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From: Aaron Turner <synfinatic@gmail.com>
Date: Mon, 1 Oct 2012 19:56:37 +0100
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Subject: Re: read-repair and deletes / forgotten deletes
To: user@cassandra.apache.org
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inline...

On Mon, Oct 1, 2012 at 7:46 PM, Hiller, Dean <Dean.Hiller@nrel.gov> wrote:
> Thanks, (actually new it was configurable) BUT what I don't get is why I
> have to run a repair.  IF all nodes became consistent on the delete, it
> should not be possible to get a forgotten delete, correct.  The forgotten
> delete will only occur if I have a node down and out for 10 days and it
> comes back online because by then the nodes no longer have the delete
> anymore and the new node has data so getting to a consistent state the
> node with data would win.
>
> Soooo, if I run repair say every 20 days, isn't it true, I would have no
> problems as long as I did not have a node outage?

Basically if you know for certain that you have 100% uptime for all
your nodes and you haven't lost any updates due to overload/etc then
you don't need to run repair.  You run repair to ensure that all the
tombstones are replicated to all the necessary nodes prior to gc_grace
so that the data doesn't come back from the dead after a compaction.
Generally speaking it's a lot safer/easier to just always run repair.

> And most importantly, does anyone know of an automated tool for running
> repairs every X days(this should really be an automated/schedulable
> thing)???

I use a cron job.  It's a good idea to use the '-pr' flag btw.  Also,
you only need to run repair against CF's which actually have deletes.

-- 
Aaron Turner
http://synfin.net/         Twitter: @synfinatic
http://tcpreplay.synfin.net/ - Pcap editing and replay tools for Unix & Windows
Those who would give up essential Liberty, to purchase a little temporary
Safety, deserve neither Liberty nor Safety.
    -- Benjamin Franklin
"carpe diem quam minimum credula postero"