Options: Set compaction_readahead_size default to 0 (#792)

HISTORY.md

@@ -16,6 +16,7 @@ Based on RocksDB 8.6.7
 
 ### Miscellaneous
 * Remove leftover references to ROCKSDB_LITE (#755).
+* Options: Set compaction_readahead_size default to 0. The current default of 2Mb is not optimal for most of our use cases. Having a value of 0 means that the FS will use its default size for prefetching (true only with https://github.com/speedb-io/speedb/pull/788).
 
 ## Hazlenut 2.7.0 (27/10/2023)
 Based on RocksDB 8.1.1

include/rocksdb/options.h

@@ -1005,10 +1005,10 @@ struct DBOptions {
   // running RocksDB on spinning disks, you should set this to at least 2MB.
   // That way RocksDB's compaction is doing sequential instead of random reads.
   //
-  // Default: 2MB
+  // Default: 0
   //
   // Dynamically changeable through SetDBOptions() API.
-  size_t compaction_readahead_size = 2 * 1024 * 1024;
+  size_t compaction_readahead_size = 0;
 
   // This is a maximum buffer size that is used by WinMmapReadableFile in
   // unbuffered disk I/O mode. We need to maintain an aligned buffer for