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Re: Kernel VS application file caching

On Thu, Jan 21, 2010 at 08:48:39PM -0500, Steven Bellovin wrote:
> Let me give a real-world example.  Between a NetBSD laptop and a NetBSD
> desktop, connected via gigE, I can run 'ttcp -s' at (if I recall
> correctly) 700M bps.  I can upload data to my office at ~2.5M bps; I
> can download at about 13M bps.  In other words, when I go in or out of
> my house, the network is by far the limiting performance factor.  For 
> anything but a floppy drive, it doesn't really matter how fast my disk
> is or how much caching happens; I can't ship data faster than the network.

This is sadly only true as long as you are doing sequential reads.
As soon as you have a lot of seeks, disk read performance suffers
greatly. Try to tar up the content of src using
        find | shuffle | tar -T -
for example.

We currently ignore POSIX_FADV_NOREUSE, which can help the outlined as
well as many similar cases. E.g. it would allow backing up a filesystem
without trashing the caches completely.


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