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From "Jonathan Ellis (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-11206) Support large partitions on the 3.0 sstable format
Date Mon, 22 Feb 2016 18:02:18 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-11206?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15157390#comment-15157390
] 

Jonathan Ellis commented on CASSANDRA-11206:
--------------------------------------------

The offset map is what allows us to deal with variable length index entries.  So you only
deserialize exactly as many IndexInfo as needed to locate the right 64KB row block.  Then
scanning for the row w/in the block is unchanged.

> Support large partitions on the 3.0 sstable format
> --------------------------------------------------
>
>                 Key: CASSANDRA-11206
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-11206
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Jonathan Ellis
>             Fix For: 3.x
>
>
> Cassandra saves a sample of IndexInfo objects that store the offset within each partition
of every 64KB (by default) range of rows.  To find a row, we binary search this sample, then
scan the partition of the appropriate range.
> The problem is that this scales poorly as partitions grow: on a cache miss, we deserialize
the entire set of IndexInfo, which both creates a lot of GC overhead (as noted in CASSANDRA-9754)
but is also non-negligible i/o activity (relative to reading a single 64KB row range) as partitions
get truly large.
> We introduced an "offset map" in CASSANDRA-10314 that allows us to perform the IndexInfo
bsearch while only deserializing IndexInfo that we need to compare against, i.e. log(N) deserializations.



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