Wednesday, July 25, 2018

Default options in MyRocks

We need to make MyRocks easier to configure -- this isn't a new idea. If you are using MyRocks with default options in mid-2018 then you are probably not using bloom filters, compression or my favorite compaction policy.

You can fix all of that by setting rocksdb_default_cf_options. I wish this were the default.
The above will enable the default compression type for all levels of the LSM tree which is Snappy in a recent MyRocks build with FB MySQL. But one of the proper distros only provides zlib and doing that for the small levels in the LSM tree (L0, L1, L2) might slow down compaction too much.

To set rocksdb_default_cf_options but disable compression use:
To set rocksdb_default_cf_options and use fast compression for all levels use this after changing $fast to one of kLZ4Compression or kSnappyCompression:
And to set rocksdb_default_cf_options with a fast compression configuration (no compression for smallest levels, fast compression for mid levels, slower compression for max level) try this after changing $fast and $slow to the appropriate values:
You can determine which compression libraries are supported by you MyRocks build by looking in $datadir/.rocksdb/LOG. This is from an FB MySQL build that I use:

Compression algorithms supported:
         kZSTDNotFinalCompression supported: 1
         kZSTD supported: 1
         kXpressCompression supported: 0
         kLZ4HCCompression supported: 1
         kLZ4Compression supported: 1
         kBZip2Compression supported: 1
         kZlibCompression supported: 1
         kSnappyCompression supported: 1

Some information on my.cnf for MyRocks is here and much more info on RocksDB is on the wiki. If you want to tune, then start with the tuning guide.

Another potential problem with the default configuration is that rocksdb_max_background_jobs=2 so there are 2 threads for compaction, you usually want more

Finally, there were some changes to MyRocks to make it better at avoiding too many tombstones and it looks like that is disabled by default:
rocksdb_compaction_sequential_deletes   0rocksdb_compaction_sequential_deletes_count_sd  OFFrocksdb_compaction_sequential_deletes_file_size 0rocksdb_compaction_sequential_deletes_window    0
Setting rocksdb_max_open_files to -1 can be great for performance but there are side-effects (more open file descriptors, more untracked memory consumption).

If you are only using MyRocks then you might want to set transaction-isolation=READ-COMMITTED because repeatable-read in MyRocks doesn't use gap locks (yet).

Finally, you probably want to set rocksdb_block_cache_size.

1 comment:

  1. Filed a feature request -