COW in Xen

COW in Xen

[Mark Ryden]

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 saw that there is a (rather small) wiki page about it :
http://wiki.xensource.com/xenwiki/COWHowTo

And also some instructions from Michael Lang for using
unionfs for COW for Xen:
http://wiki.xensource.com/xenwiki/InstructionsFromMichaelLang

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 ?

Regards,
Mark Ryden

RE: COW in Xen

[Rendon, Curtis]

> Message: 3
> Date: Mon, 5 Mar 2007 20:41:56 +0200
> From: "Mark Ryden" <markryde@gmail.com>
> Subject: [Xen-devel] COW in Xen
> To: xen-devel <xen-devel@lists.xensource.com>
> Message-ID:
> 	<dac45060703051041g69aa83fax1d983a1959bd795c@mail.gmail.com>
> 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
--
XEN of AMD

   Curtis W. Rendon  

Re: COW in Xen

[Marcel Ritter]

Rendon, Curtis schrieb:
> 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.
>   
If I got Mark right, it's not the image type qcow itself, he's
interested in,
but the COW (Copy-on-write) feature. Some time ago I tried to get
COW working with qcow images, without success. So I guess the name
is missleading, and (in Xen) there's no COW based on qcow.

I'd really like to get a simple file based COW - is this planned for
future Xen releases? Maybe someone can comment?

Bye,
   Marcel

RE: COW in Xen

[Ian Pratt]

> > 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.
> >
> If I got Mark right, it's not the image type qcow itself, he's
> interested in,
> but the COW (Copy-on-write) feature. Some time ago I tried to get
> COW working with qcow images, without success. So I guess the name
> is missleading, and (in Xen) there's no COW based on qcow.
> 
> I'd really like to get a simple file based COW - is this planned for
> future Xen releases? Maybe someone can comment?

AFAIK it works in xen-unstable, so will be in 3.0.5.

Ian

Re: COW in Xen

[Julian Chesterfield]

Marcel Ritter wrote:

>Rendon, Curtis schrieb:
>  
>
>>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.
>>  
>>    
>>
>If I got Mark right, it's not the image type qcow itself, he's
>interested in,
>but the COW (Copy-on-write) feature. Some time ago I tried to get
>COW working with qcow images, without success. So I guess the name
>is missleading, and (in Xen) there's no COW based on qcow.
>  
>
Not true. CoW functionality is supported in the current blktap driver. 
If you create a qcow disk on the host using qcow-create with a backing 
filename argument (can be either a raw image or block device) the driver 
opens the backing file for reading-only and directs all writes to the 
qcow child image.

- Julian

Re: COW in Xen

[Mark Ryden]

Hi,
First , thanks , Marcel

>If I got Mark right, it's not the image type qcow itself, he's
> interested in,
> but the COW (Copy-on-write) feature.

  You did indeed got me right; I am not talking specifically about
some format like qcow  itself but my question is general. As I said
before, it seems that there are many
Copy-on-Write solutions; at least according to the Xen wiki page on COW
(http://wiki.xensource.com/xenwiki/COWHowTo).
For example, Xen blktap, DmUserspace ,UnionFS, BenRin's Copy-on-Write
md device,Translucency, parallax, CowNFS , and LVM snapshots (and yet
I don't know yet whether MarkW XenFS should support cow).

I am not an expert on CoE. As I understand, there are two main
categories for Copy On Write solutions: file based solutions and block
based solutions.

It's obvious to me that there are some pros and cons for each method.
It seems to me that no doubt that when using a solution which should
patch the kernel
this can be thought of as a drawback by many. Also when the solution
is tightly coupled to a filesystem (like unionFS or CoWNFS) this maybe
can seem a drawback. (conmparing to block device solutions). So if
somebody can give a short analyze of pros and cons of the different
CoW techniques available for Xen
and the roadmap ahead this can make things better understood.
Regards,
Mark





On 3/6/07, Ian Pratt <m+Ian.Pratt@cl.cam.ac.uk> wrote:
> > > 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.
> > >
> > If I got Mark right, it's not the image type qcow itself, he's
> > interested in,
> > but the COW (Copy-on-write) feature. Some time ago I tried to get
> > COW working with qcow images, without success. So I guess the name
> > is missleading, and (in Xen) there's no COW based on qcow.
> >
> > I'd really like to get a simple file based COW - is this planned for
> > future Xen releases? Maybe someone can comment?
>
> AFAIK it works in xen-unstable, so will be in 3.0.5.
>
> Ian
>

Re: COW in Xen

[Mark Williamson]

> 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 saw that there is a (rather small) wiki page about it :
> http://wiki.xensource.com/xenwiki/COWHowTo

> And also some instructions from Michael Lang for using
> unionfs for COW for Xen:
> http://wiki.xensource.com/xenwiki/InstructionsFromMichaelLang
>
> 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,

I'm not sure anybody uses parallax (or if it's currently supported).

> CowNFS , and LVM snapshots (According to MichaelVrable wiki 
> as I understood it).

LVM snapshots are not good for long term COW usage - they use up dom0's memory 
and then things go wrong.  Better suited to doing backups after which the 
snapshot can be released.

> And BTW- does the XenFS has (or is intended to have) COW support ?

It's something that's well suited to its model and that I'd consider 
supporting at some stage.  Not yet though ;-)

Cheers,
Mark

> What are the pros and cons of these options ? is there a  preferred
> one when using Xen ?
>
> Regards,
> Mark Ryden
>
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> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel

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