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    SubjectRe: coLinux benchmarks
    On Mon, Apr 05, 2004 at 02:30:56PM +0100, Christoph Hellwig wrote:
    > > The VM shows better results than the host. What gives? Perhaps
    > > it is because of the combination of the host and guest's buffer
    > > cache? I'd like to know about more percise benchmarking methods
    > > for VMs.
    > How are the virtual disks for the VM implemented? If you're doing
    > direct I/O these numbers are indeed strange. If not OTOH that's
    > expected because even synchronous I/O in the guest is actually
    > async which makes it a lot faster.

    The virtual block device driver in coLinux, named cobd, is synchronous
    with the host OS highest level read()/write() functions, which
    means e.g. for a READ block I/O request in the guest,
    filp->f_op->read() is called on an open 'struct file' in the host.
    If the call blocks, the entire guest VM blocks on it.

    So, according to this, any type of I/O in the guest means synchronous
    I/O in the host unless the data is already in the guest's buffer cache.

    It's not really the implementation I am planning to stick to, but
    it sure was very easy to implement.

    BTW, the block device on the host side can be a file or any
    device that exposes read()/write() interfaces to userspace. In
    this benchmarking case it is a 3GB file that hosts an image of
    an ext3 filesystem.

    Dan Aloni
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