Support using buffered I/O with O_SYNC instead of direct I/O

docs/config/osd.en.md

@@ -31,6 +31,9 @@ them, even without restarting by updating configuration in etcd.
 - [max_flusher_count](#max_flusher_count)
 - [inmemory_metadata](#inmemory_metadata)
 - [inmemory_journal](#inmemory_journal)
+- [cached_io_data](#cached_io_data)
+- [cached_io_meta](#cached_io_meta)
+- [cached_io_journal](#cached_io_journal)
 - [journal_sector_buffer_count](#journal_sector_buffer_count)
 - [journal_no_same_sector_overwrites](#journal_no_same_sector_overwrites)
 - [throttle_small_writes](#throttle_small_writes)
@@ -255,6 +258,48 @@ is typically very small because it's sufficient to have 16-32 MB journal
 for SSD OSDs. However, in theory it's possible that you'll want to turn it
 off for hybrid (HDD+SSD) OSDs with large journals on quick devices.
 
+## cached_io_data
+
+- Type: boolean
+- Default: false
+
+Read and write *data* through Linux page cache, i.e. use a file descriptor
+opened with O_SYNC, but without O_DIRECT for I/O. May improve read
+performance for hot data and slower disks - HDDs and maybe SATA SSDs.
+Not recommended for desktop SSDs without capacitors because O_SYNC flushes
+disk cache on every write.
+
+## cached_io_meta
+
+- Type: boolean
+- Default: false
+
+Read and write *metadata* through Linux page cache. May improve read
+performance only if your drives are relatively slow (HDD, SATA SSD), and
+only if checksums are enabled and [inmemory_metadata](#inmemory_metadata)
+is disabled, because in this case metadata blocks are read from disk
+on every read request to verify checksums and caching them may reduce this
+extra read load.
+
+Absolutely pointless to enable with enabled inmemory_metadata because all
+metadata is kept in memory anyway, and likely pointless without checksums,
+because in that case, metadata blocks are read from disk only during journal
+flushing.
+
+If the same device is used for data and metadata, enabling [cached_io_data](#cached_io_data)
+also enables this parameter, given that it isn't turned off explicitly.
+
+## cached_io_journal
+
+- Type: boolean
+- Default: false
+
+Read and write *journal* through Linux page cache. May improve read
+performance if [inmemory_journal](#inmemory_journal) is turned off.
+
+If the same device is used for metadata and journal, enabling [cached_io_meta](#cached_io_meta)
+also enables this parameter, given that it isn't turned off explicitly.
+
 ## journal_sector_buffer_count
 
 - Type: integer