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Date: Wed, 12 Nov 2008 21:06:32 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [PATCH] add file: migration support
References: <gfg1l8$b56$1@ger.gmane.org>
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To: qemu-devel@nongnu.org

Charles Duffy wrote:
> This patch adds support for migration to and from file: targets, moves 
> common helpers between exec: and file: use cases from migration-exec.c 
> to migration.c, and adds a qemu_fdopen() helper parallel to qemu_fopen().
>
> Signed-off-by: Charles Duffy <charles_duffy@messageone.com>
> diff --git a/.gitignore b/.gitignore
> index e70ebab..8e28325 100644
> --- a/.gitignore
> +++ b/.gitignore
> @@ -30,4 +30,4 @@ qemu-nbd.8
>  *.tp
>  *.vr
>  *.d
> -
> +*.o
>   

Please keep this sort of stuff separate from a general patch.

> +    s->fd = open(filename, O_WRONLY|O_CREAT|O_TRUNC, 0666);
> +    if (s->fd == -1) {
> +        perror("Unable to open migration target");
> +        goto err_after_alloc;
> +    }
> +
> +    if (fcntl(s->fd, F_SETFD, O_NONBLOCK) == -1) {
> +        dprintf("Unable to set nonblocking mode on file descriptor\n");
> +        goto err_after_open;
> +    }
>   

Did you mean F_SETFL?

At any rate, this doesn't do what you think it does.  O_NONBLOCK still 
blocks with a file.  To implement a proper file: migration protocol, you 
have to use something like posix-aio to write to the file asychronously.

You can observe the problem here by setting the migration data limit to 
something very, very high (like 10GB).  That will cause the VM to become 
unresponsive until all the data is on disk which is certainly not live 
migration.

Regards,

Anthony Liguori