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https://issues.apache.org/jira/browse/HBASE-5263?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13205645#comment-13205645
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Kannan Muthukkaruppan edited comment on HBASE-5263 at 2/10/12 7:23 PM:
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Promising idea!
In terms of the implementation details, it would be nice to avoid some
pathological cases... were cold data (which was in the cache but almost on its
way out of the cache) becomes hot again. I am guessing a naive approach could
have this pitfall, but something that additionally takes into consideration the
hotness of the keys in the block and appropriately places the data in the
correct place in the blockcache LRU would not. Haven't thought through much
about the implementation details... but wanted to throw out the initial
thoughts at least.
See also related idea by Liyin here: HBASE-5639. These could be complementary
approaches.
was (Author: kannanm):
Promising idea!
In terms of the implementation details, it would be nice to avoid some
pathological cases... were cold data (which was in the cache but almost on its
way out of the cache) becomes hot again. I am guessing a naive approach could
have this pitfall, but something that additionally takes into consideration the
hotness of the keys in the block and appropriately places the data in the
correct place in the blockcache LRU would not. Haven't thought through much
about the implementation details... but wanted to throw out the initial
thoughts at least.
See also related idea by Liyin here: HBASE-5263. These could be complementary
approaches.
> Preserving cached data on compactions through cache-on-write
> ------------------------------------------------------------
>
> Key: HBASE-5263
> URL: https://issues.apache.org/jira/browse/HBASE-5263
> Project: HBase
> Issue Type: Improvement
> Reporter: Mikhail Bautin
> Assignee: Mikhail Bautin
> Priority: Minor
>
> We are tackling HBASE-3976 and HBASE-5230 to make sure we don't trash the
> block cache on compactions if cache-on-write is enabled. However, it would be
> ideal to reduce the effect compactions have on the cached data. For every
> block we are writing for a compacted file we can decide whether it needs to
> be cached based on whether the original blocks containing the same data were
> already in cache. More precisely, for every HFile reader in a compaction we
> can maintain a boolean flag saying whether the current key-value came from a
> disk IO or the block cache. In the HFile writer for the compaction's output
> we can maintain a flag that is set if any of the key-values in the block
> being written came from a cached block, use that flag at the end of a block
> to decide whether to cache-on-write the block, and reset the flag to false on
> a block boundary. If such an inclusive approach would still trash the cache,
> we could restrict the total number of blocks to be cached per an output
> HFile, switch to an "and" logic instead of "or" logic for deciding whether to
> cache an output file block, or only cache a certain percentage of output file
> blocks that contain some of the previously cached data.
> Thanks to Nicolas for this elegant online algorithm idea!
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