Thanks for sharing your experience with Linux memory management. I assume that these bits are so deeply embedded in the OS, and so well wrung out, that no one wants to touch them. There is no doubt that there is overhead - keeping track of page usage and demand, swapping, the disk I/Os, must add up to something - but they’ve been part of the landscape so long that maybe only a few crusty old realtime systems guys even have a handle on it.

The cost issue is a little trickier, IMHO. Certainly it is visible when cache is an extra cost option on a disk, an array, or an HBA. What isn’t so obvious, and is more difficult to quantify, is the engineering effort that goes into, for example, cache coherency in an array or across a network. The basic mechanisms are well understood. It is the 2% corner cases that probably absorb 60% or more of the engineering time. Maybe I’m barking up the wrong tree here and there are no significant savings to be realized. Yet it just seems like one spot that could simplify and cost-reduce massive storage.

I like your comment about the 5400 rpm drive in your new MacBook Pro. That’s one place the SSD will be a big win.

Robin

I remember thinking a similar thing about virtual memory a while ago when building myself a Linux box. So I ran it for a while and kept an eye on memory and virtual memory usage (it wasn’t operating under a major load, more like just ticking over nicely). Then I stuffed a load more memory into it and watched to see if it made any difference and to my surprise it made very little difference if any. So I dug around it a little and found that the memory management system in the 2.4 kernel was designed to use virtual memory pre-emptively even if it wasn’t required. You could have shed loads of memory available and still see quite high usage of your page file. Supposedly just in case things got extremely hectic and you didn’t have to sit around and wait until real memory was freed up.

Im sure the same will be true just about every other OS, especially where I live in the open systems world.

As for traditional monolithic arrays and their huge cash architectures…… I think we are already seeing the more modern data centres, with their unstructured data, looking for something a little different to what the more traditional data centres (banks, large government agencies etc) require. We recently talked about this in an article titled The MySpace storage monster on rupturedmonkey - http://blogs.rupturedmonkey.com/?p=53

However, and Im thinking on the spot here, although I agree that cache might not be as useful as the marketing brochure leads us to believe, having it there is still not doing us any real harm. E.g. Ive been thinking about the increasing cache capacities in monolithic arrays and wondered how much time is lost searching cache only to find your data is not there and needs to be fetched from disk. However, this is almost insignificant in comparison to actually getting the data from spinning disk.

Given the technology advancements of the last few decades, I don’t really care if my cache is under untilised, I’m more amazed that my computer still comes with a spinning disk (I know they are cheap but Im a Mac user so don’t mind paying for the good stuff). In fact here’s an analogy Ive just thought up – I have my new all singing all dancing MacBook Pro with all the mod cons and optional extras. Yet at the heart of it is a 5400RPM SATA disk. Now imagine this – I have a nice new BMW M6 (I don’t) with Satelite Navigation, DVD players in the back, climate control, all the features you can think of, but I have to drive it with my feet like the Flintstones!!! You just wouldn’t!

Anyway, apologies as Im starting to rant a little Good post though