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Jeremy Hanna commented on CASSANDRA-2897:
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fwiw - we found the read-before-write to be pretty much debilitating for write
throughput. We have a total of 5 secondary indexes, 3 on one column family.
We looked at iotop while doing heavy writes on the column family and the main
IO that was happening was on Cassandra reads. We're auditing which secondary
indexes that we really need because it affects write throughput that much.
In other words, resolving this ticket would make it so secondary indexes were
no longer a compromise wrt write performance.
> Secondary indexes without read-before-write
> -------------------------------------------
>
> Key: CASSANDRA-2897
> URL: https://issues.apache.org/jira/browse/CASSANDRA-2897
> Project: Cassandra
> Issue Type: Improvement
> Components: Core
> Affects Versions: 0.7.0
> Reporter: Sylvain Lebresne
> Priority: Minor
> Labels: secondary_index
>
> Currently, secondary index updates require a read-before-write to maintain
> the index consistency. Keeping the index consistent at all time is not
> necessary however. We could let the (secondary) index get inconsistent on
> writes and repair those on reads. This would be easy because on reads, we
> make sure to request the indexed columns anyway, so we can just skip the row
> that are not needed and repair the index at the same time.
> This does trade work on writes for work on reads. However, read-before-write
> is sufficiently costly that it will likely be a win overall.
> There is (at least) two small technical difficulties here though:
> # If we repair on read, this will be racy with writes, so we'll probably have
> to synchronize there.
> # We probably shouldn't only rely on read to repair and we should also have a
> task to repair the index for things that are rarely read. It's unclear how to
> make that low impact though.
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