make cache skipping optional
----------------------------
Key: CASSANDRA-2635
URL: https://issues.apache.org/jira/browse/CASSANDRA-2635
Project: Cassandra
Issue Type: Improvement
Reporter: Peter Schuller
Assignee: Peter Schuller
Priority: Minor
We've applied this patch locally in order to turn of page skipping; not
completely but only for compaction/repair situations where it can be directly
detrimental in the sense of causing data to become cold even though your entire
data set fits in memory.
It's better than completely disabling DONTNEED because the cache skipping does
make sense and has no relevant (that I can see) detrimental effects in some
cases, like when dumping caches.
The patch is against 0.7.5 right now but if the change is desired I can make a
patch for trunk. Also, the name of the configuration option is dubious since
saying 'false' does not actually turn it off completely. I wasn't able to
figure out a good name that conveyed the functionality in a short brief name
however.
A related concern as discussed in CASSANDRA-1902 is that the cache skipping
isn't fsync:ing and so won't work reliably on writes. If the feature is to be
retained that's something to fix in a different ticket.
A question is also whether to retain the default to true or change it to false.
I'm kinda leaning to false since it's detrimental in the "easy" cases of little
data. In "big" cases with lots of data people will have to think and tweak
anyway, so better to put the burden on that end.
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