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From Jonathan Ellis <jbel...@gmail.com>
Subject Re: KeysCached and sstable
Date Wed, 14 Apr 2010 23:47:09 GMT
On Wed, Apr 14, 2010 at 10:23 AM, Paul Prescod <paul@prescod.net> wrote:
> The inline docs say:
>
>       ~ The optional KeysCached attribute specifies
>       ~ the number of keys per sstable whose locations we keep in
>       ~ memory in "mostly LRU" order.
>
> There are a few confusing bits in that sentence.
>
>  1. Why is "keys per sstable" rather than "keys per column family". If
> I have 7 SSTable files and I set KeysCached to 10000, will I have
> 70000 keys cached? If so, why? What is the logical relationship here?

This is out of date, it's per CF now.

>  2. What makes the algorithm "mostly LRU" rather than just LRU?

it's called second-chance eviction, discussed at
http://code.google.com/p/concurrentlinkedhashmap/wiki/ProductionVersion

>  3. Is it accurate the say that the goal of the Key Cache is to avoid
> looking through a bunch off SSTable's Bloom Filters?

No.  it's to avoid deserializing the rows from the sstables and merging them.

-Jonathan

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