Compaction stats length is limited to 65536

[Longarithm]

Expected behavior

Get full compaction logs after calling DBImpl::DumpStats: https://github.com/facebook/rocksdb/blob/9ef7f70c11e26ac82bc3716d47902ad3a773fa48/db/db_impl/db_impl.cc#L893

Actual behavior

Instead, compaction stats length is limited to 65536, which results in only partial logs display (in reality we have > 50 columns):

** DB Stats **
Uptime(secs): 24069.1 total, 600.0 interval
Cumulative writes: 3279K writes, 17M keys, 3279K commit groups, 1.0 writes per commit group, ingest: 13.45 GB, 0.57 MB/s

...

** Compaction Stats [col22] **
Level    Files   Size     Score Read(GB)  Rn(GB) Rnp1(GB) Write(GB) Wnew(GB) Moved(GB) W-Amp Rd(MB/s) Wr(MB/s) Comp(sec) CompMergeCPU(sec) Comp(cnt) Avg(sec) KeyIn KeyDrop
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  L0      1/0   16.68 MB   0.5      0.0     0.0      0.0       0.0 

Steps to reproduce the behavior

Call DBImpl::DumpStats with large number of columns (200 should be enough).

Idea

This line is causing cropping compaction stats: https://github.com/facebook/rocksdb/blob/2e09a54c4fb82e88bcaa3e7cfa8ccbbbbf3635d5/logging/posix_logger.h#L100

The simplest solution could be increasing the buffer, but it could be more robust to print compaction stats separately for each column instead of collecting all stats to one string: https://github.com/facebook/rocksdb/blob/9ef7f70c11e26ac82bc3716d47902ad3a773fa48/db/db_impl/db_impl.cc#L928

[jay-zhuang]

@Longarithm make sense, either print it separately or using ostream to buffer. Are you interested in contributing a PR?

[lhsoft]

@jay-zhuang I'd like to work on it, please feel free to assign it to me:)