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From Tyler Hobbs <ty...@datastax.com>
Subject Re: TTL and gc_grace_period
Date Fri, 05 Jun 2015 15:35:35 GMT
On Fri, Jun 5, 2015 at 10:30 AM, Kévin LOVATO <klovato@alprema.com> wrote:

>
> I have a column family with data (metrics) that is never overwritten and
> only deleted using TTLs, and I am wondering if it would be reasonable to
> have a very low gc_grace_period (even 0) on that CF. I would like to do
> that mainly to save space and also to prevent tombstone scanning.
>

Yes, you can safely lower gc_grace_seconds.  You would only _not_ want to
lower gc_grace_seconds if you did deletes or overwrote cells with a lower
TTL.


>
> From what I understand of what I could read online, when an expired TTLed
> column is compacted, it is replaced by a tombstone, so having
> gc_grace_period would prevent that. Although this would allow the
> appearance of ghost/zombie columns.
>
> The question I'm trying to answer here is the following: Would those ghost
> columns be able to appear, and if so, would it be a problem, since they
> would themselves be marked as "expired"?
>

You don't need to worry about expired data being revived because every node
that has a copy of that data will have the same TTL.


-- 
Tyler Hobbs
DataStax <http://datastax.com/>

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