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From Donald Smith <>
Subject Adjusting readahead for SSD disk seeks
Date Wed, 24 Sep 2014 20:07:16 GMT
We're using cassandra as a key-value store; our values are small.  So we're thinking we don't
need much disk readahead (e.g., "blockdev -getra /dev/sda").   We're using SSDs.

When cassandra does disk seeks to satisfy read requests does it typically have to read in
the entire SStable into memory (assuming the bloom filter said yes)?  If cassandra needs to
read in lots of blocks anyway or if it needs to read the entire file during compaction then
I'd expect we might as well have a big readahead.   Perhaps there's a tradeoff between read
latency and compaction time.

Any feedback welcome.


Donald A. Smith | Senior Software Engineer
P: 425.201.3900 x 3866
C: (206) 819-5965
F: (646) 443-2333<>


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