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Re: [Qemu-devel] [PATCH] add file: migration support (r2)

From: Jamie Lokier
Subject: Re: [Qemu-devel] [PATCH] add file: migration support (r2)
Date: Thu, 13 Nov 2008 18:39:53 +0000
User-agent: Mutt/1.5.13 (2006-08-11)

Charles Duffy wrote:
> documents the limitations of O_NONBLOCK on Linux with local
> filesystem access.

That's not a Linux limitation.

All OSes ignore O_NONBLOCK for filesystem accesses, including remote
filesystems.  This is a POSIX thing.

The reason is that there's no way to select() on a file.  (You'd need
to select() on readiness to read/write a specific byte range anyway,
so the interface doesn't work.)

The nearest equivalent to select() on file I/O is AIO, which is
probably what should be used if migration file I/O is blocking QEMU.

-- Jamie

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