So what extra advantages does SSD give you?
- Persistence (don't lose data in power outage)
- Cost is still lower, and will drop very rapidly compared to RAM over time
- No upper limit to size - you'll see 1TB SSDs before you see a COTS server that accepts 1TB of RAM
- Common interface - you can move the SSD to any other computer and connect it, or even a USB<-->SATA bridge. Can't do that with RAM without checking the MB specs, removing existing memory if slots are full, etc.
- Can add multiple SSDs to one computer, whereas RAM is ultimately limited.
Why buy and SSD instead of just putting more RAM in your server machine?
When I need fast persistent storage, I use SSD.
When I need fast volatile storage I use RAM.
IF the UPS fails, or the motherboard fails, or the software crashes the OS, you lose everything in RAM.
There is simply no substitute for persistent storage.
Further, though you state the cost is similar, the cost of high performance SSDs is going to drop like a rock over the net two years.
Right now it might make sense if you have read only data, or indexes that you don't mind rebuilding, stored completely in RAM.
In cases where the cost and risk are low, you might even perform more aggressive disk caching against a slower hard drive.
But at the end of the day, if you want persistent storage AND performance, you either buy BOTH a slow hard drive and fast RAM, or you buy a high performance SSD.
In general the SSD is going to be cheaper than both the hard drive and RAM together.
But at any rate, SSDs are still niche items. You don't use an SSD unless you have specific needs.
-Adam
Best Answer
The traditional answer would be to separate the pagefile from the System drive but the X-25E's performance should generally make that unnecessary, especially on a server. If you can't put enough RAM in the system to avoid unnecessary paging, or you have applications (like Exchange 2007) that can make quite a lot of use of paging no matter what, then putting the page file on dedicated disks or on disks where the utilization is generally low remains a good idea.
However using a single X-25E for the system drive seems a bit strange to me. Whether it is a good idea or not to use SSD's for the system drive depends entirely on what you are using the server for but in most cases system drives on (properly configured) servers are not going to be the most significant IO bottleneck you want to control.
What I can say for certain though is that you should not use just one drive for system or normal data volumes. The standard practice of using two relatively small but reasonably fast drives (2x10K SAS) in RAID 1 provides sufficient performance to load the (mostly static) system drive files for most servers but guarantees you some level of failure resilience. SSD's might not be mechanical but they can, and do, still fail.
Ideally you want to use SSD's to eliminate something that is IOPS limited - your RAID 10 arrays are probably able to get close enough to matching the X-25E's transfer rate under most conditions but they wont come close to the IOPS (a couple of hundred at best versus many thousands for the X-25E). However since it's a single drive you should be very reluctant to put anything that you can't afford to lose completely on it - if it was me I'd be using it for temp file space (print spooling for example, scratch DB area for reporting etc).