Consider this a +1 for pretty much every point that Matthew made, and articulated much
better than I would have.
I wholly understand the goal with making ceph more approachable for people learning it,
and I think that is a great cause and deserves attention.
I also think Tim Serong's Linux.conf.au talk "Containers are hideously
undebuggable black boxes and we never should have invented them
<https://www.youtube.com/watch?v=pPZsN_urpqw>" was spot on with this problem
scenario, in that debugging is a very different path with multiple layers of abstraction.
He obviously details the solution(s), but I think it highlights a lot of the friction with
moving to container based ceph as compared to package based ceph.
Reed
On Jun 2, 2021, at 4:36 AM, Matthew Vernon
<mv3(a)sanger.ac.uk> wrote:
Hi,
In the discussion after the Ceph Month talks yesterday, there was a bit of chat about
cephadm / containers / packages. IIRC, Sage observed that a common reason in the recent
user survey for not using cephadm was that it only worked on containerised deployments. I
think he then went on to say that he hadn't heard any compelling reasons why not to
use containers, and suggested that resistance was essentially a user education
question[0].
I'd like to suggest, briefly, that:
* containerised deployments are more complex to manage, and this is not simply a matter
of familiarity
* reducing the complexity of systems makes admins' lives easier
* the trade-off of the pros and cons of containers vs packages is not obvious, and will
depend on deployment needs
* Ceph users will benefit from both approaches being supported into the future
We make extensive use of containers at Sanger, particularly for scientific workflows, and
also for bundling some web apps (e.g. Grafana). We've also looked at a number of
container runtimes (Docker, singularity, charliecloud). They do have advantages - it's
easy to distribute a complex userland in a way that will run on (almost) any target
distribution; rapid "cloud" deployment; some separation (via namespaces) of
network/users/processes.
For what I think of as a 'boring' Ceph deploy (i.e. install on a set of dedicated
hardware and then run for a long time), I'm not sure any of these benefits are
particularly relevant and/or compelling - Ceph upstream produce Ubuntu .debs and Canonical
(via their Ubuntu Cloud Archive) provide .debs of a couple of different Ceph releases per
Ubuntu LTS - meaning we can easily separate out OS upgrade from Ceph upgrade. And
upgrading the Ceph packages _doesn't_ restart the daemons[1], meaning that we maintain
control over restart order during an upgrade. And while we might briefly install packages
from a PPA or similar to test a bugfix, we roll those (test-)cluster-wide, rather than
trying to run a mixed set of versions on a single cluster - and I understand this
single-version approach is best practice.
Deployment via containers does bring complexity; some examples we've found at Sanger
(not all Ceph-related, which we run from packages):
* you now have 2 process supervision points - dockerd and systemd
* docker updates (via distribution unattended-upgrades) have an unfortunate habit of
rudely restarting everything
* docker squats on a chunk of RFC 1918 space (and telling it not to can be a bore), which
coincides with our internal network...
* there is more friction if you need to look inside containers (particularly if you have
a lot running on a host and are trying to find out what's going on)
* you typically need to be root to build docker containers (unlike packages)
* we already have package deployment infrastructure (which we'll need regardless of
deployment choice)
We also currently use systemd overrides to tweak some of the Ceph units (e.g. to do some
network sanity checks before bringing up an OSD), and have some tools to pair OSD /
journal / LVM / disk device up; I think these would be more fiddly in a containerised
deployment. I'd accept that fixing these might just be a SMOP[2] on our part.
Now none of this is show-stopping, and I am most definitely not saying "don't
ship containers". But I think there is added complexity to your deployment from going
the containers route, and that is not simply a "learn how to use containers"
learning curve. I do think it is reasonable for an admin to want to reduce the complexity
of what they're dealing with - after all, much of my job is trying to automate or
simplify the management of complex systems!
I can see from a software maintainer's point of view that just building one container
and shipping it everywhere is easier than building packages for a number of different
distributions (one of my other hats is a Debian developer, and I have a bunch of machinery
for doing this sort of thing). But it would be a bit unfortunate if the general thrust of
"let's make Ceph easier to set up and manage" was somewhat derailed with
"you must use containers, even if they make your life harder".
I'm not going to criticise anyone who decides to use a container-based deployment
(and I'm sure there are plenty of setups where it's an obvious win), but if I were
advising someone who wanted to set up and use a 'boring' Ceph cluster for the
medium term, I'd still advise on using packages. I don't think this makes me a
luddite :)
Regards, and apologies for the wall of text,
Matthew
[0] I think that's a fair summary!
[1] This hasn't always been true...
[2] Simple (sic.) Matter of Programming
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