Fixup HISTORY.md for 6.22 release

`git diff origin/6.21.fb origin/6.22.fb -- HISTORY.md` looked odd.
This PR fixes it up by moving items from 6.21.0 to 6.22.0 that were
never in any 6.21 release. Also mentioned the background stat collection
fix under 6.22 (previously it was mentioned under 6.21 patch releases
only).

HISTORY.md

@@ -8,11 +8,14 @@
 * fs_posix.cc GetFreeSpace() always report disk space available to root even when running as non-root.  Linux defaults often have disk mounts with 5 to 10 percent of total space reserved only for root.  Out of space could result for non-root users.
 * Subcompactions are now disabled when user-defined timestamps are used, since the subcompaction boundary picking logic is currently not timestamp-aware, which could lead to incorrect results when different subcompactions process keys that only differ by timestamp.
 * Fix an issue that `DeleteFilesInRange()` may cause ongoing compaction reports corruption exception, or ASSERT for debug build. There's no actual data loss or corruption that we find.
+* Fixed confusingly duplicated output in LOG for periodic stats ("DUMPING STATS"), including "Compaction Stats" and "File Read Latency Histogram By Level".
+* Fixed performance bugs in background gathering of block cache entry statistics, that could consume a lot of CPU when there are many column families with a shared block cache.
 
 ### New Features
 * Marked the Ribbon filter and optimize_filters_for_memory features as production-ready, each enabling memory savings for Bloom-like filters. Use `NewRibbonFilterPolicy` in place of `NewBloomFilterPolicy` to use Ribbon filters instead of Bloom, or `ribbonfilter` in place of `bloomfilter` in configuration string.
 * Allow `DBWithTTL` to use `DeleteRange` api just like other DBs. `DeleteRangeCF()` which executes `WriteBatchInternal::DeleteRange()` has been added to the handler in `DBWithTTLImpl::Write()` to implement it.
 * Add BlockBasedTableOptions.prepopulate_block_cache.  If enabled, it prepopulate warm/hot data blocks which are already in memory into block cache at the time of flush. On a flush, the data block that is in memory (in memtables) get flushed to the device. If using Direct IO, additional IO is incurred to read this data back into memory again, which is avoided by enabling this option and it also helps with Distributed FileSystem. More details in include/rocksdb/table.h.
+* Added a `cancel` field to `CompactRangeOptions`, allowing individual in-process manual range compactions to be cancelled.
 
 ## 6.21.0 (2021-05-21)
 ### Bug Fixes
@@ -24,7 +27,6 @@
 * Handle return code by io_uring_submit_and_wait() and io_uring_wait_cqe().
 * In the IngestExternalFile() API, only try to sync the ingested file if the file is linked and the FileSystem/Env supports reopening a writable file.
 * Fixed a bug that `AdvancedColumnFamilyOptions.max_compaction_bytes` is under-calculated for manual compaction (`CompactRange()`). Manual compaction is split to multiple compactions if the compaction size exceed the `max_compaction_bytes`. The bug creates much larger compaction which size exceed the user setting. On the other hand, larger manual compaction size can increase the subcompaction parallelism, you can tune that by setting `max_compaction_bytes`.
-* Fixed confusingly duplicated output in LOG for periodic stats ("DUMPING STATS"), including "Compaction Stats" and "File Read Latency Histogram By Level".
 
 ### Behavior Changes
 * Due to the fix of false-postive alert of "SST file is ahead of WAL", all the CFs with no SST file (CF empty) will bypass the consistency check. We fixed a false-positive, but introduced a very rare true-negative which will be triggered in the following conditions: A CF with some delete operations in the last a few queries which will result in an empty CF (those are flushed to SST file and a compaction triggered which combines this file and all other SST files and generates an empty CF, or there is another reason to write a manifest entry for this CF after a flush that generates no SST file from an empty CF). The deletion entries are logged in a WAL and this WAL was corrupted, while the CF's log number points to the next WAL (due to the flush). Therefore, the DB can only recover to the point without these trailing deletions and cause the inconsistent DB status.
@@ -37,7 +39,6 @@
 * Add an experimental Remote Compaction feature, which allows the user to run Compaction on a different host or process. The feature is still under development, currently only works on some basic use cases. The interface will be changed without backward/forward compatibility support.
 * RocksDB would validate total entries read in flush, and compare with counter inserted into it. If flush_verify_memtable_count = true (default), flush will fail. Otherwise, only log to info logs.
 * Add `TableProperties::num_filter_entries`, which can be used with `TableProperties::filter_size` to calculate the effective bits per filter entry (unique user key or prefix) for a table file.
-* Added a `cancel` field to `CompactRangeOptions`, allowing individual in-process manual range compactions to be cancelled.
 
 ### Performance Improvements
 * BlockPrefetcher is used by iterators to prefetch data if they anticipate more data to be used in future. It is enabled implicitly by rocksdb. Added change to take in account read pattern if reads are sequential. This would disable prefetching for random reads in MultiGet and iterators as readahead_size is increased exponential doing large prefetches.