I’ll talk about changing Cassandra CompactionStrategy on a live production Cluster.
First of all, an extract of the Cassandra documentation :
Periodic compaction is essential to a healthy Cassandra database because Cassandra does not insert/update in place. As inserts/updates occur, instead of overwriting the rows, Cassandra writes a new timestamped version of the inserted or updated data in another SSTable. Cassandra manages the accumulation of SSTables on disk using compaction. Cassandra also does not delete in place because the SSTable is immutable. Instead, Cassandra marks data to be deleted using a tombstone.
By default, Cassandra use SizeTieredCompactionStrategyi (STC). This strategy triggers a minor compaction when there are a number of similar sized SSTables on disk as configured by the table subproperty, 4 by default.
Another compaction strategy available since Cassandra 1.0 is LeveledCompactionStrategy (LCS) based on LevelDB.
Since 2.0.11, DateTieredCompactionStrategy is also available.
Depending on your needs, you may need to change the compaction strategy on a running cluster. Change this setting involves rewrite ALL sstables to the new strategy, which may take long time and can be cpu / i/o intensive.
I needed to change the compaction strategy on my production cluster to LeveledCompactionStrategy because of our workflow : lot of updates and deletes, wide rows etc.
Moreover, with the default STC, progressively the largest SSTable that is created will not be compacted until the amount of actual data increases four-fold. So it can take long time before old data are really deleted !
Note: You can test a new compactionStrategy on one new node with the write_survey bootstrap option. See the datastax blogpost about it.
The basic procedure to change the CompactionStrategy is to alter the table via cql :
If you run alter table to change to LCS like that, all nodes will recompact data at the same time, so performances problems can occurs for hours/days…
A better solution is to migrate nodes by nodes !
You need to change the compaction locally on-the-fly, via the JMX, like in write_survey mode.
I use jmxterm for that. I think I’ll write articles about all theses jmx things :)
For example, to change to LCS on mytable table with jmxterm :
A nice one-liner :
On next commitlog flush, the node will start it compaction to rewrite all it mytable sstables to the new strategy.
You can see the progression with nodetool :
You need to wait for the node to recompact all it sstables, then change the strategy to instance2, etc.
The transition will be done in multiple compactions if you have lots of data. By default new sstables will be 160MB large.
you can monitor you table with nodetool cfstats too :
You can see the 31/4 : it means that there is 31 sstables in L0, whereas cassandra try to have only 4 in L0.
Taken from the code ( src/java/org/apache/cassandra/db/compaction/LeveledManifest.java )
When all nodes have the new strategy, let’s go for the global alter table. /!\ If a node restart before the final alter table, it will recompact to default strategy (SizeTiered)!
Et voilà, I hope this article will help you :)
My latest Cassandra blogpost was one year ago… I have several in mind (jmx things !) so stay tuned !