[cowmixtoo]

I have a lot of personal success stories with SSDs but here's my current favorite.

A few months ago I had to help one of our scientists (the company is called 5AM Solutions.. they the awesome) run a bioinformatic job written in Perl and R. As it turned out, for long stretches of the processing the job required around 20 GB of memory. The one server that had all the required dependencies installed had only 8 GB at the time.

When I let the job run the first time, it started to page out memory to hard disk. The job ran for about four days, was only about 25% complete and during that time frame the server was un-useable for any other functions. Pretty much everything came to grinding halt.

Between that first run and the time our new RAM would be installed, just for grins, I gave the system 30 GB of swap space on the locally attached SSD. With that configuration the job finished in 19 hours and during that time the server was still responsive of other tasks.

When we finally added the appropriate amount of physical RAM the job took only 15 hours to complete.

It is the first time I have ever seen virtual memory be useful.

[scott_s]

"Virtual memory" is not a synonym for swap space: http://en.wikipedia.org/wiki/Virtual_memory

Virtual memory is what lets us write programs pretending that we own the entire address space, and it is very useful.

Swapping pages to disk, though, has been useful for a very long time. Yes, once your high-performance application starts swapping all the time, your performance is going to suffer by several orders of magnitude. But occasionally swapping pages in and out of disk is part of what makes modern operating systems useful. You left a large PowerPoint presentation open for several days, but never got around to working on it? Not a problem, since if the OS needs that memory, it will just swap out the pages. Without that ability, the OS would need to go around killing processes. (Which it will do if it has to, but it's a rare event because it can swap out pages.)

[cowmixtoo]

My point was that my application needed 166% more phyical memory than what I had at the time.

Paging out to a normal "disk" was not even an option, the performance ranged incredibly horrible to completely unusable.

Paging out to SSD was not only usable, it was only a percentage slower than 'real' RAM.

I think that's amazing. Having the option to use SSD as memory in a pinch is a great 'win'.

[cowmixtoo]

Hmm... I still think its valid to interchangeably use "Virtual memory" and swap space. Swap space is just where your "virtual memory" lives, right?