kernel level virtualisation requirements.

[Matthew Dillon]

    I think the reason you see such a huge degree of virtualized and
    para-virtualized environments these days is simply because service
    providers can no longer provide all the web services individual sites
    need as a single common set of managed applications.  All they can do,
    pretty much, is allow their users to install and run the application
    set they need to run for their site.  In fact, there are several layers
    of middle-management web site providers available today who simply lease
    virtual machines and build a custom environment for their clients.
    Virtualization neatly solves what would otherwise be a nightmare
    for providers.

    Despite the loss of performance you get in a virtualized environment,
    it is far, far easier to manage memory and I/O resources in a
    virtualized environment then it is in a jail, and far far easier to
    migrate such environments between physical machines without having to 
    worry about introducing incompatibilities or having to force your
    customers to upgrade and deal with the inevitable snafus that crop up
    when that happens.  You don't even have to worry about upgrades to
    physical hardware... the virtual environment doesn't have to change by
    a single byte.  I remember upgrading machines at BEST Internet.
    It was a nightmare to deal with all the customer snafus that occured
    every time we upgraded something.  I wish we had virtualization back
    then.

    Security is also a whole lot easier to manage with a virtualized
    environment.  You basically just run the virtual kernel (strangely enough,
    in a jail usually), and give the user root inside it.   The user can
    install whatever he wants...  whatever services, applications, etc.
    And if the user crashes a virtual kernel or creates a security issue,
    just that one user is effected, not the other 19 on the physical box.
    Dealing with break-ins which effect dozens, hundreds, or thousands of
    users is a nightmare for any provider.  Dealing with a break-in which
    effect one user and is basically the user's own fault is not.

    Global resources are a lot easier to manage in a virtualized environment.
    For example, it is impossible for a user running in a virtual kernel
    to use any more real system memory then was assigned that virtual
    kernel, and all I/O resources basically devolve down into just one or
    two points inside the virtual kernel (network I/O and 'raw disk' I/O),
    which are similarly ultra easy to control.  If that user blows up his
    own system, or causes it to start page-thrashing, it is his problem
    to solve and will not effect the other 19 users on the box.  Thrash
    a machine in a jailed environment and the whole machine goes to pot.
    'limit' and other resource controls just don't work well in a jailed
    environment if the intent is to avoid thrashing other environments
    running on that same machine.

    What a jailed environment offers is performance.  There is no question
    about that.   If I were designing a turnkey server with a very specific
    application set I would probably use jails.  But if I wanted to hand an
    actual login to a user in an environment where the user needs to run
    many potential services or backend run-times, I would choose a virtual
    kernel hands-down above a jail solution.  Jails are useful for many
    things, but most of those are not things which require scaling.

    Hardware is cheap, manpower is not.  Being able to avoid hiring a single
    employee is worth 20 machines at least, and hardware costs scale far
    better over time then employee costs.  That is what is driving the 
    market for virtualization.  Performance doesn't enter into the equation
    most of the time.  For anyone doing a serious deployment this is a
    huge deal.  A jailed environment just doesn't offer the same level of
    control or management, no matter how much you try to separate resources
    out inside the kernel.

    Similarly, storage is almost free these days.  Storage is not an issue,
    so the replication of environments and application installs is not an
    issue.  Not any more.

    The only real limitation for a virtualized environment is physical memory,
    and that has already been taken care of with the money you save by not
    having to hire that extra employee.

						-Matt