster.