Adaptive checkpoints of Disk Data columns in NDB Cluster

 In MySQL Cluster 8.0.20 we made it possible to write to disk data columns

at considerably higher write rates than before. In MySQL Cluster 7.6 we

introduced adaptive checkpointing of in-memory rows to ensure that restarts

can be faster.

In order to handle very high write rates (up to more than 1 GByte per second)

on disk data columns in NDB it is necessary to control write rates more

efficiently.

To get the most efficient use of disk bandwidth it is important to ensure that

write rates are kept fairly constant such that we constantly make use of the

disk bandwidth. If the load on the disks goes up and down too fast we will not

be able to make use of all the available bandwidth in the disks.

With modern NVMe devices found in e.g. the Oracle Cloud one can achieve 10-20

GByte of reads and writes to a set of 8 NVMe devices using specialised disk

benchmarks. Achieving this with a DBMS is not as easy, but we were able to

see NDB use 7 GBytes per second of reads and writes in some YCSB benchmarks.

This represents one more step in the direction of adaptive algorithms in NDB

Cluster to ensure that users always will get the optimal behaviour of their

hardware when using NDB.

Some previous steps are:

- Adaptive send assistance

  This feature was introduced in 7.5 that made it possible for all threads

  to assist the send threads to send messages on sockets. The main input to

  the adaptive algorithm is the amount of CPU time spent in each thread for

  normal execution, sleeping, assisting send threads.

- Adaptive in-memory checkpoint speed

  This feature adapts the speed of checkpoints based on the amount of write

  activity in NDB Cluster.

- Adaptive CPU spinning

  This is introduced in 8.0.20 and will be presented in separate blog.