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From Paul Chandler <p...@redshots.com>
Subject Re: TWCS and gc_grace_seconds
Date Fri, 18 Oct 2019 13:34:24 GMT
Hi Adarsh,

You will have problems if you manually delete data when using TWCS.

To fully understand why, I recommend reading this The Last Pickle post: https://thelastpickle.com/blog/2016/12/08/TWCS-part1.html
And this post I wrote that dives deeper into the problems with deletes: http://www.redshots.com/cassandra-twcs-must-have-ttls/

Thanks 

Paul

> On 18 Oct 2019, at 14:22, Adarsh Kumar <adarsh0007@gmail.com> wrote:
> 
> Thanks Jeff,
> 
> 
> I just checked with business and we have differences in having TTL. So it will be manula
purging always. We do not want to use LCS due to high IOs.
> So:
> As the use case is of time series data model, TWCS will be give some benefit (without
TTL) and with frequent deleted data
> Are there any best practices/recommendations to handle high number of tombstones 
> Can we handle this use case  with STCS also (with some configurations)
> 
> Thanks in advance
> 
> Adarsh Kumar
> 
> On Fri, Oct 18, 2019 at 11:46 AM Jeff Jirsa <jjirsa@gmail.com <mailto:jjirsa@gmail.com>>
wrote:
> Is everything in the table TTL’d? 
> 
> Do you do explicit deletes before the data is expected to expire ? 
> 
> Generally speaking, gcgs exists to prevent data resurrection. But ttl’d data can’t
be resurrected once it expires, so gcgs has no purpose unless you’re deleting it before
the ttl expires. If you’re doing that, twcs won’t be able to drop whole sstables anyway,
so maybe LCS will be less disk usage (but much higher IO)
> 
>> On Oct 17, 2019, at 10:36 PM, Adarsh Kumar <adarsh0007@gmail.com <mailto:adarsh0007@gmail.com>>
wrote:
>> 
>> 
>> Hi,
>> 
>> We have a use case of time series data with TTL where we want to use TimeWindowCompactionStrategy
because of its better management for TTL and tombstones. In this case, data we have is frequently
deleted so we want to reduce gc_grace_seconds to reduce the tombstones' life and reduce pressure
on storage. I have following questions:
>> Do we always need to run repair for the table in reduced gc_grace_seconds or there
is any other way to manage repairs in this vase
>> Do we have any other strategy (or combination of strategies) to manage frequently
deleted time-series data
>> Thanks in advance.
>> 
>> Adarsh Kumar


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