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7.3 Limitation of those technologies

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Oddly, one disadvantage of those technologies is a side effect of their major advantage: Total Independence. Each virtual server is running its own kernel. Cool. This makes the following tasks more difficult or impossible:

  • Sharing administrative tasks such as backup. The virtual servers are using volumes in the host server. The host server can't handle the files in those volumes directly without interfering with the client OS. It has to use some services of the client OS to access the file.

    The vserver solution does not have this limitation since the virtual servers are living in the same file-system, sharing the same kernel.

  • Task monitoring. The virtual servers run their own kernel. As such, the host OS can't spy on the tasks and check for intrusion for example.

  • Disk space. Virtual servers are using either volumes or full devices in the host server. This space is pre-allocated to the maximum needed by the server. You end up with a lot of wasted disk space. Imagine running 100 virtual servers this way and allocating say 10 gigs to each. You get the picture. The vserver solution is sharing a common file-system so the free disk space is available to all.

    Further, if you are running the same Linux distribution in the virtual servers, you can unify the disk space using hard link and immutable attributes. The /usr/lib/vserver/vunify was created to test that. Using information found in the rpm package the script links the files, except configuration ones.

    Testing vunify on a vserver installed with a RedHat 6.2 distribution, unifying the packages glibc, binutils, perl, and bash saved 60 megs. Quite a few packages are not changing often and could be unified.

    Vservers do not need kernel packages and hardware configuration tools. This also contribute to save disk space.

  • File system sharing

    A little the same as above. You can't share file system easily between vservers unless you use network services (often slower). Using "mount --bind", it is very easy to "map" any directory of the root server in several vservers, providing raw speed access (and even sharing the disk cache).

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