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From "Matt Corgan (Created) (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-4676) Prefix Compression - Trie data block encoding
Date Tue, 25 Oct 2011 23:14:32 GMT
Prefix Compression - Trie data block encoding
---------------------------------------------

                 Key: HBASE-4676
                 URL: https://issues.apache.org/jira/browse/HBASE-4676
             Project: HBase
          Issue Type: New Feature
          Components: io
    Affects Versions: 0.94.0
            Reporter: Matt Corgan


The HBase data block format has room for 2 significant improvements for applications that
have high block cache hit ratios.  

First, there is no prefix compression, and the current KeyValue format is somewhat metadata
heavy, so there can be tremendous memory bloat for many common data layouts, specifically
those with long keys and short values.

Second, there is no random access to KeyValues inside data blocks.  This means that every
time you double the datablock size, average seek time (or average cpu consumption) goes up
by a factor of 2.  The standard 64KB block size is ~10x slower for random seeks than a 4KB
block size, but block sizes as small as 4KB cause problems elsewhere.  Using block sizes of
256KB or 1MB or more may be more efficient from a disk access and block-cache perspective
in many big-data applications, but doing so is infeasible from a random seek perspective.

The PrefixTrie block encoding format attempts to solve both of these problems.  Some features:

* trie format for row key encoding completely eliminates duplicate row keys and encodes similar
row keys into a standard trie structure which also saves a lot of space
* the column family is currently stored once at the beginning of each block.  this could easily
be modified to allow multiple family names per block
* all qualifiers in the block are stored in their own trie format which caters nicely to wide
rows.  duplicate qualifers between rows are eliminated.  the size of this trie determines
the width of the block's qualifier fixed-width-int
* the minimum timestamp is stored at the beginning of the block, and deltas are calculated
from that.  the maximum delta determines the width of the block's timestamp fixed-width-int

The block is structured with metadata at the beginning, then a section for the row trie, then
the column trie, then the timestamp deltas, and then then all the values.  Most work is done
in the row trie, where every leaf node (corresponding to a row) contains a list of offsets/references
corresponding to the cells in that row.  Each cell is fixed-width to enable binary searching
and is represented by [1 byte operationType, X bytes qualifier offset, X bytes timestamp delta
offset].

If all operation types are the same for a block, there will be zero per-cell overhead.  Same
for timestamps.  Same for qualifiers when i get a chance.  So, the compression aspect is very
strong, but makes a few small sacrifices on VarInt size to enable faster binary searches in
trie fan-out nodes.

A more compressed but slower version might build on this by also applying further (suffix,
etc) compression on the trie nodes at the cost of slower write speed.  Even further compression
could be obtained by using all VInts instead of FInts with a sacrifice on random seek speed
(though not huge).

One current drawback is the current write speed.  While programmed with good constructs like
TreeMaps, ByteBuffers, binary searches, etc, it's not programmed with the same level of optimization
as the read path.  Work will need to be done to optimize the data structures used for encoding
and could probably show a 10x increase.  It will still be slower than delta encoding, but
with a much higher decode speed.  I have not yet created a thorough benchmark for write speed
nor sequential read speed.

Though the trie is reaching a point where it is internally very efficient (probably within
half or a quarter of its max read speed) the way that hbase currently uses it is far from
optimal.  The KeyValueScanner and related classes that iterate through the trie will eventually
need to be smarter and have methods to do things like skipping to the next row of results
without scanning every cell in between.  When that is accomplished it will also allow much
faster compactions because the full row key will not have to be compared as often as it is
now.

Current code is on github.  The trie code is in a separate project than the slightly modified
hbase.  There is an hbase project there as well with the DeltaEncoding patch applied, and
it builds on top of that.

https://github.com/hotpads/hbase/tree/delta-encoding-plus-trie
https://github.com/hotpads/hbase-prefix-trie

I'll follow up later with more implementation ideas.

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