"install devtools" in F22 live boot, device-mapper implosion

[Fabian Deutsch]

On Fri, Jul 31, 2015 at 5:28 AM, Adam Williamson
<adamwill at fedoraproject.org> wrote:
> On Thu, 2015-07-30 at 21:11 -0600, Chris Murphy wrote:
>> User over on users@ list reports booting Fedora 22 Workstation (live
>> media), and doing:
>>
>> # dnf install "Developer Tools"
>>
>> Implodes.
>>
>> I've reproduced the problem, and the gist is that there's a dm
>> snapshot that's too small for this task, it gets full quickly, the
>> file system face plants, and the kernel oopses. No recovery is
>> possible. Long version [1].
>>
>> Question 1: Should this work better and be a supported use case?
>
> My understanding has always been that installing packages on the live
> media is an unsupported, 'if you're lucky it might work for a few small
> ones' kind of thing.
>
> If someone wants to try and make it work better that's great, of
> course.

We could try to also use rd.live.fsimg=1 in addition to root=live:.
This will not give us more RAM, but can help sometimes.

IIRC root=live: will create loop mounts for the squashfs and create a
dm-snapshot to provide a writable fs.
The problem with this method is that it does not free memory after it
has been claimed. That means the memory consumption is determined by
the number of changes, not by the actual requirements.
In other words: Creating a 500MB file will acquire 500MB more, but
deleting it does not give that memory back.
This behavior can slowly eat up the memory over time.

If rd.live.fsimg=1 is used in addition, then the fs within the
squashfs will be copied to a tmpfs and going from there.
The benefit of this method is, that unused memory will be given back
to the system.
When we take the example from above, then the 500MB will be freed
again, once the file got removed.
The drawback is that you need enough memory to at least hold the
complete uncompressed liveimg.

- fabian

-- 
Fabian Deutsch <fdeutsch at redhat.com>
RHEV Hypervisor
Red Hat