[Xen-devel] Questions on Copy-on-Write

["Yan-Ching CHU" <cs0u210a@xxxxxxxxxxxxxxx>]

Thanks for your input Bin.

I would love to know your progress of CoW for linux kernel 2.4, will you
integrate it into Xen later? Any plan for a Linux 2.6 port? Any
papers/materials I can refer to?

Regarding the Copy-on-Write NFS is the group using it with Xen? Any one has
experience/comments of using it with Xen?

Cheers,
Yan-Ching CHU




----- Original Message ----- 
From: "Bin Ren" <br260@xxxxxxxxx>
To: "Yan-Ching CHU" <cs0u210a@xxxxxxxxxxxxxxx>
Cc: "Devel Xen" <xen-devel@xxxxxxxxxxxxxxxxxxxxx>
Sent: Thursday, January 01, 1970 1:10 AM
Subject: Re: Problems about Xen's Howto


On 2 Feb 2004, at 11:23, Yan-Ching CHU wrote:

> Hi Bin,
>
> I have got some problem starting new domains following the Xen
> Howto v1.1 that I hope you can help. In both the README and your howto
> there is no explicit statement about rather new domains can/should be
> started using another filesystem. I wonder if I use the same root as
> Domain0 is using to start new domains, rather the original filesystem
> will be destroyed due to multiple writers to the filesystem.

I'm sorry I didn' t make it clear. You are absolutely right that each
domain
should use a different root filesystem.

> If I really need new fileysystem for every new domain, what minimum
> set of services/files about the boot process (inittab, init rc,
> daemons) do I need to have? The whole thing in the DemoCD is quite big
> (especially with /usr) that I don't want to install each copy of them
> to every domain I want to start. Any ideas/comments?

One independent root filesystem for each domain requires too much space,
this is a major problem of today's Xen that we are trying to tackle.
The method
is to use Copy on Write, e.g. different domains share the same files as
many
as possible and retains modified files separately. CoW can be done both
in
user space and in kernel space.

Russ has implemented a CoW NFS and I'm at the last staging of writing a
CoW multi-disk driver for linux kernel 2.4 series.

Before CoW features become stable and robust, you can:

(1) duplicate root filesystems for each domain (Yes, I can hear you
screaming...)
(2) share /usr among all the domains to save a lot of space (Yes, let's
relax...)
(3) get a new hard disk and go to either (1) or (2)

Thanks,
Bin



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