From: Christoph Hellwig <hch@lst.de>
To: Kevin Wolf <kwolf@redhat.com>
Cc: aliguro@us.ibm.com, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>,
Stefan Hajnoczi <stefanha@gmail.com>,
qemu-devel@nongnu.org, ryanh@us.ibm.com,
Zhi Yong Wu <zwu.kernel@gmail.com>,
Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
Subject: Re: [Qemu-devel] Why qemu write/rw speed is so low?
Date: Sun, 11 Sep 2011 15:32:54 +0200 [thread overview]
Message-ID: <20110911133254.GA4055@lst.de> (raw)
In-Reply-To: <4E6A1CD7.70300@redhat.com>
On Fri, Sep 09, 2011 at 04:04:07PM +0200, Kevin Wolf wrote:
> > need to read in the existing 4 KB, modify the 512 bytes in place, and
> > write out the 4 KB block again. This is read-modify-write. In this
> > worst-case scenario a 512 byte write turns into a 4 KB read followed
> > by a 4 KB write.
>
> But that should only happen with a 4k sector size, otherwise there's no
> reason for RMW.
The might not be a need for RMW, but if you're doing 512 byte writes to a
sparse file on 4k filesystem that filesystem will have to serialize the
I/O to prevent races from happening during block allocation.
next prev parent reply other threads:[~2011-09-11 13:32 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-09-09 9:44 [Qemu-devel] Why qemu write/rw speed is so low? Zhi Yong Wu
2011-09-09 10:38 ` Stefan Hajnoczi
2011-09-09 13:48 ` Zhi Yong Wu
2011-09-09 13:54 ` Stefan Hajnoczi
2011-09-09 14:04 ` Kevin Wolf
2011-09-09 15:27 ` Stefan Hajnoczi
2011-09-11 13:32 ` Christoph Hellwig [this message]
2011-09-09 14:09 ` Zhi Yong Wu
2011-09-13 2:38 ` Zhi Yong Wu
2011-09-13 2:52 ` Zhi Yong Wu
2011-09-13 7:14 ` Stefan Hajnoczi
2011-09-13 9:25 ` Zhi Yong Wu
2011-09-13 10:14 ` Stefan Hajnoczi
2011-09-13 10:27 ` Zhi Yong Wu
2011-09-14 2:42 ` Zhi Yong Wu
2011-09-14 14:17 ` Stefan Hajnoczi
2011-09-15 9:04 ` Zhi Yong Wu
2011-09-13 7:15 ` Stefan Hajnoczi
2011-09-13 8:31 ` Zhi Yong Wu
2011-09-13 8:49 ` Stefan Hajnoczi
2011-09-13 8:54 ` Zhi Yong Wu
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