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Date: Thu, 19 Feb 2009 17:59:50 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] migration: adding migration to/from a file (v2)
References: <499DADBB.4080008@redhat.com>
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To: qemu-devel@nongnu.org
Cc: uril@redhat.com

M. Warner Losh wrote:
> In message: <20090219202849.GE22319@shareable.org>
>             Jamie Lokier <jamie@shareable.org> writes:
> : Anthony Liguori wrote:
> : > >Sure looks like a bug.
> : > I wish!  It's Unix suckiness.
> : 
> : Windows is the same.
> : It's a more of a conceptual problem than it looks, not merely an API bug.
> : 
> : It comes down to "what would 'readable' and 'writable' mean on a file?".
>
> "Would a read or write operation block?" is a better way to look at
> the interface that select() or poll() provides.  For a regular file,
> the answer is "no" since the writes are so fast and often
> asynchronous...
>   

Except when they aren't..

The real issue is that read/write offers streaming semantics, not random 
access.  You cannot guarantee that a read is going to complete unless 
you do read ahead.  So the semantics would be something like pread(fd, 
buf, X) = EAGAIN (kernel starts the operation for X), later, pread(fd, 
buf, X) = OK.  Sort of a weird interface.

For write, it's even more bizarre because you can't "write-ahead".  If 
you're dealing with O_SYNC or O_DIRECT, there's simply no semantic that 
makes sense.

So fundamentally, read/write is a bad interface for random IO.

Regards,

Anthony Liguori

> Warner
>
>
>