fragile Live Fedora ? Device-mapper Snapshot vs. Layered Fs solution (AUFS or Overlayfs)

[Chris Murphy]

On Thu, Jul 30, 2015 at 5:26 PM, derek <denc716 at gmail.com> wrote:
> On Thu, Jul 30, 2015 at 11:24 AM, Chris Murphy <lists at colorremedies.com> wrote:
>> Could be this:
>> https://bugzilla.redhat.com/show_bug.cgi?id=1220950
>>
>> However I don't see the same snapshot message in that bug, that you get.
>>
>
> No, it's different.
>
> This bug is as easy to reproduce: run Fedora 22 Live Mode on laptop or VM,
> it boots default into liveuser, try a `sudo dnf install 'Development
> Tools'` then kernel crash,
> from console I collected the dm-0 IO Errors.

Yes it's reproducible. The problem is not inherently with
device-mapper. It's a configuration issue where the snapshot is too
small and it's a fixed size, doesn't grow. So it fills up, the file
system face plants, then kernel oopses.

The untested workaround for this for baremetal install is to re-create
the installer media using livecd-iso-to-disk with the
--overlay-size-mb option and set to something like 1000. Now there
will be a live overlay of sufficient size for this installation.

For VM, just install the OS and then install the tools post-install.


> This is always reproducible that makes me think Fedora Live Mode is
> not usable at all, or maybe no users at all.

Your snark amuses me. I might try to recruit you for QA.


> On Thu, Jul 30, 2015 at 11:28 AM, Chris Murphy <lists at colorremedies.com> wrote:
>>
>> Also, overlayfs is really recently added to the kernel. It's much much
>> newer than device mapper stuff. So I wouldn't call it immature. You
>> might be thinking of the thin provisioning snapshots which is much
>> newer, but still has been around longer than overlayfs. But I don't
>> think the live media is using thin snapshots (?) I think they're the
>> conventional (thick) type.
>
> The fact is with overlayfs (or aufs from 2014 Ubuntu release) Live
> Linux you can apt-get install whatever (write to /) as much data as
> possible; till it becomes full and returns -ENOSPACE (and continues
> usable if clean some data), on this Fedora Live OS with device-mapper
> and simple sudo dnf install a few packages crashes kernel. Which is
> more mature?

Feel free to ask a kernel developer. Based on development time and
testing, device-mapper has a lot more of both compared to overlayfs.
On that basis alone I think device-mapper is more mature by a lot.

The failure of your use case with Fedora is a configuration related
issue rather than device-mapper vs overlayfs. Whether your use case
should be better supported I don't know, so I asked on the devel@
list. And in the footnote are the technical details on why this fails.

https://lists.fedoraproject.org/pipermail/devel/2015-July/212956.html

-- 
Chris Murphy