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From "Jonathan Ellis (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-11206) Support large partitions on the 3.0 sstable format
Date Mon, 22 Feb 2016 17:21:18 GMT


Jonathan Ellis commented on CASSANDRA-11206:

The offset map is written AFTER the serialized IndexInfo, but since we write out the size
of both up front, we can still access the map without deserializing everything.  Here's the
code from RIE that writes it out:

           ... [write out the IndexInfo and compute offsets map as we go] ...
           for (int off : offsets)

(There is no code yet that reads it back in because CASSANDRA-9738 got put on the back burner.)

Thus the offsets map starts at the total size, minus count * sizeof(int).

So we can read the middle offsetmap entry, deserialize the IndexInfo it points to, compare
with the row we're looking for, and repeat until bsearch is done.

> Support large partitions on the 3.0 sstable format
> --------------------------------------------------
>                 Key: CASSANDRA-11206
>                 URL:
>             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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