From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:50581) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1VH946-0007Sf-A3 for qemu-devel@nongnu.org; Wed, 04 Sep 2013 05:08:00 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1VH93x-0008NR-LN for qemu-devel@nongnu.org; Wed, 04 Sep 2013 05:07:54 -0400 Date: Wed, 4 Sep 2013 17:07:48 +0800 From: xuanmao_001 References: <2013090411470490667813@163.com>, <20130904080040.GA3562@dhcp-200-207.str.redhat.com> Mime-Version: 1.0 Message-ID: <2013090417074860959027@163.com> Content-Type: multipart/alternative; boundary="----=_001_NextPart581381320813_=----" Subject: Re: [Qemu-devel] question about qemu disk cache mode Reply-To: xuanmao_001 List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: qemu-devel , qemu-discuss This is a multi-part message in MIME format. ------=_001_NextPart581381320813_=---- Content-Type: text/plain; charset="gb2312" Content-Transfer-Encoding: base64 SSB1bmRlcnN0YW5kIHRoZSBwaHlzaWNhbCBkaXNrIGNhY2hlIGFuZCBob3N0IHBhZ2UgY2FjaGUu DQoNCkkgd2FudCB0byB0aGUgZGlmZmVyZW5jZSBiZXR3ZWVuIGd1ZXN0IGRpc2sgd3JpdGUgY2Fj aGUgYW5kIGhvc3QgcGFnZSBjYWNoZSB0aGF0IGRlc2NyaWJlZCB3aXRoICJDYWNoaW5nIG1vZGVz IGluIFFlbXUiIGluDQpodHRwczovL2V2ZW50cy5saW51eGZvdW5kYXRpb24ub3JnL3NsaWRlcy8y MDExL2xpbnV4Y29uLWphcGFuL2xjajIwMTFfaGFqbm9jemkucGRmDQoNCg0KZ2l2ZSBtZSBzb21l IG1vcmUgaW5mb3JtYXRpb24gcGxlYXNlLCB0aGFua3MuDQoNCg0KDQoNCnh1YW5tYW9fMDAxDQoN CkZyb206IEtldmluIFdvbGYNCkRhdGU6IDIwMTMtMDktMDQgMTY6MDANClRvOiB4dWFubWFvXzAw MQ0KQ0M6IHFlbXUtZGV2ZWw7IHFlbXUtZGlzY3Vzcw0KU3ViamVjdDogUmU6IHF1ZXN0aW9uIGFi b3V0IHFlbXUgZGlzayBjYWNoZSBtb2RlDQpBbSAwNC4wOS4yMDEzIHVtIDA1OjQ3IGhhdCB4dWFu bWFvXzAwMSBnZXNjaHJpZWJlbjoNCj4gRGVhciBxZW11ZXJzOg0KPiAgDQo+IG15IHFlbXUta3Zt IHZlcnNpb24gaXMgMS4wLjENCj4gSSB3b3VsZCBsaWtlIHRvIGZpZ3VyZSBvdXQgdGhlIHFlbXUg ZGlzayBjYWNoZSBtb2RlLiBJIGhhdmUgdmlzaXRlZCB0aGUNCj4gcWVtdS1vcHRpb25zLmh4DQo+ IHRoZXJlIGlzIHR3byBjYWNoZSB0aGF0IEkgZGlkbid0IHVuZGVyc3RhbmQ6IHRoZSBob3N0IGNh Y2hlIHBhZ2UgYW5kIHRoZSBxZW11DQo+IGRpc2sgd3JpdGUgY2FjaGUuDQo+ICANCj4gSXMgImhv c3QgcGFnZSBjYWNoZSIgb25seSB1c2VkIGZvciByZWFkLiBhbmQgInFlbXUgZGlzayB3cml0ZSBj YWNoZSIgdXNlZCBmb3INCj4gd3JpdGluZy4NCj4gIA0KPiB3aGljaCBjYWNoZSB0aGUgZGF0YSBy ZWFjaGVkIGZpcnN0PyBob3N0IHBhZ2UgY2FjaGUgb3IgcWVtdSBkaXNrIHdyaXRlIGNhY2hlPw0K DQpZb3UncmUgcHJvYmFibHkgbWlzdW5kZXJzdGFuZGluZyB0aGUgbGF0dGVyLCBJIGFzc3VtZSB0 aGF0IHdoYXQgeW91J3ZlDQpyZWFkIGFib3V0IGlzIHRoZSAiZGlzayBjYWNoZSIsIG5vdCBhICJx ZW11IGRpc2sgY2FjaGUiLiBUaGlzIGlzIHRoZQ0KY2FjaGUgb24geW91ciBoYXJkd2FyZSwgdGhl IHBoeXNpY2FsIGhhcmQgZGlzay4gVGhlIGhvc3QgcGFnZSBjYWNoZSBpcw0KdGhlIGNhY2hpbmcg dGhhdCB0aGUgTGludXgga2VybmVsIGRvZXMgZm9yIGV2ZXJ5IGZpbGUgKHVubGVzcyBpdCdzDQpi eXBhc3NlZCB3aXRoIE9fRElSRUNULCB3aXRoIGlzIGV4cG9zZWQgYXMgY2FjaGU9bm9uZS9kaXJl Y3RzeW5jIGluDQpxZW11KS4gTm9uZSBvZiB0aGlzIGlzIGltcGxlbWVudGVkIGluIG9yIHNwZWNp ZmljIHRvIHFlbXUuDQoNCldoZW4geW91IHdyaXRlIGRhdGEsIGl0IHJlYWNoZXMgdGhlIHBhZ2Ug Y2FjaGUgaW4gdGhlIGtlcm5lbCBmaXJzdCAoaWYNCml0IGlzIHVzZWQgYXQgYWxsKSwgYW5kIHRo ZW4gdGhlIGNhY2hlIG9uIHRoZSBoYXJkIGRpc2suDQoNCkhvcGUgdGhpcyBoZWxwcy4NCg0KS2V2 aW4= ------=_001_NextPart581381320813_=---- Content-Type: text/html; charset="gb2312" Content-Transfer-Encoding: quoted-printable
I understand the physical disk cache and host page cache.
 
I want to the difference between guest disk write cache and host page= cache=20 that described with "Caching modes in Qemu" in
 
xuanmao_001
 
From: Kevin Wolf<= /DIV>
Date: 2013-09-04 16:00
Subject: Re: question about qemu disk cache=20 mode
Am 04.09.2013 um 05:47 hat xuanmao_001 = geschrieben:
> Dear qemuers:
>  
> my qemu-kvm version is 1.0.1
> I would like to figure out th= e qemu disk cache mode. I have visited&= nbsp;the
> qemu-options.hx
> there is two cache that I did= n't understand: the host cache page and = ;the qemu
> disk write cache.
>  
> Is "host page cache" only used&nbs= p;for read. and "qemu disk write cache" = ;used for
> writing.
>  
> which cache the data reached first= ? host page cache or qemu disk write&nb= sp;cache?
 
You're probably misunderstanding the latter, = ;I assume that what you've
read about is the "disk cache", not&nbs= p;a "qemu disk cache". This is the
cache on your hardware, the physical ha= rd disk. The host page cache is
the caching that the Linux kernel does&= nbsp;for every file (unless it's
bypassed with O_DIRECT, with is exposed = ;as cache=3Dnone/directsync in
qemu). None of this is implemented in&n= bsp;or specific to qemu.
 
When you write data, it reaches the&nbs= p;page cache in the kernel first (if
it is used at all), and then the&n= bsp;cache on the hard disk.
 
Hope this helps.
 
Kevin
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