Re: [Xen-devel] COW in Xen

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> 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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-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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