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From Edward Capriolo <edlinuxg...@gmail.com>
Subject Re: nodetool repair
Date Sun, 15 Jul 2012 13:47:46 GMT
Great job sleuthing.

Originally repair did not have a -pr. When you run the standard repair
the node compares it's data with its neighbours and vice versa. They
also send each other updates. Since you are supposed to repair every
node < gc_grace submitting a full repair to each node would create
duplicated work, since a repair on node A has an effect on node B and
node C.

If you want to understand this some more you should run
compactionstats and netstats across your cluster while a repair is
going on, then you can see what effect the commands have on other
nodes.

I will try to write up some documentation on it as well because -pr is
a nice feature. Many may not even be expressly aware of it.

On Sat, Jul 14, 2012 at 2:00 PM, Michael Theroux <mtheroux2@yahoo.com> wrote:
> Hello,
>
> I'm looking at nodetool repair with the "-pr", vs. non "-pr" option.  Looking around,
I'm seeing a lot of conflicting information out there.  Almost universally, the recommendation
is to run nodetool repair with the "-pr" for any day-to-day maintenance.
>
> This is my understanding of how it works.  I appreciate any corrections to my misinformation.
>
> nodetool repair -pr
>
> - This performs a repair on the "primary range" of the node.  The primary range is essentially
the part of the ring that the node is responsible for.  When this command is run, synchronization
of replicas will occur for the rows that this node is responsible for.  If replicas are missing
from that node's neighbors for those rows, they will be replicated.
>
> nodetool repair
>
> - This is where I see a lot of conflicting information.  I see a lot of answers in which
there is a suggestion that this command will perform a repair across the entire cluster. 
However, I don't believe this is true from my observations (and some of the items I read seems
to agree with this).  Instead, this command performs synchronization of your primary range,
but also for other ranges that this node maybe responsible for in a replica capacity.  The
way I'm thinking about it is that the -pr option causes repairs to push information from its
primary range to replicas.  Without -pr, nodetool replair does a push, and pull for its neighbors
that this node maybe a replica for.  This makes sense to me, as people recommend running nodetool
repair after a node has been down.  This is to allow the downed node to get any missed information
that should have been replicated to it while it was down.
>
> I'm sure there lots of flaws to the above understanding as I'm cobbling it together.
 I appreciate the feedback,
>
> -Mike

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