On 03/04/2016 08:36 PM, Shyam wrote:
On 03/04/2016 07:30 AM, Pranith Kumar Karampuri wrote:
On 03/04/2016 05:47 PM, Bipin Kunal wrote:
HI Pranith,
Thanks for starting this mail thread.
Looking from a user perspective most important is to get a "good copy"
of data. I agree that people use replication for HA but having stale
data with HA will not have any value.
So I will suggest to make auto quorum as default configuration even
for 2-way replication.
If user is willing to lose data at the cost of HA, he always have
option disable it. But default preference should be data and its
integrity.
I think we need to consider *maintenance* activities on the volume,
like replacing a brick in a replica pair, or upgrading one half of the
replica and then the other, at which time the replica group would
function read-only, if we choose 'auto' in a 2-way replicated state,
is this correct?
Yes.
Having said the above, we already have the option in place, right? I.e
admins can already choose 'auto', it is just the default that we are
discussing. This could also be tackled via documentation/best
practices ("yeah right! who reads those again?" is a valid comment here).
Yes. I just sent a reply to Jeff, where I told it is better to have
interactive question at the time of creating 2-way replica volume which
gives this information :-).
I guess we need to be clear (in documentation or otherwise) what they
get when they choose one over the other (like the HA point below and
also upgrade concerns etc.), irrespective of how this discussion ends
(just my 2 c's).
Totally agree. We will give an interactive question above, a link which
gives detailed explanation.
That is the point. There is an illusion of choice between Data integrity
and HA. But we are not *really* giving HA, are we? HA will be there only
if second brick in the replica pair goes down. In your typical
@Pranith, can you elaborate on this? I am not so AFR savvy, so unable
to comprehend why HA is available if only when the second brick goes
down and is not when the first does. Just helps in understanding the
issue at hand.
Because it is client side replication there is a fixed *leader* i.e. 1st
brick.
As a side note. We recently had a discussion with NSR team (Jeff, avra).
We will be using some infra for NSR to implement server side afr as well
with leader election etc.
Pranith
deployment, we can't really give any guarantees about what brick will go
down when. So I am not sure if we can consider it as HA. But I would
love to hear what others have to say about this as well. If majority of
users say they need it to be auto, you will definitely see a patch :-).
Pranith
Thanks,
Bipin Kunal
On Fri, Mar 4, 2016 at 5:43 PM, Ravishankar N <ravishankar@xxxxxxxxxx>
wrote:
On 03/04/2016 05:26 PM, Pranith Kumar Karampuri wrote:
hi,
So far default quorum for 2-way replication is 'none' (i.e.
files/directories may go into split-brain) and for 3-way replication
and
arbiter based replication it is 'auto' (files/directories won't go
into
split-brain). There are requests to make default as 'auto' for 2-way
replication as well. The line of reasoning is that people value data
integrity (files not going into split-brain) more than HA
(operation of
mount even when bricks go down). And admins should explicitly change
it to
'none' when they are fine with split-brains in 2-way replication. We
were
wondering if you have any inputs about what is a sane default for
2-way
replication.
I like the default to be 'none'. Reason: If we have 'auto' as quorum
for
2-way replication and first brick dies, there is no HA.
+1. Quorum does not make sense when there are only 2 parties. There
is no
majority voting. Arbiter volumes are a better option.
If someone wants some background, please see 'Client quorum' and
'Replica 2
and Replica 3 volumes' section of
http://gluster.readthedocs.org/en/latest/Administrator%20Guide/arbiter-volumes-and-quorum/
-Ravi
If users are fine with it, it is better to use plain distribute
volume
rather than replication with quorum as 'auto'. What are your
thoughts on the
matter? Please guide us in the right direction.
Pranith
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