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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