incubator-cassandra-user mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Brandon Williams <dri...@gmail.com>
Subject Re: Dynamic Snitch / Read Path Questions
Date Mon, 13 Dec 2010 20:15:32 GMT
On Sun, Dec 12, 2010 at 10:49 AM, Daniel Doubleday <daniel.doubleday@gmx.net
> wrote:

> Hi again.
>
> It would be great if someone could comment whether the following is true or
> not.
> I tried to understand the consequences of using
> |-Dcassandra.dynamic_snitch=true for the read path |and that's what I came
> up with:
>
> 1) If using CL > 1 than using the dynamic snitch will result in a data read
> from node with the lowest latency (little simplified) even if the proxy node
> contains the data but has a higher latency that other possible nodes which
> means that it is not necessary to do load-based balancing on the client
> side.
>

No.  If the coordinator node is part of the replica set, the dynamic snitch
will fall back to the wrapped snitch for ordering, since it does not track
latencies of itself.  This likely means it will return the data.


> 2) If using CL =1 than the proxy node will always return the data itself
> even when there is another node with less load.
>

Yes, same as above.


> 3) Digest requests will be sent to all other living peer nodes for that key
> and will result in a data read on all nodes to calculate the digest. The
> only difference is that the data is not sent back but IO-wise it is just as
> expensive.
>

Yes.


> The next one goes a little further:
>
> We read / write with quorum / rf = 3.
>
> It seems to me that it wouldn't be hard to patch the StorageProxy to send
> only one read request and one digest request. Only if one of the requests
> fail we would have to query the remaining node. We don't need read repair
> because we have to repair once a week anyways and quorum guarantees
> consistency. This way we could reduce read load significantly which should
> compensate for latency increase by failing reads. Am I missing something?
>

Just turn off read repair in that case.

-Brandon

Mime
View raw message