Data & Architecture DBA

1．  关于cassandra的读性能分析的一篇文章：

     Mike Perham  continues his series now explaining: “reads and […] why they are slow”.

So what happens with a Cassandra read?

• a client makes a read request to a random node
• the node acts as a proxy determining the nodes having copies of data
• the node request the corresponding data from each node
• the client can select the strength of the read consistency:
• single read => the request returns once it gets the first response, but data can be stale
• quorum read => the request returns only after the majority responded with the same value

Mark mentions a couple of corner cases related to this behavior that is more complicated.

• the node also performs read repair of any inconsistent response
• each node reading data uses either Memtable (in-memory) or SSTables (disk)

Mike and Jonathan provide a very detailed explanation of the read performance:

Mike: To scan the SSTable, Cassandra uses a row-level column index and bloom filter to find the necessary blocks on disk, deserializes them and determines the actual data to return. There’s a lot of disk IO here which ultimately makes the read latency higher than a similar DBMS.

Jonathan: The reason uncached reads are slower in Cassandra is not because the SSTable is inherently io-intensive (it’s actually better than b-tree based storage on a 1:1 basis) but because in the average case you’ll have to merge row fragments from 2-4 SSTables to complete the request, since SSTables are not update-in-place.

It is also important to note that Cassandra employs row caching that addresses reads latency.

2．  关于cassandra的写性能分析的一篇文章:

An interesting explanation of how Cassandra write ops are happening:

• client submits its write request to a single, random Cassandra node
• the node behavior is similar to a proxy writing data to the cluster
• writes are replicated to N nodes according to the replication placement strategy (the details of RackAwareStrategy are quite interesting)
• each of the N nodes performs 2 actions when receiving a write (in the form of RowMutation):
• append the mutation to the commit log for transactional purposes
• update an in-memory Memtable structure with the change

There are also a couple of asynchronous operations:

• Memtable is written to disk in a structure called SSTable
• SSTables corresponding to a column family are merged into a raw ColumnFamily datafile.

参考文档：

 http://nosql.mypopescu.com/post/474623402/cassandra-reads-performance-explained

 http://nosql.mypopescu.com/post/454521259/cassandra-write-operation-performance-explained