...making Linux just a little more fun!
[ In reference to "Right To Your Own Devices" in LG#114 ]
Kapil Hari Paranjape [kapil at imsc.res.in]
Hello,
----- Forwarded message from lee hughes <toxicnaan@gmail.com> -----
Date: Sun, 24 Feb 2008 15:09:19 +0000 From: lee hughes <toxicnaan@gmail.com> To: kapil@imsc.res.in 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 -----
Kapil Hari Paranjape [kapil at imsc.res.in]
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. --
Kapil Hari Paranjape [kapil at imsc.res.in]
----- Forwarded message from lee hughes <toxicnaan@gmail.com> -----
Date: Sun, 24 Feb 2008 19:14:07 +0000 From: lee hughes <toxicnaan@gmail.com> To: Kapil Hari Paranjape <kapil@imsc.res.in> 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 -----
Kapil Hari Paranjape [kapil at imsc.res.in]
Hello,
On Sun, 24 Feb lee hughes <toxicnaan@gmail.com> 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. --