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Re: [Qemu-devel] migration: adding migration to/from a file (v2)

From: Anthony Liguori
Subject: Re: [Qemu-devel] migration: adding migration to/from a file (v2)
Date: Thu, 19 Feb 2009 17:59:50 -0600
User-agent: Thunderbird (X11/20090105)

M. Warner Losh wrote:
In message: <address@hidden>
            Jamie Lokier <address@hidden> 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

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.


Anthony Liguori


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