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Re: [Qemu-devel] [PATCH 0/2] improve qemu-img conversion performance

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH 0/2] improve qemu-img conversion performance
Date: Thu, 8 Sep 2011 08:56:28 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

On Wed, Sep 07, 2011 at 04:06:51PM -0700, Yehuda Sadeh wrote:
> The following set of patches improve the qemu-img conversion process
> performance. When using a higher latency backend, small writes have a
> severe impact on the time it takes to do image conversion. 
> We switch to using async writes, and we avoid splitting writes due to
> holes when the holes are small enough.
> Yehuda Sadeh (2):
>   qemu-img: async write to block device when converting image
>   qemu-img: don't skip writing small holes
>  qemu-img.c |   34 +++++++++++++++++++++++++++-------
>  1 files changed, 27 insertions(+), 7 deletions(-)
> -- 

This has nothing to do with the patch itself, but I've been curious
about the existence of both a QEMU and a Linux kernel rbd block driver.

The I/O latency with qemu-img has been an issue for rbd users.  But they
have the option of using the Linux kernel rbd block driver, where
qemu-img can take advantage of the page cache instead of performing
direct I/O.

Does this mean you intend to support both QEMU block/rbd.c and Linux
drivers/block/rbd.c?  As a user I would go with the Linux kernel driver
instead of the QEMU block driver because it offers page cache and host
block device features.  On the other hand a userspace driver is nice
because it does not require privileges.


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