MyRocks: use less IO on writes to have more IO for reads

Holiday is almost here and I wrote a long blog post on write-efficiency yesterday so this one will be short. A longer version of this is in progress because this is an interesting result for me to explain. We assume that an LSM is less efficient for reads because it is more efficient for writes and it is hard to be optimal for all of read, write & space efficiency.

For real workloads it is complicated and for now I include benchmarks in "real workloads".  Here is one interesting result from my IO-bound tests of Linkbench. The summary is that when you spend less on IO to write back changes then you can spend more on IO to handle user queries. That benefit is more apparent on slower storage (disk array) than on faster storage (MLC NAND flash) because slower storage is more likely to be the bottleneck.

IO-bound Linkbench means that I used a server with 50G of RAM and ran Linkbench with maxid1=1B (1B nodes). The MyRocks database was ~400G and the InnoDB database was ~1.6T. Both MyRocks and InnoDB used MySQL 5.6.26. The workload is IO-heavy and the database working set is not cached.

The interesting result is that the difference between MyRocks and InnoDB becomes larger as storage gets slower. Another way to describe this is that InnoDB loses more performance than MyRocks when moving from faster to slower storage. I assume this is because MyRocks uses less IO capacity for writing back database changes so it has more IO capacity for handling user queries.

                Transactions per second

                MyRocks InnoDB  MyRocks/InnoDB

Disk array      2195    414     5.3

Slow SSD        23484   10143   2.3

Fast SSD        28965   21414   1.4

The random operations per second provided by the storage devices above is approximately 1k for the disk array, 10k for the slow SSD and more than 100k for the fast SSD.