[linux-lvm] read-only media in a LVM?

Sean P. Kane spkane at genomatica.com
Mon Jun 23 18:54:02 UTC 2003

I'd love to have a unionfs, but my understanding is that no such beast
exists for Linux 2.4 currently and won't likely appear until after the
2.6 release. If this is not the case, I would love to hear about it. Is
there some decent (although maybe beta) support for a unionfs in the 2.5
kernels? I am using the LVM2 backport to 2.4 currently to test the
device-mapper approach out in the meantime, but if there is a better way
to do it, I'd love to hear about it.


Sean P. Kane
spkane at genomatica.com
Lead Infrastructure Architect
Genomatica, Inc.
"When we destroy something man has created we call it vandalism......
 When we destroy something that Gaia has created we call it progress."

-----Original Message-----
From: Kevin P. Fleming [mailto:kpfleming at cox.net] 
Sent: Monday, June 23, 2003 15:41
To: linux-lvm at sistina.com
Subject: Re: [linux-lvm] read-only media in a LVM?

Sean P. Kane wrote:

> Ok, let me clarify a bit. I mis-spoke, when I spoke of memory I meant 
> ramdisk (fs). This is basically what I was actually visualizing in my
> head:
> 1) Boot cdrom into a Linux instance.
> 2) Mount a read-write filesystem into memory (this is why I asked the 
> size issue, obviously coping the whole cdrom to memory wouldn't be
> practical)
> 3) Create a snapshot (I'm am really new to lvm, so I'm still learning 
> what all this means), which would be a small reference table of some 
> sort, plus 32k blocks of changed data. 4)This would then allow 
> "changes" to the cdrom while running in that session until the ram 
> disk filled up? Correct?
> 5) If a file is created then deleted in an environment like this will 
> the space it was using in the snapshot fs be released?

This is not really an application for LVM snapshots at all. Snapshots
continue to use more and more memory if those files are created and
unless the underlying filesystem is very intelligent about space
(which the iso9660 filesystem is very likely not).

This is really an application for a "union" mount. This could also be
done with 
a "bind" mount, if the files that need changing can be isolated to a
number of directories. If they are spread all over the filesystem, or
locations are not known ahead of time, a "union" mount would do what you
overlaying a tmpfs ram-based filesystem on top of the CD-ROM filesystem.

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