Alan Robertson wrote:
Kevin Tomlinson wrote:
On Tue, 2007-11-06 at 10:25 -0700, Alan Robertson wrote:
We now have the ComponentFail test in CTS. Thanks Lars for getting
it going!
And, in the process, it's showing up some kinds of problems that we
hadn't been looking for before. A couple examples of such problems
can be found here:
http://old.linux-foundation.org/developer_bugzilla/show_bug.cgi?id=1762
http://old.linux-foundation.org/developer_bugzilla/show_bug.cgi?id=1732
The question that comes up is this:
For problems that should "never" happen like death of one of our
core/key processes, is an immediate reboot of the machine the right
recovery technique?
<snip>
Here's the issue:
The solution as I see it is to do one of:
a) reboot the node and clear the problem with certainty
I'm well aware that saying "yeah, we did it that way" isn't Good Form,
but, well, we did it that way (also we did 'd', see below). I refer to
existing proprietary HA products in which I was involved designing and
implementing.
What we found was that certain processes were indistinguishable from the
node itself and their failure was therefore near impossible to deal with
cleanly. The problem was, as described here, that the OS and
applications/services would continue on but the other nodes in the
cluster would see it as a node failure and take recovery actions (as you
describe here).
This was admin-controllable, as we did offer something else...
b) continue on and risk damaging your disks.
c) write some new code to recover from specific cases more
gracefully and then test it thoroughly.
d) Try and figure out how to propagate the failure to the
top layer of the cluster, and hope you get the notice
there soon enough so that it can "freeze" the cluster
before the code reacts to the apparent failure
and begins to try and recover from it.
Our architecture was that our clients were in a 'process group' across
the cluster (group services) where each was connected to our server
process via a unix domain socket on the same node. Across the cluster
these process group peers were assumed to be controlling resources and
they mediated recovery actions through group services.
The unix-domain socket breaking was (well, still is) a defined condition
that the client was told to handle as our process dying. They were
told in this case to immediately clean up and assume that their peers
would assume that the local process was gone since the node had died and
would be doing takeovers.
This was meant to allow some hope of avoiding taking the node down for
real, depending on the application space. The intent was that the local
client would notice the death immediately, while the remote ones would
take some time (i.e., lack of heartbeats, etc.) to notice.
In cases where the node wasn't taken down, via inittab or similar our
processes would get automatically restarted, and we'd reintegrate into
the cluster. The local client process(es) would be expected to
reconnect and have to rejoin their group(s). Our interface manual
described all of this.
If for some reason our processes couldn't restart (or inittab gave up
because of too many retries) that node would stay out of the cluster.
In the current code, sometimes you'll get behavior (a) and sometimes
you'll get behavior (b) and sometimes you'll get behavior (c).
In the particular case described by bug 1762, failure to reboot the node
did indeed start the same resource twice. In a cluster where you have
shared disk (like yours for example), that would probably trash the
filesystem. Not a good plan unless you're tired of your current job
;-). I'd like to take most/all of the cases where you might get
behavior (b) and cause them to use behavior (a).
If writing correct code and testing it were free, then (c) would
obviously be the right choice.
Quite honestly, I don't know how to do (d) in a reliable way at all.
It's much more difficult than it sounds. Among other reasons, it relies
on the components you're telling to freeze things to work correctly.
Since resource freezes happen at the top level of the system, and the
top layers need all the layers under them to work correctly, getting
this right seems to be the kind of approach you could make into your
life's work - and still never get it right.
You're right. In the scheme I described above we (group services)
simply washed our hands of what our clients (layers above) were able to
do and get right... We didn't write those. We offered this as the only
thing we could think of, with the hope that some clients could do things
correctly. It assumes that the OS, for example, is still working so the
client can take dependable actions.
And if they weren't confident they could enable node rebooting in this
case and let recovery happen 'normally'.
Case (c) has to be handled on a case by case basis, where you write and
test the code for a particular failure case. IMHO the only feasible
_general_ answer is (a).
There are an infinite number of things that can go wrong. So, having a
reliable and general strategy to deal with the WTF's of the world is a
good thing. Of course, for those cases where we have a (c) behavior
would not be affected by this change in general policy.
Peter
--
Peter R. Badovinatz aka 'Wombat'
STG Telecommunications, Linux & Standards
preferred: [EMAIL PROTECTED] / alternate: [EMAIL PROTECTED]
These are my opinions and absolutely not official opinions of IBM, Corp.
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