Re: raptor or cache?

On Sun, 15 Jan 2006 12:07:24 -0800, ivowel wrote:

> I want to build a fast linux machine now. Among my choices are whether
> to put more money into memory ($70/GB), or more money into a Raptor HD
> RAID-0. I believe linux uses a cache write-back scheme, which is good
> for me. I do not like the idea of data sitting for hours on an
> external hard drive, but data that is written to "when convenient,"
> where convenient is a number in the low minutes. Still, I understand
> that battery-backed RAM drives on PCI-E cards are on the horizon. I
> guess I could set up an automatic rsync.
> Now, realistically, almost all my linux operating system fits into 1GB.
> All of my user files that I am working on on any given day would fit
> into another 1GB. I pretty much know where the these preferred areas
> are. Alas, what would be useful in such a situation would be the
> ability to designate some files or directory areas as "preferred to
> retain in cache", others as "normal cache", and others as "do not
> cache". for example, under gentoo, I do not want my 6GB /usr/portage
> tree to be cached. and, given the choice between discarding a
> preferred and normal buffer, I would like the algorithm to prefer to
> dump the normal cache.
> Possible?
> sincerely,
> /iaw

Generally more memory helps a lot more then a faster drive. A 10000 RPM
disk is only useful if you are accessing a lot of small files and you
aren't ever accessing them again. Linux uses the available RAM to cache
files so as long as you have enough RAM you'll hardly ever do a disk
access. In my experience 2G is sufficient for a desktop system. You can
live with 1G but at 2G you never page out your file cache. Above 2G is
useful only if you have some application that uses a huge amount of RAM.