Asynchronous File I/O on Linux

[gaf-mNDKBlG2WHs at public.gmane.org (Jerry Feldman)]

On 05/16/2010 11:56 AM, Richard Pieri wrote:
> On May 16, 2010, at 10:06 AM, Mark Woodward wrote:
>  =20
>> Well, it is "async" in the view that I want to issue multiple read=20
>> requests simultaneously from the same execution context (thread or=20
>> process). What I want to test is *if* I can issue multiple read reques=
ts=20
>>    =20
> That's definitely not asynchronous I/O.  Asynchronous means that the ca=
lling process doesn't wait for the write call to finish.  That is, the pr=
ocess runs asynchronously to the write.  Reads are always synchronous.
>
> What you described is either concurrent or parallel (I'm not precisely =
sure which term is most appropriate) reads.  Can you do it?  Sure, just a=
s I described: create as many file handles to the file that you need with=
 open().  Each file handle has its own unique read/write pointer to the f=
ile.
>  =20
Additionally, Linux (and Unix) always buffer file I/O. When a file is
open, there will be several buffers containing data for a file.
Essentially, if you are reading (assume a random piece of data), if that
block happens to be in memory already you will retrieve it from the
buffers, otherwise it will cause a physical read. Additionally, when you
write, you are not guaranteed that your data will be immediately written
to disk. Certainly, you can control whether a file is buffered or not,
but I would avoid using unbuffered files, and I would avoid flushing the
data to disk if you want performance.

--=20
Jerry Feldman <gaf-mNDKBlG2WHs at public.gmane.org>
Boston Linux and Unix
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