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https://issues.apache.org/jira/browse/HBASE-7958?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13589061#comment-13589061
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Jesse Yates commented on HBASE-7958:
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The notable piece here is that we can put this 'stats scanner' in as we write
the compaction. It sees every key that gets written, so it can build perfect
stats as of the compaction. The only limitation then is that its not 'per
region', but 'per-CF per region'; it would be a much larger overhaul to combine
all the information across all the compactions in the region - doable, but
probably not worth it as you actually lose information and only gain a little
savings in terms of math effort when calculating per-region, cross CF key
distributions.
(For calculating key distributions, you basically sum the area under the
histogram across the columns to determine the 'volume' of keys in the given
range)
> Statistics per-column family per-region
> ---------------------------------------
>
> Key: HBASE-7958
> URL: https://issues.apache.org/jira/browse/HBASE-7958
> Project: HBase
> Issue Type: New Feature
> Affects Versions: 0.96.0
> Reporter: Jesse Yates
> Fix For: 0.96.0
>
>
> Originating from this discussion on the dev list:
> http://search-hadoop.com/m/coDKU1urovS/Simple+stastics+per+region/v=plain
> Essentially, we should have built-in statistics gathering for HBase tables.
> This allows clients to have a better understanding of the distribution of
> keys within a table and a given region. We could also surface this
> information via the UI.
> There are a couple different proposals from the email, the overview is this:
> We add in something on compactions that gathers stats about the keys that are
> written and then we surface them to a table.
> The possible proposals include:
> *How to implement it?*
> # Coprocessors -
> ** advantage - it easily plugs in and people could pretty easily add their
> own statistics.
> ** disadvantage - UI elements would also require this, we get into dependent
> loading, which leads down the OSGi path. Also, these CPs need to be installed
> _after_ all the other CPs on compaction to ensure they see exactly what gets
> written (doable, but a pain)
> # Built into HBase as a custom scanner
> ** advantage - always goes in the right place and no need to muck about with
> loading CPs etc.
> ** disadvantage - less pluggable, at least for the initial cut
> *Where do we store data?*
> # .META.
> ** advantage - its an existing table, so we can jam it into another CF there
> ** disadvantage - this would make META much larger, possibly leading to
> splits AND will make it much harder for other processes to read the info
> # A new stats table
> ** advantage - cleanly separates out the information from META
> ** disadvantage - should use a 'system table' idea to prevent accidental
> deletion, manipulation by arbitrary clients, but still allow clients to read
> it.
> Once we have this framework, we can then move to an actual implementation of
> various statistics.
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