From: Stefan Hajnoczi <stefanha@redhat.com>
To: lihuiba <magazine.lihuiba@163.com>
Cc: kwolf@redhat.com, qiujian@meituan.com,
Fam Zheng <famz@redhat.com>,
qemu-devel@nongnu.org, mengcong@meituan.com
Subject: Re: [Qemu-devel] extremely low IOPS performance of QCOW2 image format on an SSD RAID1
Date: Mon, 23 Jun 2014 16:25:10 +0800 [thread overview]
Message-ID: <20140623082510.GA9640@stefanha-thinkpad.redhat.com> (raw)
In-Reply-To: <6db7bace.5114.146c761fa69.Coremail.magazine.lihuiba@163.com>
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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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next prev parent reply other threads:[~2014-06-23 8:25 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-06-23 2:06 [Qemu-devel] extremely low IOPS performance of QCOW2 image format on an SSD RAID1 lihuiba
2014-06-23 3:01 ` Fam Zheng
2014-06-23 3:14 ` lihuiba
2014-06-23 3:22 ` Fam Zheng
2014-06-23 6:20 ` lihuiba
2014-06-23 8:25 ` Stefan Hajnoczi [this message]
[not found] ` <225b854d.98bc.146ccc39a83.Coremail.magazine.lihuiba@163.com>
2014-06-24 10:15 ` Kevin Wolf
2014-06-23 7:30 ` Stefan Hajnoczi
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