Off-heap BucketCache

If the BucketCache is enabled, it stores data blocks, leaving the on-heap cache free for storing indexes and Bloom filters. The physical location of the BucketCache storage can be either in memory (off-heap) or in a file stored in a fast disk.

Off-heap: This is the default configuration.
File-based: You can use the file-based storage mode to store the BucketCache on an SSD or FusionIO device,

You can configure a column family to keep its data blocks in the L1 cache instead of the BucketCache, using the HColumnDescriptor.cacheDataInL1(true) method or by using the following syntax in HBase Shell:

hbase> alter 'myTable', CONFIGURATION => {CACHE_DATA_IN_L1 => 'true'}}

This table summaries the important configuration properties for the BucketCache. To configure the BucketCache. The table is followed by three diagrams that show the impacts of different blockcache settings.

Table 1. BucketCache Configuration Properties
Property	Default	Description
`hbase.bucketcache.combinedcache.enabled`	`true`	When `BucketCache` is enabled, use it as a L2 cache for `LruBlockCache`. If set to true, indexes and Bloom filters are kept in the `LruBlockCache` and the data blocks are kept in the `BucketCache`.
`hbase.bucketcache.ioengine`	none (`BucketCache` is disabled by default)	Where to store the contents of the `BucketCache`. Its value can be `offheap`, `file:PATH`, `mmap:PATH` or `pmem:PATH` where PATH is the path to the file that host the file-based cache.
`hfile.block.cache.size`	0.4	A float between 0.0 and 1.0. This factor multiplied by the Java heap size is the size of the L1 cache. In other words, the percentage of the Java heap to use for the L1 cache.
`hbase.bucketcache.size`	not set	When using `BucketCache`, this is a float that represents one of two different values, depending on whether it is a floating-point decimal less than 1.0 or an integer greater than 1.0. If less than 1.0, it represents a percentage of total heap memory size to give to the cache. If greater than 1.0, it represents the capacity of the cache in megabytes
`hbase.bucketcache.bucket.sizes`	`4, 8, 16, 32, 40, 48, 56, 64, 96, 128, 192, 256, 384, 512` KB	A comma-separated list of sizes for buckets for the `BucketCache` if you prefer to use multiple sizes. The sizes should be multiples of the default blocksize, ordered from smallest to largest. The sizes you use will depend on your data patterns. This parameter is experimental.
`-XX:MaxDirectMemorySize`	`MaxDirectMemorySize = BucketCache + 1`	A JVM option to configure the maximum amount of direct memory available for the JVM. It is automatically calculated and configured based on the following formula: `MaxDirectMemorySize = BucketCache size + 1 GB` for other features using direct memory, such as DFSClient. For example, if the BucketCache size is 8 GB, it will be `-XX:MaxDirectMemorySize=9G`.

Figure 1. Default LRUCache, L1 only block cache `hbase.bucketcache.ioengine=NULL`

Figure 2. Default LRUCache, L1 only block cache `hbase.bucketcache.ioengine=offheap`