[arch-releng] [archiso] saying good-bye to union-fs-method?

Thomas Bächler thomas at archlinux.org
Tue May 31 12:07:36 EDT 2011

Am 31.05.2011 17:08, schrieb Gerardo Exequiel Pozzi:
> On 05/31/2011 07:54 AM, Thomas Bächler wrote:
>> Am 31.05.2011 03:21, schrieb Gerardo Exequiel Pozzi:
>>> Hi
>>> * With 2.6.39 there is no aufs2 support, at least for a long time[#1].
>>> * I think that back in time with unionfs is not desirable, needs a
>>> complete kernel patch.
>>> * union-fs-fuse does not work good.
>>> What about using the method like in Fedora? (basically a device-mapper
>>> with a snapshot in tmpfs).
>>> This change from file-to-file level to block-to-block logic.
>> This actually sounds pretty cool. However, I don't understand how it
>> works: The squashfs file system is a read-only one, how can we put that
>> into a block-level snapshot?
> Yes, but there are at least one downside: since is not an union-fs,
> there is no concept of layers. We are currently using "overlay" thats
> overlap some files on the layer "root-image".

This is not problematic.

> About your question: The squasfs image contains only one file... an
> image of and 4 GiB ext4. There is another small squasfs image with
> another one file inside that is a "lvm snapshot". So dm device is made
> via these images loopback mounted...

Some of this doesn't make sense to me right now (especially the second
squashfs image).

However: With an ext4 loopback inside the squashfs image you loose one
squashfs feature: metadata compression. I don't know how bad that will
be though.

I'll summarize what I think is going on: You mount the squashfs (loop0),
and set up the ext4 image inside as loop1. You then create a large
sparse file on tmpfs and set it up as loop2. Then, you create a snapshot
device (can't use LVM here, can one use the device-mapper snapshot
target directly?) with loop1 as read-only and loop2 as read-write layer.
You then mount that device. Sounds doable, but not optimal (a VFS-based
solution would be way cooler).

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