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From "Benedict (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-6916) Preemptive opening of compaction result
Date Mon, 21 Apr 2014 23:10:15 GMT


Benedict commented on CASSANDRA-6916:

Thanks, everything looks good to me. Once we double check against stock 2.1 with populate
page cache enabled to check there's no surprises, I'm happy to commit.

> Preemptive opening of compaction result
> ---------------------------------------
>                 Key: CASSANDRA-6916
>                 URL:
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>            Reporter: Benedict
>            Assignee: Benedict
>            Priority: Minor
>              Labels: performance
>             Fix For: 2.1
>         Attachments: 6916-stock2_1.mixed.logs.tar.gz, 6916v3-preempive-open-compact.logs.gz,
> Related to CASSANDRA-6812, but a little simpler: when compacting, we mess quite badly
with the page cache. One thing we can do to mitigate this problem is to use the sstable we're
writing before we've finished writing it, and to drop the regions from the old sstables from
the page cache as soon as the new sstables have them (even if they're only written to the
page cache). This should minimise any page cache churn, as the old sstables must be larger
than the new sstable, and since both will be in memory, dropping the old sstables is at least
as good as dropping the new.
> The approach is quite straight-forward. Every X MB written:
> # grab flushed length of index file;
> # grab second to last index summary record, after excluding those that point to positions
after the flushed length;
> # open index file, and check that our last record doesn't occur outside of the flushed
length of the data file (pretty unlikely)
> # Open the sstable with the calculated upper bound
> Some complications:
> # must keep running copy of compression metadata for reopening with
> # we need to be able to replace an sstable with itself but a different lower bound
> # we need to drop the old page cache only when readers have finished

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