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"Cassandra can write 50GB of data in 0.12 milliseconds ..."

These numbers have been taken from p.21 of the linked PDF by the Facebook engineer Avinash Lakshman where it shows this MySQL vs Cassandra comparison:

"MySQL > 50GB Data Writes Average : ~300ms Reads Average : ~350ms Cassandra > 50GB Data Writes Average : 0.12ms Reads Average : 15ms"

Which I'm guessing actually means: with a database table which is more than 50GB total size they have measured individual row accesses at these speeds. Maybe Avinash has accidentally transposed the read and write figures - I can't think why the read would be slower than the write.

15ms to write a row to disk seems possible given that a fast modern disk

http://www.seagate.com/docs/pdf/datasheet/disc/ds_cheetah_15...

has a latency of 2ms and seek time of 4ms and a sustained transfer rate of about 100MB/s. The track to track time is only 0.4ms so maybe if you just wrote all the data to disk serial-log-style you could reconstruct from the log after a failure and handle all reads from memory. I don't know Cassandra. Obviously, from these figures, the disk couldn't do a row read in 0.12ms.




Cassandra uses log-structured merge storage, so reads really _are_ slower than writes. (But still much faster than MySQL!)

You can (usually) make reads faster by throwing things like memcached at the problem. Writes are harder. So I think this is the right tradeoff for a modern system.

/Cassandra dev




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