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Date: Wed, 1 Apr 2015 10:25:53 +0000 (UTC)
From: "Marcus Eriksson (JIRA)" <jira@apache.org>
To: commits@cassandra.apache.org
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Subject: [jira] [Resolved] (CASSANDRA-8360) In DTCS, always compact SSTables
 in the same time window, even if they are fewer than min_threshold
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     [ https://issues.apache.org/jira/browse/CASSANDRA-8360?page=3Dcom.atla=
ssian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Marcus Eriksson resolved CASSANDRA-8360.
----------------------------------------
    Resolution: Fixed

will commit fix as part of CASSANDRA-8359

> In DTCS, always compact SSTables in the same time window, even if they ar=
e fewer than min_threshold
> -------------------------------------------------------------------------=
--------------------------
>
>                 Key: CASSANDRA-8360
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-8360
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Bj=C3=B6rn Hegerfors
>            Assignee: Bj=C3=B6rn Hegerfors
>            Priority: Minor
>             Fix For: 3.0, 2.1.4, 2.0.14
>
>         Attachments: cassandra-2.0-CASSANDRA-8360.txt
>
>
> DTCS uses min_threshold to decide how many time windows of the same size =
that need to accumulate before merging into a larger window. The age of an =
SSTable is determined as its min timestamp, and it always falls into exactl=
y one of the time windows. If multiple SSTables fall into the same window, =
DTCS considers compacting them, but if they are fewer than min_threshold, i=
t decides not to do it.
> When do more than 1 but fewer than min_threshold SSTables end up in the s=
ame time window (except for the current window), you might ask? In the curr=
ent state, DTCS can spill some extra SSTables into bigger windows when the =
previous window wasn't fully compacted, which happens all the time when the=
 latest window stops being the current one. Also, repairs and hints can put=
 new SSTables in old windows.
> I think, and [~jjordan] agreed in a comment on CASSANDRA-6602, that DTCS =
should ignore min_threshold and compact tables in the same windows regardle=
ss of how few they are. I guess max_threshold should still be respected.
> [~jjordan] suggested that this should apply to all windows but the curren=
t window, where all the new SSTables end up. That could make sense. I'm not=
 clear on whether compacting many SSTables at once is more cost efficient o=
r not, when it comes to the very newest and smallest SSTables. Maybe compac=
ting as soon as 2 SSTables are seen is fine if the initial window size is s=
mall enough? I guess the opposite could be the case too; that the very newe=
st SSTables should be compacted very many at a time?


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