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Re: [Qemu-devel] [PATCH] qcow2: Bring synchronous read/write back to lif

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH] qcow2: Bring synchronous read/write back to life
Date: Thu, 08 Oct 2009 09:30:47 -0500
User-agent: Thunderbird (X11/20090825)

Kevin Wolf wrote:
When the synchronous read and write functions were dropped, they were replaced
by generic emulation functions. Unfortunately, these emulation functions don't
provide the same semantics as the original functions did.

The original bdrv_read would mean that we read some data synchronously and that
we won't be interrupted during this read. The latter assumption is no longer
true with the emulation function which needs to use qemu_aio_poll and therefore
allows the callback of any other concurrent AIO request to be run during the

Perhaps you could create a mechanism to freeze the qcow2 image by queuing all completions within qcow2 until the image was unfrozen. This would have the same effect switching to synchronous read/write.

You may also have to queue new read/write requests...

Introducing sync read/write seems like a major step backwards to me.


Anthony Liguori

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