From: Eric Blake <eblake@redhat.com>
To: Liu Qing <liuqing@huayun.com>,
qemu-devel@nongnu.org, qemu block <qemu-block@nongnu.org>
Subject: Re: [Qemu-devel] reduce write bandwidth of qcow2 driver while allocating new cluster
Date: Mon, 28 Aug 2017 10:46:34 -0500 [thread overview]
Message-ID: <7327a956-aef3-a8da-13ff-53073039802f@redhat.com> (raw)
In-Reply-To: <20170828055631.GB3258@host-172-16-90-85.openstacklocal>
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[adding qemu-block]
On 08/28/2017 12:56 AM, Liu Qing wrote:
> Dear list,
> Recently I used fio to test qcow2 driver in the guest os, and found out
> that when a new cluster is allocated the 4K IO will occupy 64K(default cluster
> size) bandwith.
> From the code qcow2 driver will fill the unused part of new allocated
> cluster with 0 in perform_cow. These 0s are set in qcow2_co_readv when the read
> destination is not allocated and it has no backing file. Could I forbidden any
> further write in copy_sectors if the copy source is not allocated and it has
> no backing file? So only the requested data is written to the cluster. Function
> copy_sectors is only used by perform_cow in the master branch.
There have already been discussions on optimizing COW writes in a manner
similar to what you are describing; for example,
https://lists.gnu.org/archive/html/qemu-devel/2017-08/msg00109.html
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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next prev parent reply other threads:[~2017-08-28 15:46 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-08-28 5:56 [Qemu-devel] reduce write bandwidth of qcow2 driver while allocating new cluster Liu Qing
2017-08-28 15:46 ` Eric Blake [this message]
2017-08-29 2:56 ` Liu Qing
2017-08-30 10:15 ` Anton Nefedov
2017-08-31 6:55 ` Liu Qing
2017-09-04 13:17 ` Anton Nefedov
2017-09-05 3:32 ` Liu Qing
2017-08-28 21:40 ` John Snow
2017-08-29 3:05 ` Liu Qing
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