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From "Harish Doddi (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-2635) make cache skipping optional
Date Sat, 03 Mar 2012 00:21:57 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-2635?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13221377#comment-13221377
] 

Harish Doddi commented on CASSANDRA-2635:
-----------------------------------------

Only caveat is that we need 2x the memory with traditional compaction for this to work. With
leveled compaction this might work better.

---Yes I am assuming going forward we will be using leveled compaction rather than size tiered.
Hence unless the user unsets this flag, the default value is true which should work for traditional
implementation.

Updated the patch
                
> 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: Harish Doddi
>            Priority: Minor
>         Attachments: CASSANDRA-2635-075.txt, CASSANDRA-2635-trunk.txt
>
>
> 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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