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RE: [Xen-devel] COW in Xen

  • To: xen-devel@xxxxxxxxxxxxxxxxxxx
  • From: "Rendon, Curtis" <Curtis.Rendon@xxxxxxx>
  • Date: Mon, 5 Mar 2007 13:53:39 -0600
  • Delivery-date: Mon, 05 Mar 2007 11:52:55 -0800
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: AcdfXpyyfG/7/xINSriTEK0yvnQRuAAAE94w
  • Thread-topic: [Xen-devel] COW in Xen

> Message: 3
> Date: Mon, 5 Mar 2007 20:41:56 +0200
> From: "Mark Ryden" <markryde@xxxxxxxxx>
> Subject: [Xen-devel] COW in Xen
> To: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
> Message-ID:
>       <dac45060703051041g69aa83fax1d983a1959bd795c@xxxxxxxxxxxxxx>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> Hello,
> I had posted this question in Xen-user mailing list three 
> days ago and I did not
> get a single answer. On a second thought, maybe xen-devel is more
> appropriate so
> I am trying again in the hope that I will receive an answer:
> Does anybody have an experience (good or bad) with using
> COW (Copy-On-Write) storage in Xen ?

I've been using qcow images generated by qemu-img for Linux and
MSWIndows HVM guests, and I've been getting good results. The image
files are much smaller that the equivalently sized raw images.

I simply create them (via gemi-img create -f qcom 10G) and use the
disk=['file:...'] line in my guest configuration files.

> My question is :
> There are some altenatives which are discussed in that wiki page:
> unionfs, blktap ,using qemu: see
> http://lxr.xensource.com/lxr/source/tools/blktap/,
> BenRin's Copy-on-Write md device,Translucency , DmUserspace
> ,parallax, CowNFS , and LVM snapshots (According to MichaelVrable wiki
> as I understood it).
> And BTW- does the XenFS has (or is intended to have) COW support ?
> What are the pros and cons of these options ? is there a  preferred
> one when using Xen ?

I can't answer all the questions you've posted as I haven't experimented
that much. The pros are smaller image files, the cons are those of the
speed loss in a loopback file image, and mounting the file directly as a
loopback within linux is a problem. The latter is possible I believe,
but after playing with it a bit I had to move on to testing XEN...


   Curtis W. Rendon  

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