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Subject: Re: How to determine if repair need to be run
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From: Jonathan Colby <jonathan.colby@gmail.com>
In-Reply-To: <AANLkTi=tRx9qyEwz4TQfaLM5FNtNw1m6U398Jn+e45JJ@mail.gmail.com>
Date: Thu, 31 Mar 2011 23:24:03 +0200
Cc: user@cassandra.apache.org
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To: Peter Schuller <peter.schuller@infidyne.com>

Peter -

Thanks a lot for elaborating on repairs.    Still, it's a bit fuzzy to =
me why it is so important to run a repair before the GCGraceSeconds =
kicks in.   Does this mean a delete does not get "replicated" ?   In =
other words when I delete something on a node, doesn't cassandra set =
tombstones on its replica copies?

And technically, isn't repair only needed for cases where things weren't =
properly propogated in the cluster?  If all writes are written to the =
right replicas, and all deletes are written to all the replicas, and all =
nodes were available at all times, then everything should work as =
designed -  without manual intervention, right?

Thanks again.


On Mar 31, 2011, at 6:17 PM, Peter Schuller wrote:

>> silly question, would every cassandra installation need to have =
manual repairs done on it?
>>=20
>> It would seem cassandra's "read repair" and regular compaction would =
take care of keeping the data clean.
>>=20
>> Am I missing something?
>=20
> See my previous posts in this thread for the distinct reasons to run
> repair. Except in special circumstances where you know exactly what
> you're doing (mainly that no deletes are performed), you are
> *required* to run repair often enough for GGraceSeconds:
>=20
>   =
http://wiki.apache.org/cassandra/Operations#Frequency_of_nodetool_repair
>=20
> It seems that there needs to be some more elaborate documentation
> about this somewhere to point to since there seems to be confusion.
>=20
> Regular compaction does *not* imply repair. Read repair only works if
> (1) you touch all data within GCGraceSeconds, and (2) you touch it in
> such a way that read repair is enabled (e.g., not range scans), and
> (3) no node ever happens to be down, flap, or drop a request when you
> touch the data in question.
>=20
> Basically, unless you are really sure what you're doing - run repair.
>=20
> --=20
> / Peter Schuller