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From "Peter Schuller (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-2498) Improve read performance in update-intensive workload
Date Mon, 18 Apr 2011 20:10:05 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-2498?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13021220#comment-13021220
] 

Peter Schuller commented on CASSANDRA-2498:
-------------------------------------------

Sounds good; a trade-off to keep in mind is that it optimizes for throughput (number of sstables
touched) at the cost of latency (because of the inability to submit read requests concurrently
since they become dependent). Probably makes sense for most workloads.

> Improve read performance in update-intensive workload
> -----------------------------------------------------
>
>                 Key: CASSANDRA-2498
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-2498
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Jonathan Ellis
>            Priority: Minor
>              Labels: ponies
>             Fix For: 1.0
>
>
> Read performance in an update-heavy environment relies heavily on compaction to maintain
good throughput. (This is not the case for workloads where rows are only inserted once, because
the bloom filter keeps us from having to check sstables unnecessarily.)
> Very early versions of Cassandra attempted to mitigate this by checking sstables in descending
generation order (mostly equivalent to descending mtime): once all the requested columns were
found, it would not check any older sstables.
> This was incorrect, because data timestamp will not correspond to sstable timestamp,
both because compaction has the side effect of "refreshing" data to a newer sstable, and because
hintead handoff may send us data older than what we already have.
> Instead, we could create a per-sstable piece of metadata containing the most recent (client-specified)
timestamp for any column in the sstable.  We could then sort sstables by this timestamp instead,
and perform a similar optimization (if the remaining sstable client-timestamps are older than
the oldest column found in the desired result set so far, we don't need to look further).
Since under almost every workload, client timestamps of data in a given sstable will tend
to be similar, we expect this to cut the number of sstables down proportionally to how frequently
each column in the row is updated. (If each column is updated with each write, we only have
to check a single sstable.)
> This may also be useful information when deciding which SSTables to compact.
> (Note that this optimization is only appropriate for named-column queries, not slice
queries, since we don't know what non-overlapping columns may exist in older sstables.)

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