storage/engine: log RocksDB compaction stats

[petermattis]

Periodically log RocksDB compaction stats which include details such as
write amplification per level.

[cockroach-teamcity]

This change is 

[petermattis]

For an example of what this output looks like, see https://github.com/facebook/rocksdb/wiki/RocksDB-Tuning-Guide#rocksdb-statistics.

[tbg]

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Reviewed 5 of 5 files at r1.
Review status: all files reviewed at latest revision, 1 unresolved discussion, some commit checks failed.

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pkg/storage/engine/rocksdb.go, line 832 at r1 (raw file):

}

// GetCompactionStats returns the internal RocksDB compaction stats.

Since you're eating CI anyway, mind adding the link to RocksDB's explanation in the comment?

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Comments from Reviewable

[petermattis]

Review status: all files reviewed at latest revision, 1 unresolved discussion, some commit checks failed.

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pkg/storage/engine/rocksdb.go, line 832 at r1 (raw file):

Previously, tschottdorf (Tobias Schottdorf) wrote…

Since you're eating CI anyway, mind adding the link to RocksDB's explanation in the comment?

Done.

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Comments from Reviewable

[tbg]

Reviewed 1 of 1 files at r2.
Review status: all files reviewed at latest revision, 1 unresolved discussion, some commit checks pending.

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Comments from Reviewable