From: Stefan Hajnoczi <stefanha@gmail.com>
To: 浩樊啊 <fanh2010@foxmail.com>
Cc: qemu-devel <qemu-devel@nongnu.org>
Subject: Re: [Qemu-devel] 回复： some confusion on qemu i/o pocess and the qcow2format
Date: Wed, 2 Dec 2015 21:29:50 +0800 [thread overview]
Message-ID: <20151202132950.GA29511@stefanha-x1.localdomain> (raw)
In-Reply-To: <tencent_04C6AF085D5F3396311ECC83@qq.com>
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On Tue, Dec 01, 2015 at 03:14:14PM +0800, 浩樊啊 wrote:
> I read the source code of qcow2.
> I can see bdrv_co_do_readv---bdrv_co_do_preadv---bdrv_aligned_preadv---drv->bdrv_co_readv (here drv.bdrv_co_readv=qcow2_co_readv)
> but when I read qcow2_co_readv->bdrv_co_readv->bdrv_co_do_readv->bdrv_co_do_preadv
> I am confused about this. qcow2_co_readv use bdrv_co_do_preadv again?
The qcow2 BlockDriverState uses a file BlockDriverState to do the actual
I/O. This way the qcow2 image format can be used on POSIX files, iSCSI
LUNs, Gluster files, etc.
It means that qcow2 code does not call preadv(2)/pwritev(2) directly but
instead uses another BlockDriverState to perform the actual I/O. In
QEMU we keep "image format" block drivers (e.g. qcow2, vmdk, vhdx)
separate from "protocol" block drivers (e.g. POSIX, Win32, Gluster,
Sheepdog, iSCSI).
Stefan
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next prev parent reply other threads:[~2015-12-02 13:30 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-11-20 3:21 [Qemu-devel] some confusion on qemu i/o pocess and the qcow2 format =?gb18030?B?usa3rrCh?=
2015-11-25 9:44 ` Stefan Hajnoczi
[not found] ` <tencent_04C6AF085D5F3396311ECC83@qq.com>
2015-12-02 13:29 ` Stefan Hajnoczi [this message]
2015-12-21 2:48 ` [Qemu-devel] =?gb18030?b?u9i4tKO6ICC72Li0o7ogIHNvbWVjb25mdXNpb24g?= =?gb18030?q?on_qemu_i/o_pocess_and_the_qcow2format?= =?gb18030?B?usa3rrCh?=
2015-12-22 6:35 ` [Qemu-devel] 回复： 回复： someconfusion on qemu i/o pocess and the qcow2format Stefan Hajnoczi
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