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From Kevin Burton <bur...@spinn3r.com>
Subject Re: Fetching ONE cell with a row cache hit takes 1 second on an idle box?
Date Wed, 02 Jul 2014 01:50:39 GMT
so.. caching the *queries* ?

it seems like a better mechanism would be to cache the actually logical
row…, not the physical row.

Query caches just don't work in production,  If you re-word your query, or
structure it a different way, you get a miss…

In my experience.. query caches have a 0% hit rate.


On Tue, Jul 1, 2014 at 6:40 PM, Robert Coli <rcoli@eventbrite.com> wrote:

> On Tue, Jul 1, 2014 at 6:06 PM, Kevin Burton <burton@spinn3r.com> wrote:
>
>> you know.. one thing I failed to mention.. .is that this is going into a
>> "bucket" and while it's a logical row, the physical row is like 500MB …
>> according to compaction logs.
>>
>> is the ENTIRE physical row going into the cache as one unit?  That's
>> definitely going to be a problem in this model.  500MB is a big atomic unit.
>>
>
> Yes, the row cache is a row cache. It caches what the storage engine calls
> rows, which CQL calls "partitions." [1] Rows have to be assembled from all
> of their row fragments in Memtables/SSTables.
>
> This is a big part of why the "off-heap" row cache's behavior of
> invalidation on write is so bad for its overall performance. Updating a
> single column in your 500MB row invalidates it and forces you to assemble
> the entire 500MB row from disk.
>
> The only valid use case for the current off-heap row cache seems to be :
> very small, very uniform in size, very hot, and very rarely modified.
>
> https://issues.apache.org/jira/browse/CASSANDRA-5357
>
> Is the ticket for replacing the row cache and its unexpected
> characteristics with something more like an actual query cache.
>
> also.. I assume it's having to do a binary search within the physical row
>> ?
>
>
> Since the column level bloom filter's removal in 1.2, the only way it can
> get to specific columns is via the index.
>
> =Rob
> [1] https://issues.apache.org/jira/browse/CASSANDRA-6632
>



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