Benjamin Roth created CASSANDRA-13241:
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Summary: Lower default chunk_length_in_kb from 64kb to 4kb
Key: CASSANDRA-13241
URL: https://issues.apache.org/jira/browse/CASSANDRA-13241
Project: Cassandra
Issue Type: Wish
Components: Core
Reporter: Benjamin Roth
Having a too low chunk size may result in some wasted disk space. A too high
chunk size may lead to massive overreads and may have a critical impact on
overall system performance.
In my case, the default chunk size lead to peak read IOs of up to 1GB/s and avg
reads of 200MB/s. After lowering chunksize (of course aligned with read ahead),
the avg read IO went below 20 MB/s, rather 10-15MB/s.
The risk of (physical) overreads is increasing with lower (page cache size) /
(total data size) ratio.
High chunk sizes are mostly appropriate for bigger payloads pre request but if
the model consists rather of small rows or small resultsets, the read overhead
with 64kb chunk size is insanely high. This applies for example for (small)
skinny rows.
Please also see here:
https://groups.google.com/forum/#!topic/scylladb-dev/j_qXSP-6-gY
To give you some insights what a difference it can make (460GB data, 128GB RAM):
- Latency of a quite large CF: https://cl.ly/1r3e0W0S393L
- Disk throughput: https://cl.ly/2a0Z250S1M3c
- This shows, that the request distribution remained the same, so no "dynamic
snitch magic": https://cl.ly/3E0t1T1z2c0J
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