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From "Lars Hofhansl (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-9794) KeyValues / cells backed by buffer fragments
Date Thu, 17 Oct 2013 18:04:42 GMT

    [ https://issues.apache.org/jira/browse/HBASE-9794?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13798193#comment-13798193

Lars Hofhansl commented on HBASE-9794:

I am reminded of my writing network protocols in OCaml in a former life :)

> 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 the KV internals then 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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