Accumulo [tablet servers][tserver] have block caches that buffer data in memory to limit reads from disk. This caching has the following benefits:
Each tablet server has an index and data block cache that is shared by all hosted tablets (see the [tablet server diagram][tserver] to learn more). A typical Accumulo read operation will perform a binary search over several index blocks followed by a linear scan of one or more data blocks. If these blocks are not in a cache, they will need to be retrieved from [RFiles] in HDFS. While the index block cache is enabled for all tables, the data block cache has to be enabled for a table by the user. It is typically only enabled for tables where read performance is critical.
The index and data block caches are configured for tables by the following properties:
false)true)While the index block cache is enabled by default for all Accumulo tables, users must enable the data block cache by setting {% plink table.cache.block.enable %} to true in the shell:
config -t mytable -s table.cache.block.enable=true
Or programmatically using [TableOperations.setProperty()][tableops]:
client.tableOperations().setProperty("mytable", "table.cache.block.enable", "true");
The size of the index and data block caches (which are shared by all tablets of tablet server) can be changed from their defaults by setting the following properties:
[tserver]: {% durl getting-started/design#tablet-server-1 %} [RFiles]: {% durl getting-started/design#rfile %} [tableops]: {% jurl org.apache.accumulo.core.client.admin.TableOperations#setProperty-java.lang.String-java.lang.String-java.lang.String- %}