Re: [Qemu-devel] extremely low IOPS performance of QCOW2 image format on an SSD RAID1

[Stefan Hajnoczi]

On Mon, Jun 23, 2014 at 02:20:25PM +0800, lihuiba wrote:
> I think I have found the reason:
> There's a cache in qemu that accelerates the transform of virtual LBA to 
> cluster offset of qcow2 image.
> The cache has a fixed size of 16x8192=128k in my configuration, which 
> corresponds to a 8GB (128K*64KB)
> mapping size. So when the "working set" of fio exceeds 8GB, the transform wil 
> be degraded to reading
> external table, and the performances goes extremely low.

Can you confirm that making L2_CACHE_SIZE much bigger solves the
problem?

You also have the optional of specifying the cluster size when creating
the qcow2 image file.  A larger cluster size reduces the amount of
metadata overhead and therefore increases cache hits.

Stefan

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