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From "Terje Marthinussen (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-2540) Data reads by default
Date Fri, 29 Apr 2011 06:37:03 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-2540?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13026880#comment-13026880
] 

Terje Marthinussen commented on CASSANDRA-2540:
-----------------------------------------------

Would it make sense to make dynamic snitch "react" to "digest but not data" error directly
to improve recovery time and why do we need to wait for the 100ms to make dynamic snitch work?

Couldn't we add a load balancer like function that would detect if a node has a significant
number of outstanding requests on node 1 vs. 2 and 3, then send to node 2 instead?

Overall, I am not asking for a round robin load balancer though... (not good for caching)

The digest function may also in some cases maybe be made dynamic based on the size of the
data being read. That is, just send the data for small data sizes and use digests for large
responses?

No, I don't know all the details on how this part of the code work, so my suggestions may
be totally wrong :)

I do wonder however if we get enough of these timeouts to actually be a problem. If there
is a couple of delays in latencies for a few seconds 2-3 times a week/month, no problem. 

However, if we have so many of these errors that people see them many times a day, it would
seem like this is a performance problem somewhere in cassandra which should be fixed rather
than applying some patchwork which hide it... 

> Data reads by default
> ---------------------
>
>                 Key: CASSANDRA-2540
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-2540
>             Project: Cassandra
>          Issue Type: Wish
>            Reporter: Stu Hood
>            Priority: Minor
>
> The intention of digest vs data reads is to save bandwidth in the read path at the cost
of latency, but I expect that this has been a premature optimization.
> * Data requested by a read will often be within an order of magnitude of the digest size,
and a failed digest means extra roundtrips, more bandwidth
> * The [digest reads but not your data read|https://issues.apache.org/jira/browse/CASSANDRA-2282?focusedCommentId=13004656&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13004656]
problem means failing QUORUM reads because a single node is unavailable, and would require
eagerly re-requesting at some fraction of your timeout
> * Saving bandwidth in cross datacenter usecases comes at huge cost to latency, but since
both constraints change proportionally (enough), the tradeoff is not clear
> Some options:
> # Add an option to use digest reads
> # Remove digest reads entirely (and/or punt and make them a runtime optimization based
on data size in the future)
> # Continue to use digest reads, but send them to {{N - R}} nodes for (somewhat) more
predicatable behavior with QUORUM
> \\
> The outcome of data-reads-by-default should be significantly improved latency, with a
moderate increase in bandwidth usage for large reads.

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