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From Daning Wang <dan...@netseer.com>
Subject Re: Weird problem with empty CF
Date Thu, 29 Sep 2011 14:27:18 GMT
Jonathan/Aaron,

Thank you guy's reply, I will change GCGracePeriod to 1 day to see what will
happen.

Is there a way to purge tombstones at anytime? because if tombstones affect
performance, we want them to be purged right away, not after GCGracePeriod.
We know all the nodes are up, and we can do repair first to make sure the
consistency before purging.

Thanks,

Daning


On Wed, Sep 28, 2011 at 5:22 PM, aaron morton <aaron@thelastpickle.com>wrote:

> if I had to guess I would say it was spending time handling tombstones. If
> you see it happen again, and are interested, turn the logging up to DEBUG
> and look for messages from something starting with "Slice"
>
> Minor (automatic) compaction will, over time, purge the tombstones. Until
> then reads must read discard the data deleted by the tombstones. If you
> perform a big (i.e. 100k's ) delete this can reduce performance until
> compaction does it's thing.
>
> My second guess would be read repair (or the simple consistency checks on
> read) kicking in. That would show up in the "ReadRepairStage" in TPSTATS
>
> it may have been neither of those two things, just guesses. If you have
> more issues let us know and provide some more info.
>
> Cheers
>
>
> -----------------
> Aaron Morton
> Freelance Cassandra Developer
> @aaronmorton
> http://www.thelastpickle.com
>
> On 29/09/2011, at 6:35 AM, Daning wrote:
>
> > I have an app polling a few CFs (select first N * from CF), there were
> data in CFs but later were deleted so CFs were empty for a long time. I
> found Cassandra CPU usage was getting high to 80%, normally it uses less
> than 30%. I issued the select query manually and feel the response is slow.
> I have tried nodetool compact/repair for those CFs but that does not work.
> later, I issue 'truncate' for all the CFs and CPU usage gets down to 1%.
> >
> > Can somebody explain to me why I need to truncate an empty CF? and what
> else I could do to bring the CPU usage down?
> >
> > I am running 0.8.6.
> >
> > Thanks,
> >
> > Daning
> >
>
>

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