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Date: Tue, 05 May 2009 16:51:39 +0300
From: Avi Kivity <avi@redhat.com>
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Subject: Re: [Qemu-devel] barriers: block layer preparations
References: <20090505120804.GA30651@lst.de> <20090505120817.GA30721@lst.de>
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To: Christoph Hellwig <hch@lst.de>
Cc: qemu-devel@nongnu.org

Christoph Hellwig wrote:
> Add a flag to BlockDriverState to advertise barriers support, and add a flags
> argument to bdrv_aio_readv/writev to allow passing down the barrier flag.
>
> Note that the flags argument to bdrv_aio_readv is for now actually superflous
> because write barriers obviously only apply to writes.  I kept the read side
> argument so the API is symmetric and we can easily add more flags to both
> of them.
>
>
> @@ -1498,7 +1498,7 @@ static void qcow_aio_write_cb(void *opaq
>      acb->hd_aiocb = bdrv_aio_writev(s->hd,
>                                      (acb->cluster_offset >> 9) + index_in_cluster,
>                                      &acb->hd_qiov, acb->n,
> -                                    qcow_aio_write_cb, acb);
> +                                    qcow_aio_write_cb, acb, 2);
>   

2?

An alternative approach is to add a new op, bdrv_aio_barrier(), 
submitted immediately after the write.  It's probably more complicated 
overall.

A barrier for read can be meaningful if the guest wishes to read a 
known-to-be-stable write.  I don't think any guest can make use of this 
though.

-- 
error compiling committee.c: too many arguments to function