Are Cassandra read and write to disks are asynchronous?

I read in Cassandra documentation (https://cassandra.apache.org/doc/latest/cassandra/operating/hardware.html that Cassandra write to sstable in async mode. I want to ask that whether all writes are async to disc. Also what about reads are those also async or depends on case to case basis.

By default, writes to disk are asynchronous - and there are two different writes to consider.

The first is the commit log which by default is asynchronous. The behaviour can be altered by changing some settings, the more frequently you sync, the lower performance / throughput you could expect on writes.

• commitlog_sync - this value is either periodic (default), group or batch
• commitlog_sync_period - default value of 10 seconds.

So by default, it will sync every 10 seconds, making the changes on that node durable. Each node sync's to disk independently, so across 3 replicas, you can expect the data to by synced every 3.33ms on average to once of the node's disks.

The second is the memtables being flushed, e.g. the data currently held in memory, they can flush at any time, when sufficient data is held in memory to exceed a threshold. There are a number of settings which also influence this behaviour:

• memtable_heap_space
• memtable_offheap_space
• memtable_cleanup_threshold

Unless you have some specific problems / evidence of a flush bottleneck, I would not alter these from the defaults to start with.

In the question you mention reads - they are synchronous (for the most part) - in that if you request the data in a query, it will perform the read.
In 3.11 and earlier versions of Cassandra, there was the potential for a read_repair_chance to trigger an asynchronous read for repair purposes.

Strictly speaking, I believe you are talking about two different things.

The "write" you are referring to relates to the commitlog being persisted disk with the Unix fsync() function to make them durable. This is a completely different concept to the client/driver read/write requests being asynchronous.

By default, Cassandra's commitlog is buffered by the operating system in a cache the is periodically flushed to disk with fsync() every 10 seconds.

On the other hand, you can configure your application to perform asynchronous reads or writes so that client requests are not blocked waiting for either the result or acknowledgement from the coordinator.

So to answer your question directly, not all reads or writes are asynchronous. It depends on how you've written your application. Our recommendation is to use the drivers' asynchronous API to maximise the performance of your cluster. Cheers!