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From "Andrew Purtell (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-9794) KeyValues / cells backed by buffer fragments
Date Thu, 17 Oct 2013 17:54:44 GMT
Andrew Purtell created HBASE-9794:

             Summary: KeyValues / cells backed by buffer fragments
                 Key: HBASE-9794
                 URL: https://issues.apache.org/jira/browse/HBASE-9794
             Project: HBase
          Issue Type: Brainstorming
            Reporter: Andrew Purtell

There are various places in the code where we see comments to the effect "would be great if
we had a scatter gather API for KV", appearing at places where we rewrite KVs on the server,
for example in HRegion where we process appends and increments.

KeyValues are stored in buffers of fixed length. This approach has performance advantages
for the common case where KVs are not manipulated on their way from disk to RPC. The disadvantage
of this approach is any manipulation of tags requires the creation of a new buffer to hold
the result, and a copy of the KV data into the new buffer. Appends and increments are typically
a small percentage of overall workload so this has been fine up to now.
KeyValues can now carry metadata known as tags. Tags are stored contiguously with the rest
of the KeyValue. Applications wishing to use tags (like per cell security) change the equation
by wanting to rewrite KVs significantly more often. 

We should consider backing KeyValue with an alternative structure that can better support
rewriting portions of its data, appends to existing buffers, scatter-gather copies, possibly
even copy-on-write.

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