Andrew Purtell created HBASE-9794:
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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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