Todd Lipcon commented on KUDU-1567:

Put some work on this here: https://gerrit.cloudera.org/#/c/4470/

> Short default for log retention increases write amplification
> -------------------------------------------------------------
>                 Key: KUDU-1567
>                 URL: https://issues.apache.org/jira/browse/KUDU-1567
>             Project: Kudu
>          Issue Type: Improvement
>          Components: perf, tserver
>    Affects Versions: 0.10.0
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
> Currently the maintenance manager prioritizes flushes over compactions if the 
> flush operations are retaining WAL segments. The goal here is to prevent the 
> amount of in-memory data from getting so large that restarts would be 
> incredibly slow. However, it has a somewhat unintuitive negative effect on 
> performance:
> - with the default of retaining just two segments, flushes become highly 
> prioritized when the MRS only has ~128MB of data, regardless of the 
> "flush_threshold_mb" configuration
> - this creates lots of overlapping rowsets in the case of random-write 
> applications
> - because flushes are prioritized over compactions, compactions rarely run
> - the frequent flushes, combined with low priority of compactions, means that 
> after a few days of constant inserts, we often end up with average "bloom 
> lookups per op" metrics of 50-100, which is quite slow even if the blooms fit 
> in cache.

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