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From Daniel Doubleday <daniel.double...@gmx.net>
Subject Re: Row cache
Date Thu, 30 Jun 2011 16:03:15 GMT
Here's my understanding of things ... (this applies only for the regular heap implementation
of row cache)

> Why Cassandra does not cache a row that was requested few times? 

What does the cache capacity read. Is it > 0?

> What the ReadCount attribute in ColumnFamilies indicates and why it remains zero. 

Hm I had that too one time (read count wont go up while there were reads). But I didn't have
the time to debug.

> How can I know from where Cassandra read a row (from MEMTable,RowCache or SSTable)? 

It will always read from 
row cache or 
memtable(s) and sstable(s)

jmx should tell you (hits go up)

> does the following correct? In read operation Cassandra looks for the row in the MEMTable
- if not found it looks in the row-cache - if not found it looks in SSTable (after looking
in the key-cache to optimize the access to the SSTable)? 

No. 

If row cache capacity is > 0 then a read will check if the row is in cache if not it read
the entire row and cache it. Then / or if row was in cache already it will read from there
and apply the respective filter to the cached CF.   Writes update memtable and row cache when
the row is cached. I must admit that I still dont quite understand why there's no race here.
I haven't found any cache lock. So someone else should explain why a concurrent read / write
cannot produce a lost update in the cached row.

If capacity is 0 then it will read from the current memtable, the memtable(s) that are being
flushed and all sstables that may contain the row (filtered by bloom filter)

Hope that's correct and helps.

Cheers,
Daniel


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