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Re: [Qemu-devel] [PATCH 0/4] qcow2: Improve cluster refcount update

Subject: Re: [Qemu-devel] [PATCH 0/4] qcow2: Improve cluster refcount update
From: Anthony Liguori
Date: Thu, 06 Nov 2008 12:53:11 -0600
Laurent Vivier wrote:
This series of patches improves qcow2 performance with O_DIRECT
when the qcow2 file is empty and we begin to fill it.

I really dislike this series and any future series that does similar things. The real problem is that non-aligned accesses are so slow, but there's no reason that they must be so slow. It's because we're doing synchronous IO operations instead of using posix-aio like we should. That's the real problem. Just doing a memory copy to an aligned buffer is not going to cause that much performance delay.

We shouldn't be adding more cruft to the code base to avoid fixing the real problem. I think the current implementation of O_DIRECT needs to be rewritten.


Anthony Liguori

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