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From "Marcus Eriksson (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-8635) STCS cold sstable omission does not handle overwrites without reads
Date Thu, 22 Jan 2015 08:26:35 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-8635?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14287107#comment-14287107
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Marcus Eriksson commented on CASSANDRA-8635:
--------------------------------------------

Hmm, yeah, this is probably the wrong approach - compacting cold data with hot data is likely
to always be a bad solution. Maybe we should just do a standard compaction of any overlapping
cold data if there are no 'hot' compactions to do? Ie, if hot sstables are empty, just return
the most overlapping cold ones?

bq. scrap the whole don't-compact-cold-sstables in STCS
Not all data models fit DTCS though and this gives users atleast a small benefit without remodelling
their data. Do we expose any metrics on how many sstables we consider being cold?

> STCS cold sstable omission does not handle overwrites without reads
> -------------------------------------------------------------------
>
>                 Key: CASSANDRA-8635
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-8635
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>            Reporter: Tyler Hobbs
>            Assignee: Marcus Eriksson
>            Priority: Critical
>             Fix For: 2.1.3
>
>         Attachments: 0001-Include-cold-sstables-in-compactions-if-they-overlap.patch
>
>
> In 2.1, STCS may omit cold SSTables from compaction (CASSANDRA-6109).  If data is regularly
overwritten or deleted (but not enough to trigger a single-sstable tombstone purging compaction),
data size on disk may continuously grow if:
> * The table receives very few reads
> * The reads only touch the newest SSTables
> Basically, if the overwritten data is never read and there aren't many tombstones, STCS
has no incentive to compact the sstables.  We should take sstable overlap into consideration
as well as coldness to address this case.



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