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    • Julien Muchembled's avatar
    • Julien Muchembled's avatar
    • Julien Muchembled's avatar
      mariadb: build RocksDB without fallocate · dc16dc04
      Julien Muchembled authored
      https://github.com/facebook/rocksdb/commit/7291a3f813e563efbd6870465b1063a115480373
      is not enough.
      
      Anway, given how SST files are written, fallocate
      is unlikely to be useful, even less on SSD.
      
      RocksDB seems to have options to disable at runtime (see env/io_posix.cc)
      but I see nothing for that in MariaDB configuration.
      
      Steps to reproduce:
      
      With a Btrfs partition (no specific mkfs option and for example 2GB size)
      and following MariaDB configuration:
      
        [mysqld]
        datadir = /tmp/datadir
        disable-log-bin
        plugin-load = ha_rocksdb
        rocksdb_default_cf_options = compression=kNoCompression;bottommost_compression=kZSTD
      
      execute the following queries with `mysql -uroot`:
      
      > USE test;
      > CREATE TABLE t(x MEDIUMBLOB) ENGINE=RocksDB;
      > INSERT INTO t VALUES(repeat('.',16777215));
      > INSERT INTO t SELECT x FROM t UNION ALL SELECT x FROM t;
      > INSERT INTO t SELECT x FROM t UNION ALL SELECT x FROM t;
      > INSERT INTO t SELECT x FROM t UNION ALL SELECT x FROM t;
      
      Disk usage can be checked with:
      
      > SHOW ENGINE RocksDB STATUS \G
      
      Level    Files   Size     ...
      ------------------------- ...
        L0      1/0   144.00 MB ...
        L2      1/0   288.00 MB ...
       Sum      2/0   432.00 MB ...
      
      and this can be compacted as follows:
      
      > SET GLOBAL rocksdb_compact_cf='default';
      
      > SHOW ENGINE RocksDB STATUS \G
      
      Level    Files   Size     ...
      ------------------------- ...
        L0      0/0    0.00 KB  ...
        L1      0/0    0.00 KB  ...
        L2      1/0   16.28 KB  ...
       Sum      1/0   16.28 KB  ...
      
      Now using `compsize` (on Debian, it's provided by btrfs-compsize package):
      
      # with -DWITH_FALLOCATE=NO
      
      $ compsize /tmp/datadir/#rocksdb/000029.sst
      Type       Perc     Disk Usage   Uncompressed Referenced
      TOTAL      100%       20K          20K          20K
      none       100%       20K          20K          20K
      
      # without -DWITH_FALLOCATE=NO
      
      $ compsize /tmp/datadir/#rocksdb/000029.sst
      Type       Perc     Disk Usage   Uncompressed Referenced
      TOTAL      100%       35M          35M          20K
      none       100%       35M          35M          20K
      dc16dc04
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