[jira] [Reopened] (CASSANDRA-8434) L0 should have a separate configurable bloom filter false positive ratio

["Benedict (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/CASSANDRA-8434?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Benedict reopened CASSANDRA-8434:
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Except that is not what I proposed.  I proposed having L0 always have a lower FP, ie that of STCS since this is the compaction strategy used for L0. This would translate into higher average values during overload, but not via any automation.  It's a bit anti democratic to declare the memory burden a showstopper without any dialogue, or apparently reading the ticket.

I have little vested interest in the outcome of this decision, but as in my last interaction, I abhor bad decision making processes; if a good process reaches a bad decision that's comparatively fine by me.

On a database smaller than 4Tb just 3 L0 files increases server read load by 10%.  On a database with tables smaller than 25Gb per node it is 15%.  This overload scales linearly with L0 file count. So you only need a very small number of files in L0 to start overloading the server by tens of percent of its normal workload, significantly lengthening the time period the server is overloaded for and hence how large L0 grows. As a result, the *number* of BF grows larger and you may easily reach equivalent memory pressure increase, eliminating any perceived advantage on that front. 

Conversely, on a cluster with any appreciable amount of data stored, a few extra sstables with larger BFs will make perhaps fractions of a percent difference to total memory consumption.

> L0 should have a separate configurable bloom filter false positive ratio
> ------------------------------------------------------------------------
>
>                 Key: CASSANDRA-8434
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-8434
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Benedict
>             Fix For: 2.1.x
>
>
> In follow up to CASSANDRA-5371. We now perform size-tiered file selection for compaction if L0 gets too far behind, however as far as I can tell we stick with the CF configured false positive ratio, likely inflating substantially the number of files we visit on average until L0 is cleaned up. Having a a different bf fp for L0 would solve this problem without introducing any significant burden when L0 is not overloaded.



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