Re: O.T. optimising file placement

Patricia Shanahan <>
Thu, 23 Feb 2012 21:45:33 -0800
On 2/23/2012 3:16 PM, Martin Gregorie wrote:

On Wed, 22 Feb 2012 21:40:19 -0500, Arne Vajh??j wrote:

Most OS'es support async IO.

Yes, I know, but its not relevant to a single-threaded process since its
logic generally requires it to wait for a read or write to complete
before it continues[1]. Hence my comment that this prevents head movement
being optimized unless a lot of processes are active because there's only
one outstanding IOP per process.

[1] unless you're deliberately doing async i/o using poll() or
     select() (in C) or nio (in Java), in which case the process is often
     best regarded as a half-way house between single and multi-threaded

There are some exceptions to this. For example, if you are reading a
file sequentially, the OS may prefetch blocks you have not yet
requested, and have multiple reads outstanding as a result.

Depending on the OS and how the IO is being handled, a write may appear
to be complete from the program's point of view once the data has been
copied to a kernel buffer. The OS may be writing out modified blocks,
including swap space blocks, at any time.


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