Where to find it? You can find compaction stats in following ways:

1. RocksDB dump statistics to LOG file every stats_dump_period_sec seconds. This is 600 by default, which means that stats will be dumped every 10 minutes in LOG files.
2. You can get the same data in the application by calling db->GetProperty("rocksdb.stats");

In both ways, the outputs look like this:

** Compaction Stats **
Level Files  Size(MB) Score Read(GB)  Rn(GB) Rnp1(GB) Write(GB) Wnew(GB) Moved(GB) W-Amp Rd(MB/s) Wr(MB/s) Comp(sec) Comp(cnt) Avg(sec) Stall(sec) Stall(cnt) Avg(ms)     KeyIn   KeyDrop
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
L0      2/0        15   0.5     0.0     0.0      0.0      32.8     32.8       0.0   0.0      0.0     23.0    1457      4346    0.335       0.00          0    0.00             0        0
L1     22/0       125   1.0   163.7    32.8    130.9     165.5     34.6       0.0   5.1     25.6     25.9    6549      1086    6.031       0.00          0    0.00    1287667342        0
L2    227/0      1276   1.0   262.7    34.4    228.4     262.7     34.3       0.1   7.6     26.0     26.0   10344      4137    2.500       0.00          0    0.00    1023585700        0
L3   1634/0     12794   1.0   259.7    31.7    228.1     254.1     26.1       1.5   8.0     20.8     20.4   12787      3758    3.403       0.00          0    0.00    1128138363        0
L4   1819/0     15132   0.1     3.9     2.0      2.0       3.6      1.6      13.1   1.8     20.1     18.4     201       206    0.974       0.00          0    0.00      91486994        0
Sum  3704/0     29342   0.0   690.1   100.8    589.3     718.7    129.4      14.8  21.9     22.5     23.5   31338     13533    2.316       0.00          0    0.00    3530878399        0
Int     0/0         0   0.0     2.1     0.3      1.8       2.2      0.4       0.0  24.3     24.0     24.9      91        42    2.164       0.00          0    0.00      11718977        0
Flush(GB): accumulative 32.786, interval 0.091
Stalls(secs): 0.000 level0_slowdown, 0.000 level0_numfiles, 0.000 memtable_compaction, 0.000 leveln_slowdown_soft, 0.000 leveln_slowdown_hard
Stalls(count): 0 level0_slowdown, 0 level0_numfiles, 0 memtable_compaction, 0 leveln_slowdown_soft, 0 leveln_slowdown_hard

** DB Stats **
Uptime(secs): 128748.3 total, 300.1 interval
Cumulative writes: 1288457363 writes, 14173030838 keys, 357293118 batches, 3.6 writes per batch, 3055.92 GB user ingest, stall micros: 7067721262
Cumulative WAL: 1251702527 writes, 357293117 syncs, 3.50 writes per sync, 3055.92 GB written
Interval writes: 3621943 writes, 39841373 keys, 1013611 batches, 3.6 writes per batch, 8797.4 MB user ingest, stall micros: 112418835
Interval WAL: 3511027 writes, 1013611 syncs, 3.46 writes per sync, 8.59 MB written

Compaction stats for the compactions executed between levels N and N+1 are reported at level N+1 (compaction output). Here is the quick reference:

• Level - for leveled compaction the level of the LSM. For universal compaction all files are in L0. Sum has the values aggregated over all levels. Int is like Sum but limited to the data from the last reporting interval.
• Files - this has two values as (a/b). The first is the number of files in the level. The second is the number of files currently doing compaction for that level.
• Score: for levels other than L0 the score is (current level size) / (max level size). Values of 0 or 1 are okay, but any value greater than 1 means that level needs to be compacted. For L0 the score is computed from the current number of files and number of files that triggers a compaction.
• Read(GB): Total bytes read during compaction between levels N and N+1. This includes bytes read from level N and from level N+1
• Rn(GB): Bytes read from level N during compaction between levels N and N+1
• Rnp1(GB): Bytes read from level N+1 during compaction between levels N and N+1
• Write(GB): Total bytes written during compaction between levels N and N+1
• Wnew(GB): New bytes written to level N+1, calculated as (total bytes written to N+1) - (bytes read from N+1 during compaction with level N)
• Moved(GB): Bytes moved to level N+1 during compaction. In this case there is no IO other than updating the manifest to indicate that a file which used to be in level X is now in level Y
• W-Amp: (total bytes written to level N+1) / (total bytes read from level N). This is the write amplification from compaction between levels N and N+1
• Rd(MB/s): The rate at which data is read during compaction between levels N and N+1. This is (Read(GB) * 1024) / duration where duration is the time for which compactions are in progress from level N to N+1.
• Wr(MB/s): The rate at which data is written during compaction. See Rd(MB/s).
• Rn(cnt): Total files read from level N during compaction between levels N and N+1
• Rnp1(cnt): Total files read from level N+1 during compaction between levels N and N+1
• Wnp1(cnt): Total files written to level N+1 during compaction between levels N and N+1
• Wnew(cnt): (Wnp1(cnt) - Rnp1(cnt)) -- Increase in file count as result of compaction between levels N and N+1
• Comp(sec): Total time spent doing compactions between levels N and N+1
• Comp(cnt): Total number of compactions between levels N and N+1
• Avg(sec): Average time per compaction between levels N and N+1
• Stall(sec): Total time writes were stalled because level N+1 was uncompacted (compaction score was high)
• Stall(cnt): Total number of writes stalled because level N+1 was uncompacted
• Avg(ms): Average time in milliseconds a write was stalled because level N+1 was uncompacted
• KeyIn: number of records compared during compaction
• KeyDrop: number of records dropped (not written out) during compaction

After the per-level compaction stats, we also output some general stats. General stats are reported for both cumulative and interval. Cumulative stats report total values from RocksDB instance start. Interval stats report values since the last stats output.

• Uptime(secs): total -- number of seconds this instance has been running, interval -- number of seconds since the last stats dump.
• Cumulative/Interval writes: total -- number of Put calls; keys -- number of entries in the WriteBatches from the Put calls; batches -- number of group commits where each group commit makes persistent one or more Put calls (with concurrency there can be more than 1 Put call made persistent at one point in time); per batch -- average number of bytes in a single batch; ingest -- total bytes written into DB (not counting compactions); stall micros - number of microseconds writes have been stalled when compaction gets behind
• Cumulative/Interval WAL: writes -- number of writes logged in the WAL; syncs - number of times fsync or fdatasync has been used; writes per sync - ratio of writes to syncs; GB written - number of GB written to the WAL
• Stalls: total count and seconds of each stall type since beginning of time: level0_slowdown -- Stall because of level0_slowdown_writes_trigger. level0_numfiles -- Stall because of level0_stop_writes_trigger. memtable_compaction -- Stall because all memtables were full, flush process couldn't keep up. leveln_slowdown -- Stall because of soft_rate_limit and hard_rate_limit