Summary:
**Context:**
[PR11406](https://github.com/facebook/rocksdb/pull/11406/) caused more frequent read during db open reading files with no `tail_size` in the manifest as part of the upgrade to 11406. This is due to that PR introduced
- [smaller](https://github.com/facebook/rocksdb/pull/11406/files#diff-57ed8c49db2bdd4db7618646a177397674bbf25beacacecb104070071d30129fR833) prefetch tail buffer size compared to pre-11406 for small files (< 52 MB) when `tail_prefetch_stats` infers tail size to be 0 (usually happens when the stats does not have much historical data to infer early on)
- more read (up to # of partitioned filter/index) when such small prefetch tail buffer does not contain all the partitioned filter/index needed in CacheDependencies() since the [fallback logic](https://github.com/facebook/rocksdb/pull/11406/files#diff-d98f1a83de24412ad7f3527725dae7e28851c7222622c3cdb832d3cdf24bbf9fR165-R179) that prefetches all partitions at once will be [skipped](url) when such a small prefetch tail buffer is passed in
**Summary:**
- Revert the fallback prefetch buffer size change to preserve existing behavior fully during upgrading in `BlockBasedTable::PrefetchTail()`
- Use passed-in prefetch tail buffer in `CacheDependencies()` only if it has a smaller offset than the the offset of first partition filter/index, that is, at least as good as the existing prefetching behavior
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11516
Test Plan:
- db bench
Create db with small files prior to PR 11406
```
./db_bench -db=/tmp/testdb/ --partition_index_and_filters=1 --statistics=1 -benchmarks=fillseq -key_size=3200 -value_size=5 -num=1000000 -write_buffer_size=6550000 -disable_auto_compactions=true -compression_type=zstd`
```
Read db to see if post-pr has lower read qps (i.e, rocksdb.file.read.db.open.micros count) during db open.
```
./db_bench -use_direct_reads=1 --file_opening_threads=1 --threads=1 --use_existing_db=1 --seed=1682546046158958 --partition_index_and_filters=1 --statistics=1 --db=/tmp/testdb/ --benchmarks=readrandom --key_size=3200 --value_size=5 --num=100 --disable_auto_compactions=true --compression_type=zstd
```
Pre-PR:
```
rocksdb.file.read.db.open.micros P50 : 3.399023 P95 : 5.924468 P99 : 12.408333 P100 : 29.000000 COUNT : 611 SUM : 2539
```
Post-PR:
```
rocksdb.file.read.db.open.micros P50 : 593.736842 P95 : 861.605263 P99 : 1212.868421 P100 : 2663.000000 COUNT : 585 SUM : 345349
```
_Note: To control the starting offset of the prefetch tail buffer easier, I manually override the following to eliminate the effect of alignment_
```
class PosixRandomAccessFile : public FSRandomAccessFile {
virtual size_t GetRequiredBufferAlignment() const override {
- return logical_sector_size_;
+ return 1;
}
```
- CI
Reviewed By: pdillinger
Differential Revision: D46472566
Pulled By: hx235
fbshipit-source-id: 2fe14ac8d489d15b0e08e6f8fe4f46d5f110978e
Hui Xiao