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https://issues.apache.org/jira/browse/CASSANDRA-1956?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13205673#comment-13205673
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Vijay commented on CASSANDRA-1956:
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Alright, What i was trying to do here is to get the feedback from everyone on
all the use cases and try to fit it into the one cache,
I did some fair amount of research to see if there is any better option and
there wasn't one, the closest concept which i got to was something like a block
cache or Linux Page cache.... When there is updates to those blocks we can find
those and update those.
1) The problem shows up only when you have a wide row, which means most
probably the user is doing a range queries
2) If the user has a wide row then most probably he has a large number of
writes into the row, but if we invalidate the row cache for every updates then
it might not be useful and also the first read will have to read multiple SST's.
3) Lets say user has a 100 columns to query and he queries in this case
(specially with composite type columns where the column names can be larger
than the value), then we can possibly run into memory pressure.
4) Having whole row in memory is absolutely required case and we are supporting
it (setting min and max number of columns in a block will help it).
5) the above solution can work seamlessly well for narrow rows when the block
size is reasonably big.
Head and Tail is basically a optimization for the Reverse/Forward queries which
is supported if you have 1 M rows and your block size is 500 and your count is
100 and you are reading from reverse.
> Convert row cache to row+filter cache
> -------------------------------------
>
> Key: CASSANDRA-1956
> URL: https://issues.apache.org/jira/browse/CASSANDRA-1956
> Project: Cassandra
> Issue Type: Improvement
> Components: Core
> Reporter: Stu Hood
> Assignee: Vijay
> Priority: Minor
> Fix For: 1.2
>
> Attachments: 0001-1956-cache-updates-v0.patch,
> 0001-commiting-block-cache.patch, 0001-re-factor-row-cache.patch,
> 0001-row-cache-filter.patch, 0002-1956-updates-to-thrift-and-avro-v0.patch,
> 0002-add-query-cache.patch
>
>
> Changing the row cache to a row+filter cache would make it much more useful.
> We currently have to warn against using the row cache with wide rows, where
> the read pattern is typically a peek at the head, but this usecase would be
> perfect supported by a cache that stored only columns matching the filter.
> Possible implementations:
> * (copout) Cache a single filter per row, and leave the cache key as is
> * Cache a list of filters per row, leaving the cache key as is: this is
> likely to have some gotchas for weird usage patterns, and it requires the
> list overheard
> * Change the cache key to "rowkey+filterid": basically ideal, but you need a
> secondary index to lookup cache entries by rowkey so that you can keep them
> in sync with the memtable
> * others?
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