Albert P Tobey created CASSANDRA-10249:
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Summary: Reduce over-read for standard disk io by 16x
Key: CASSANDRA-10249
URL: https://issues.apache.org/jira/browse/CASSANDRA-10249
Project: Cassandra
Issue Type: Improvement
Reporter: Albert P Tobey
Fix For: 2.1.x
On read workloads, Cassandra 2.1 reads drastically more data than it emits over
the network. This causes problems throughput the system by wasting disk IO and
causing unnecessary GC.
I have reproduce the issue on clusters and locally with a single instance. The
only requirement to reproduce the issue is enough data to blow through the page
cache. The default schema and data size with cassandra-stress is sufficient for
exposing the issue.
With stock 2.1.9 I regularly observed anywhere from 300:1 to 500 disk:network
ratio. That is to say, for 1MB/s of network IO, Cassandra was doing 300-500MB/s
of disk reads, saturating the drive.
After applying this patch https://gist.github.com/tobert/10c307cf3709a585a7cf
the ratio fell to around 100:1 on my local test rig.
I tested with 512 byte reads as well, but got the same performance, which makes
sense since all HDD and SSD made in the last few years have a 4K block size
(many of them lie and say 512).
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