(CC'ing ceph-users)
On 07/28/2014 12:34 PM, Marc wrote:
Hi,
This said, if out of 3 monitors you have 2 monitors down, your cluster
will cease functioning (no admin commands, no writes or reads served).
This is not entirely true. (At least) RBDs will continue being fully
functional even if the mon quorum is lost. This only applies to RBDs
that are already mounted (qemu) at the time of quorum loss though.
Meaning: (K)VMs running off of Ceph will remain fully functional even if
the mon quorum is lost (assuming you havent lost too many OSDs at the
same time).
True. Clients will maintain the connections they have to OSDs for about
15 minutes or so, at which point timeouts will go off and all work will
be halted. New clients won't be able to do this though, as they have to
grab maps from the monitors prior to connecting to OSDs, and the monitor
will not serve those requests if quorum is not in place.
-Joao
On 28/07/2014 12:22, Joao Eduardo Luis wrote:
On 07/28/2014 08:49 AM, Christian Balzer wrote:
Hello,
On Sun, 27 Jul 2014 18:20:43 -0400 Robert Fantini wrote:
Hello Christian,
Let me supply more info and answer some questions.
* Our main concern is high availability, not speed.
Our storage requirements are not huge.
However we want good keyboard response 99.99% of the time. We
mostly do
data entry and reporting. 20-25 users doing mostly order , invoice
processing and email.
* DRBD has been very reliable , but I am the SPOF . Meaning that when
split brain occurs [ every 18-24 months ] it is me or no one who knows
what to do. Try to explain how to deal with split brain in advance....
For the future ceph looks like it will be easier to maintain.
The DRBD people would of course tell you to configure things in a way
that
a split brain can't happen. ^o^
Note that given the right circumstances (too many OSDs down, MONs down)
Ceph can wind up in a similar state.
I am not sure what you mean by ceph winding up in a similar state. If
you mean regarding 'split brain' in the usual sense of the term, it does
not occur in Ceph. If it does, you have surely found a bug and you
should let us know with lots of CAPS.
What you can incur though if you have too many monitors down is cluster
downtime. The monitors will ensure you need a strict majority of
monitors up in order to operate the cluster, and will not serve requests
if said majority is not in place. The monitors will only serve requests
when there's a formed 'quorum', and a quorum is only formed by (N/2)+1
monitors, N being the total number of monitors in the cluster (via the
monitor map -- monmap).
This said, if out of 3 monitors you have 2 monitors down, your cluster
will cease functioning (no admin commands, no writes or reads served).
As there is no configuration in which you can have two strict
majorities, thus no two partitions of the cluster are able to function
at the same time, you do not incur in split brain.
If you are a creative admin however, you may be able to enforce split
brain by modifying monmaps. In the end you'd obviously end up with two
distinct monitor clusters, but if you so happened to not inform the
clients about this there's a fair chance that it would cause havoc with
unforeseen effects. Then again, this would be the operator's fault, not
Ceph itself -- especially because rewriting monitor maps is not trivial
enough for someone to mistakenly do something like this.
-Joao
--
Joao Eduardo Luis
Software Engineer | http://inktank.com | http://ceph.com
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