ML: I've often heard that Virtuozzo scales even better and allows for even higher VPS/VE density than OpenVZ. Why is this true and what feature differences between OpenVZ and Virtuozzo account for this?
Kir: There is a VZFS filesystem in Virtuozzo but not in OpenVZ. It is similar to unionfs or aufs, the idea here is to make a single copy of files for applications, binaries, libraries and data. If there is a single copy of such files on disk for different VEs, there will be a single copy in memory holding the read-only code and data. It helps to save some memory and thus increase running VE density a bit -- but only the in cases where all (or most of) the VEs are identical (i.e. have the exactly same versions of programs installed).
So, VZFS is not a silver bullet and it comes with a price -- not conforming to POSIX by adding an ability to have additional permission and ownership attributes for symlinks. Because of that Virtuozzo ships with its own modified versions of tar, cpio, rpm, and deb which are patched to understand the added filesystem semantics. Besides, it complicates template tools. We do not want that sort of complexity in OpenVZ.
(с) http://www.montanalinux.org/openvz-kir-interview.html
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