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From MichaƂ Michalski <mich...@opera.com>
Subject Re: cassandra disk access
Date Wed, 07 Aug 2013 08:27:05 GMT

> 2. when cassandra lookups a key in sstable (assuming bloom-filter and other
> "stuff" failed, also assuming the key is located in this single sstable),
> cassandra DO NOT USE sequential I/O. "She" probably will read the
> hash-table slot or similar structure, then cassandra will do another disk
> seek in order to get the value (and probably the key). Also probably there
> will need another seek, if there is key collision there will need
> additional seeks.

It will use the Index Sample (RAM) first, then it will use "full" Index 
(disk) and finally it will read data from SSTable (disk). There's no 
such thing like "collision" in this case.

> 3. once the data (e.g. the row) is located, a sequential read for entire
> row will occur. (Once again I assume there is single well compacted
> sstable). Also if disk is not fragmented, the data will be placed on disk
> sectors one after the other.

Yes, this is how I understand it too.

M.


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