Block Compressors

• none

• Snappy

• LZO

• LZ4

• GZ

Data Block Encoding Types

• Prefix - Often, keys are very similar. Specifically, keys often share a common prefix and only differ near the end. For instance, one key might be RowKey:Family:Qualifier0 and the next key might be RowKey:Family:Qualifier1. In Prefix encoding, an extra column is added which holds the length of the prefix shared between the current key and the previous key. Assuming the first key here is totally different from the key before, its prefix length is 0. The second key's prefix length is 23, since they have the first 23 characters in common.

  Obviously if the keys tend to have nothing in common, Prefix will not provide much benefit.

  The following image shows a hypothetical ColumnFamily with no data block encoding.

  Figure E.1. ColumnFamily with No Encoding

  A ColumnFamily with no encoding>

  
  

  Here is the same data with prefix data encoding.

  Figure E.2. ColumnFamily with Prefix Encoding

  A ColumnFamily with prefix encoding

  
  

• Diff - Diff encoding expands upon Prefix encoding. Instead of considering the key sequentially as a monolithic series of bytes, each key field is split so that each part of the key can be compressed more efficiently. Two new fields are added: timestamp and type. If the ColumnFamily is the same as the previous row, it is omitted from the current row. If the key length, value length or type are the same as the previous row, the field is omitted. In addition, for increased compression, the timestamp is stored as a Diff from the previous row's timestamp, rather than being stored in full. Given the two row keys in the Prefix example, and given an exact match on timestamp and the same type, neither the value length, or type needs to be stored for the second row, and the timestamp value for the second row is just 0, rather than a full timestamp.

  Diff encoding is disabled by default because writing and scanning are slower but more data is cached.

  This image shows the same ColumnFamily from the previous images, with Diff encoding.

  Figure E.3. ColumnFamily with Diff Encoding

  A ColumnFamily with diff encoding

  
  

• Fast Diff - Fast Diff works similar to Diff, but uses a faster implementation. It also adds another field which stores a single bit to track whether the data itself is the same as the previous row. If it is, the data is not stored again. Fast Diff is the recommended codec to use if you have long keys or many columns. The data format is nearly identical to Diff encoding, so there is not an image to illustrate it.

• Prefix Tree encoding was introduced as an experimental feature in HBase 0.96. It provides similar memory savings to the Prefix, Diff, and Fast Diff encoder, but provides faster random access at a cost of slower encoding speed. Prefix Tree may be appropriate for applications that have high block cache hit ratios. It introduces new 'tree' fields for the row and column. The row tree field contains a list of offsets/references corresponding to the cells in that row. This allows for a good deal of compression. For more details about Prefix Tree encoding, see HBASE-4676. It is difficult to graphically illustrate a prefix tree, so no image is included. See the Wikipedia article for Trie for more general information about this data structure.