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Posted to user@cassandra.apache.org by Kevin Burton <bu...@spinn3r.com> on 2011/08/20 02:41:30 UTC

memory overhead of vector clocks vs timestamps and running *without* either to save memory?

I have a few questions which I can't seem to find answers to...

I know that the memory overhead of timestamps is 8 bytes per row/column.
  What is the memory overhead of vector clocks?

Is it possible (at least in theory) to run without timestamps on your
values?  I'm fine with last writer wins semantics and the memory overhead
here seems very high.

I understand that the timestamps might also be for repartitioning which adds
complexity.

Thanks!

-- 

Founder/CEO Spinn3r.com

Location: *San Francisco, CA*
Skype: *burtonator*

Skype-in: *(415) 871-0687*

Re: memory overhead of vector clocks vs timestamps and running *without* either to save memory?

Posted by Jonathan Ellis <jb...@gmail.com>.
The problem with naive last write wins is that writes don't always
arrive at each replica in the same order.  So no, that's a
non-starter.

Vector clocks are a series of (client id, clock) entries, and usually
a timestamp so you can prune old entries.  Obviously implementations
can vary, but to pick a specific example, Voldemort [1] uses 2 bytes
per client id, a variable number (at least one) of bytes for the
clock, and 8 bytes for the timestamp.

[1] https://github.com/voldemort/voldemort/blob/master/src/java/voldemort/versioning/VectorClock.java

On Fri, Aug 19, 2011 at 7:41 PM, Kevin Burton <bu...@spinn3r.com> wrote:
> I have a few questions which I can't seem to find answers to...
> I know that the memory overhead of timestamps is 8 bytes per row/column.
>   What is the memory overhead of vector clocks?
> Is it possible (at least in theory) to run without timestamps on your
> values?  I'm fine with last writer wins semantics and the memory overhead
> here seems very high.
> I understand that the timestamps might also be for repartitioning which adds
> complexity.
> Thanks!
>
> --
>
> Founder/CEO Spinn3r.com
>
> Location: San Francisco, CA
> Skype: burtonator
>
> Skype-in: (415) 871-0687
>



-- 
Jonathan Ellis
Project Chair, Apache Cassandra
co-founder of DataStax, the source for professional Cassandra support
http://www.datastax.com