Only fallback to RocksDB internal prefetching on unsupported FS prefe…

…tching (#11897)

Summary:
**Context/Summary:**
facebook/rocksdb#11631 introduced an undesired fallback behavior to RocksDB internal prefetching even when FS prefetching return non-OK status other than "Unsupported". We only want to fall back when FS prefetching is not supported.

Pull Request resolved: facebook/rocksdb#11897

Test Plan: CI

Reviewed By: ajkr

Differential Revision: D49667055

Pulled By: hx235

fbshipit-source-id: fa36e4e5d6dc9507080217035f9d6ff8e4abda28

table/block_based/block_prefetcher.cc

@@ -37,11 +37,12 @@ void BlockPrefetcher::PrefetchIfNeeded(const BlockBasedTable::Rep* rep,
       if (s.ok()) {
         readahead_limit_ = offset + len + compaction_readahead_size_;
         return;
+      } else if (!s.IsNotSupported()) {
+        return;
       }
     }
     // If FS prefetch is not supported, fall back to use internal prefetch
-    // buffer. Discarding other return status of Prefetch calls intentionally,
-    // as we can fallback to reading from disk if Prefetch fails.
+    // buffer.
     //
     // num_file_reads is used  by FilePrefetchBuffer only when
     // implicit_auto_readahead is set.
@@ -123,8 +124,6 @@ void BlockPrefetcher::PrefetchIfNeeded(const BlockBasedTable::Rep* rep,
   }
 
   // If prefetch is not supported, fall back to use internal prefetch buffer.
-  // Discarding other return status of Prefetch calls intentionally, as
-  // we can fallback to reading from disk if Prefetch fails.
   IOOptions opts;
   Status s = rep->file->PrepareIOOptions(read_options, opts);
   if (!s.ok()) {

unreleased_history/bug_fixes/fallback_only_unsupported.md

@@ -0,0 +1 @@
+Fixed a bug where compaction read under non direct IO still falls back to RocksDB internal prefetching after file system's prefetching returns non-OK status other than `Status::NotSupported()`