Linux – SSD for swap on Ubuntu server

This is mostly an educated guess, as I have not tested such a configuration.

I would say using a small, fast hard drive for the OS and programs would be a good idea. This does not have to be an SSD by any means. It could help if you need to be stingy on power, but would cost much more, and have less capacity, than using a 100GB standard HD. As another answer says, Vista and developer tools would likely not fit in 32GB unless you were very careful which components from each are installed. Even then, maybe not.

My recommendation would be to save the money and get a 80-120GB main drive that is fast, then a larger, slower drive for data. If you decide to go cheap on the second, slower drive, be sure to run backups on the data periodically. A 10k or 15k spindle speed drive is generally not a great idea in a desktop workstation, as it adds heat, power draw, and vibration to the system for very little real-world performance gain, in most cases.

As for what to watch out for if you decide to go SSD, watch for manufacturer and read the warranty terms. Also, look at real-world benchmarks, as some SSDs have been found to be unbearably slow.

Given the performance levels you are trying to hit then you should bear a couple of things in mind.

The earlier suggestion to format the drive holding your data set with an NTFS cluster size > 25k is a good one, 32k seems like a good choice for your use case. The purpose of doing this is to ensure you're avoiding have to deal with fragmentation if at all possible, and lowering the file system overhead associated with reading in a single file.

Also I'd suggest taking a look at your RAID stripe size. If the nature of your data set results in (mostly) sequential IO then a larger stripe size is more beneficial, if it's random then a smaller stripe size will be smarter, just don't make it any smaller than your files system cluster size. Given the description of what you're trying to do I'd say your IO profile is mostly random so a stripe size of 64k would be fine but it might be worth experimenting with.

What's very important is to make sure the partition is aligned - you will want to use Diskpart.exe on your systems, generally setting an alignment offset of 64k does the trick with standard cluster and RAID stripe sizes but a 1024k offset is used in Vista\Windows7\W2K8 as it will ensure alignment even with larger stripe & cluster sizes. There is a very good article on SQL server performance from Microsoft here that explains the reasons why this is important to do for high performance drives\arrays. The short version of this is that a poorly aligned partition can degrade IO performance by 15-30%.

For SSD's the same general rules apply but the underlying behavior of reads\writes is vastly different. Rather than dealing with 512byte disk sectors at the most fundamental level IO on SSD's uses much larger blocks. Reads tend to be in fixed page sizes of between 4k and 128k, writes involve buffering and large erase block sizes (in the megabyte range). The key thing for you (where read IO is important) is that you want to set your RAID stripe size to be a multiple of the read page size for the disk type you select (I think the Intel X-25's all use a 128k read block) and you want to set your alignment offset to some number that ensures it is greater than that. The standard recommendation of a 64k partition offset would be a poor choice if the SSD has a 128k read page size for example. Because of the asymmetric nature of SSD writes it isn't easy to optimize SSD RAID arrays for write performance but that's a whole 'nother story and involves lots of cache.

There's a nice article on optimizing SSD RAID here on the OCZForum, it's aimed at enthusiast setups but they get the general message right for anyone trying to roll their own SSD RAID from off the shelf kit as far as I can tell.

Finally if your read IO pattern is predominantly sequential the 8 15k drives can (in theory) hit somewhere in the ballpark of 800Meg/sec if not more. Two SSD's, even Intel X-25E's, will only hit about half that. I'd guess that your read IO pattern is biased enough towards random IO to negate this but under the right circumstances your 8 15k SCSI drives can be a lot faster than the two SSD's. This is borne out by your testing. Looking at those numbers though I'd say that some work with partition alignment and stripe sizes will help quite a bit.