Creating or altering tables to use DSE In-Memory
Use CQL directives to create and alter tables to use DSE In-Memory and dse.yaml to limit the size of tables.
Creating a table to use DSE In-Memory
To create a table that uses DSE In-Memory, add a CQL directive to the CREATE TABLE statement. Use the compaction directive in the statement to specify the MemoryOnlyStrategy class and disable the key and row caches.
CREATE TABLE customers (
uid text,
fname text,
lname text,
PRIMARY KEY (uid)
) WITH compaction= { 'class': 'MemoryOnlyStrategy' }
AND caching = {'keys':'NONE', 'rows_per_partition':'NONE'};
Altering an existing table to use DSE In-Memory
Use the ALTER TABLE statement to change a traditional table to use in-memory, or to change an in-memory table to a traditional table.
For example, use the DESCRIBE command for a table named employee.
Verify that employee is a traditional table because the output of the DESCRIBE command does not include a line that looks something like:
compaction={'class': 'MemoryOnlyStrategy'} >
Alter the employee table to use DSE In-Memory:
ALTER TABLE employee WITH compaction= { 'class': 'MemoryOnlyStrategy' }
AND caching = {'keys':'NONE', 'rows_per_partition':'NONE'};
After you alter the table, rewrite existing SSTables:
nodetool upgradesstables -a <keyspacename> <tablename>Use the --jobs option to set the number of SSTables that upgrade simultaneously.
The default setting is 2, which minimizes impact on the cluster.
Set to 0 to use all available compaction threads.
In cqlsh, use the DESCRIBE TABLE command to view table properties:
DESCRIBE TABLE employee;This output shows that the table uses DSE In-Memory:
CREATE TABLE employee (
uid text PRIMARY KEY,
fname text,
lname text
) WITH bloom_filter_fp_chance = 0.01
AND caching = '{"keys":"NONE", "rows_per_partition":"NONE"}'
AND comment = ''
AND compaction = {'min_threshold': '4', 'class': 'org.apache.cassandra.db.compaction.MemoryOnlyStrategy',
'max_threshold': '32'}
AND default_time_to_live = 0
AND gc_grace_seconds = 864000
AND max_index_interval = 2048
AND memtable_flush_period_in_ms = 0
AND min_index_interval = 128
AND nodesync = {'enabled' : 'true'}
AND speculative_retry = '99.0PERCENTILE';
When memtable_flush_period_in_ms=0, the memtable will flush when:
-
the flush threshold is met
-
on shutdown
-
on nodetool flush
-
when commitlogs get full
Limiting the size of tables
Use the max_memory_to_lock_fraction or max_memory_to_lock_mb configuration option in the dse.yaml file to specify how much system memory to use for all in-memory tables.
max_memory_to_lock_fraction |
Specify a fraction of the system memory. The default value of 0.20 specifies to use up to 20% of system memory. |
max_memory_to_lock_mb |
Specify a maximum amount of memory in megabytes (MB). |
Disabling caching on tables
DataStax recommends disabling caching on tables that use the DSE In-Memory option. If caching is not disabled, a warning is logged. Set the table caching property to disable both types of caching:
ALTER TABLE customers WITH caching = {'keys':'NONE',
'rows_per_partition':'NONE'};
