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Talkback:114/kapil.html (2)

[ In reference to "Right To Your Own Devices" in LG#114 ]

Kapil Hari Paranjape [kapil at imsc.res.in]


Tue, 26 Feb 2008 07:00:01 +0530

Hello,

----- Forwarded message from lee hughes <[email protected]> -----

Date: Sun, 24 Feb 2008 15:09:19 +0000
From: lee hughes <[email protected]>
To: [email protected]
Subject: tkb:Device Mapper?
Helllo,

There still not any good device mapper documentation out there, you page was a great help to me!

I am trying to do this.

I have a loopback filesystem, that I am trying to snapshot.

so

myfile--------/dev/loop0

cow1--------/dev/loop1 cow2--------/dev/loop2 cow3--------/dev/loop3 cow4--------/dev/loop4

you document allowed me to a snapshot of /dev/loop0, and cow the changes to /dev/loop1

can, I do the same with loop2,3,4?

i.e. I have 4 file system cow, from the same filesystem?

hope you can help, thanks for a great article.

----- End forwarded message -----


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Kapil Hari Paranjape [kapil at imsc.res.in]


Tue, 26 Feb 2008 07:01:35 +0530

Hello,

On Sun, 24 Feb 2008, lee hughes wrote:

> There still not any good device mapper documentation out there, you page was
> a great help to me!

Glad to be of help.

> I am trying to do this.
> 
>  I have a loopback filesystem, that I am trying to snapshot.
> 
> so
> 
> myfile--------/dev/loop0
> 
> cow1--------/dev/loop1
> cow2--------/dev/loop2
> cow3--------/dev/loop3
> cow4--------/dev/loop4
> 
> you document allowed me to a snapshot of /dev/loop0, and cow the changes to
> /dev/loop1
> 
> can, I do the same with loop2,3,4?
> 
> i.e. I have 4 file system cow, from the same filesystem?

As long as you write only to the "cow" devices in each case (i.e. /dev/loop0 is "read-only") there is no problem.

By the way, since you raised this question based on the article in LG (I presume) it would be nice if I could re-direct this discussion the "TAG" mailing list of that publication. Do I have your permission?

Regards,

Kapil. --


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Kapil Hari Paranjape [kapil at imsc.res.in]


Tue, 26 Feb 2008 07:02:51 +0530

----- Forwarded message from lee hughes <[email protected]> -----

Date: Sun, 24 Feb 2008 19:14:07 +0000
From: lee hughes <[email protected]>
To: Kapil Hari Paranjape <[email protected]>
Subject: tkb:Re: Device Mapper?
Hmm, I still don't understand it myself!!

so, can my other device use the same cow file? or do I have to one cow file per filesystem I want to mount r/w?

yeah, TAG Mailing, you have my permission...!

----- End forwarded message -----


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Kapil Hari Paranjape [kapil at imsc.res.in]


Tue, 26 Feb 2008 07:11:24 +0530

Hello,

On Sun, 24 Feb lee hughes <[email protected]> wrote:

> Hmm, I still don't understand it myself!!
> 
> so, can my other device use the same cow file? or do I have to one cow file
> per filesystem
> I want to mount r/w?

You need different cow files/devices.

Each cow contains the record of all changes that you want to make to the read-only device (call it a bull[1]) device that it goes with.

So you can have many cows to one bull. But ultimately it is the cow which you are reading/writing from and so it determines the file-system you are actually viewing. (Which is not so un-PC after all!)

[1] Bull for unwrita-bull (sorry couldn't resist!)

Regards,

Kapil. --


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