Reply to two commanets from "when virtualization is too expensive".
1st was from grantm. Thanks for pointing out the OpenVZ. It's much more professional tool than debootstraping and chrooting, but comes with more configuration and setting-up complexity. It's possible to share memory using OpenVZ which is the resource that we never have enough.
2nd was from Alias. Alias is pointing out the deduplication of disk data. Disk space is kind of cheep these days, comparing to memory, bandwidth and cpu processing costs when dealing with huge data. The deduplication can offer, besides saving the disk space, a better cache efficiency, that will increase disk read speed, that is never fast enough. ZFS should soon (first quarter of 2010) support deduplication. It seems that there will be no native ZFS support for Linux because of the licencing problems
`cp -al some-folder/ some-folder.copy && cd some-folder.copy && cow-shell` will copy "some-folder" via creating hardlinks which is really fast. Than any write operation under this shell and folder will result in removing the hardlink and creating a new file => copy-on-write. This works good with chroot-ed systems to test new Perl module versions or do some other experiments.