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Re: [Qemu-discuss] Understanding QEMU Cache Modes and Emulated Disk Writ

From: Fam Zheng
Subject: Re: [Qemu-discuss] Understanding QEMU Cache Modes and Emulated Disk Write Cache
Date: Wed, 17 Aug 2016 19:43:13 +0800
User-agent: Mutt/1.5.21 (2010-09-15)

On Wed, 08/17 17:26, wyang wrote:
> On 2016年08月17日 15:50, Fam Zheng wrote:
> >
> >I am confused about the difference between the guest disk write cache and the
> >host page cache. Aren't these effectively the same (in both cases storing the
> >dirty data in the host's RAM)? Is there a difference between how one gets
> >Yes they are the same thing.
> >
> >Fam
> >
> Fam, if they are the same thing, how to understand "cache=none" as
> the host page cache is disabled and guest disk write cache is
> enabled for the guest when setting none?

Even though host page cache is disabled with O_DIRECT, the host disk can have a
write back cache, which matters in the case of power loss. That's why we have
to report WCE=1 to guest, so it knows FLUSH is needed.


> http://www.ibm.com/support/knowledgecenter/linuxonibm/liaat/liaatbpkvmguestcache.htm
> Thanks
> Wei

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