How new memtables create in cassandra node ? Are the created for every write basis's ? is there any max number we defined for cassandra memtablse in memory ?
There are as many memtables as there are tables. That is the total number of tables in all keyspaces, regardless of how many keyspaces there are.
The total memory allocated for memtables are determined by the 2 parameters in
Cassandra uses the total of these 2 to determine the threshold for triggering memtable flushes to disk.
For more info, see the common memtable settings in cassandra.yaml.
UPDATE - The new writes (mutations) are temporarily stored in memtables but are also persisted to disk in the
commitlog. When the a memtable is flushed to disk (written to an SSTable), the corresponding
commitlog segment gets truncated since the mutations in it have already been persisted to disk.
But when a node goes down, the memtables are lost but the mutations they held are on disk. On startup, Cassandra checks if there are uncommitted mutations and replays the
You can find out more about it in the Cassandra write path. Cheers!
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