No file system prefetching when Options::compaction_readahead_size is…

… 0 (#11887)

Summary:
**Context/Summary:**

facebook/rocksdb#11631 introduced `readahead()` system call for compaction read under non direct IO. When `Options::compaction_readahead_size` is 0, the `readahead()` will issued with a small size (i.e, the block size, by default 4KB)

Benchmarks shows that such readahead() call regresses the compaction read compared with "no readahead()" case (see Test Plan for more).

Therefore we decided to not issue such `readhead() ` when `Options::compaction_readahead_size` is 0.

Pull Request resolved: facebook/rocksdb#11887

Test Plan:
Settings: `compaction_readahead_size = 0, use_direct_reads=false`
Setup:
```
TEST_TMPDIR=../ ./db_bench -benchmarks=filluniquerandom -disable_auto_compactions=true -write_buffer_size=1048576 -compression_type=none -value_size=10240 && tar -cf ../dbbench.tar -C ../dbbench/ .
```
Run:
```
for i in $(seq 3); do rm -rf ../dbbench/ && mkdir -p ../dbbench/ && tar -xf ../dbbench.tar -C ../dbbench/ . && sudo bash -c 'sync && echo 3 > /proc/sys/vm/drop_caches' && TEST_TMPDIR=../ /usr/bin/time ./db_bench_{pre_PR11631|PR11631|PR11631_with_improvementPR11887} -benchmarks=compact -use_existing_db=true -db=../dbbench/ -disable_auto_compactions=true -compression_type=none ; done |& grep elapsed
```

pre-PR11631("no readahead()" case):

PR11631:

PR11631+this improvement:

Reviewed By: ajkr

Differential Revision: D49607266

Pulled By: hx235

fbshipit-source-id: 2efa0dc91bac3c11cc2be057c53d894645f683ef

table/block_based/block_prefetcher.cc

@@ -22,7 +22,7 @@ void BlockPrefetcher::PrefetchIfNeeded(const BlockBasedTable::Rep* rep,
   const size_t offset = handle.offset();
 
   if (is_for_compaction) {
-    if (!rep->file->use_direct_io()) {
+    if (!rep->file->use_direct_io() && compaction_readahead_size_ > 0) {
       // If FS supports prefetching (readahead_limit_ will be non zero in that
       // case) and current block exists in prefetch buffer then return.
       if (offset + len <= readahead_limit_) {