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Ariel Weisberg commented on CASSANDRA-13241:
--------------------------------------------
Attached is an implementation of a compact integer sequence that requires 37%
less space compared to storing a sequence of 8-byte values. The maximum safe
span between values is 419430 bytes. This means we can comfortably fit 32k
compressed blocks even with the fudge factor required because compressors can
very slightly increase data size in the worst case. I checked LZ4, Snappy, and
Deflate, and if only loaded this representation for 32k block sizes we would be
fine.
I can also have it detect when it fails and load the old less space efficient
representation instead.
If we want to go ahead and use a more efficient representation in addition to
tuning the value to 16k as discussed in IRC then I can clean this up, add tests
etc.
> 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
> Assignee: Ariel Weisberg
> Priority: Major
> Attachments: CompactIntegerSequence.java
>
>
> 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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