Add BlobDB GC cutoff parameter to db_bench (#6211)

Summary:
The patch makes it possible to set the BlobDB configuration option
`garbage_collection_cutoff` on the command line. In addition, it changes
the `db_bench` code so that the default values of BlobDB related
parameters are taken from the defaults of the actual BlobDB
configuration options (note: this changes the the default of
`blob_db_bytes_per_sync`).
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6211

Test Plan: Ran `db_bench` with various values of the new parameter.

Differential Revision: D19166895

Pulled By: ltamasi

fbshipit-source-id: 305ccdf0123b9db032b744715810babdc3e3b7d5

HISTORY.md

@@ -15,6 +15,7 @@
 ### New Features
 * It is now possible to enable periodic compactions for the base DB when using BlobDB.
 * BlobDB now garbage collects non-TTL blobs when `enable_garbage_collection` is set to `true` in `BlobDBOptions`. Garbage collection is performed during compaction: any valid blobs located in the oldest N files (where N is the number of non-TTL blob files multiplied by the value of `BlobDBOptions::garbage_collection_cutoff`) encountered during compaction get relocated to new blob files, and old blob files are dropped once they are no longer needed. Note: we recommend enabling periodic compactions for the base DB when using this feature to deal with the case when some old blob files are kept alive by SSTs that otherwise do not get picked for compaction.
+* `db_bench` now supports the `garbage_collection_cutoff` option for BlobDB.
 
 ## 6.6.0 (11/25/2019)
 ### Bug Fixes

tools/db_bench_tool.cc

@@ -750,27 +750,40 @@ DEFINE_bool(use_blob_db, false,
             "Open a BlobDB instance. "
             "Required for large value benchmark.");
 
-DEFINE_bool(blob_db_enable_gc, false, "Enable BlobDB garbage collection.");
+DEFINE_bool(blob_db_enable_gc,
+            rocksdb::blob_db::BlobDBOptions().enable_garbage_collection,
+            "Enable BlobDB garbage collection.");
 
-DEFINE_bool(blob_db_is_fifo, false, "Enable FIFO eviction strategy in BlobDB.");
+DEFINE_double(blob_db_gc_cutoff,
+              rocksdb::blob_db::BlobDBOptions().garbage_collection_cutoff,
+              "Cutoff ratio for BlobDB garbage collection.");
 
-DEFINE_uint64(blob_db_max_db_size, 0,
+DEFINE_bool(blob_db_is_fifo, rocksdb::blob_db::BlobDBOptions().is_fifo,
+            "Enable FIFO eviction strategy in BlobDB.");
+
+DEFINE_uint64(blob_db_max_db_size,
+              rocksdb::blob_db::BlobDBOptions().max_db_size,
               "Max size limit of the directory where blob files are stored.");
 
 DEFINE_uint64(
     blob_db_max_ttl_range, 0,
     "TTL range to generate BlobDB data (in seconds). 0 means no TTL.");
 
-DEFINE_uint64(blob_db_ttl_range_secs, 3600,
+DEFINE_uint64(blob_db_ttl_range_secs,
+              rocksdb::blob_db::BlobDBOptions().ttl_range_secs,
               "TTL bucket size to use when creating blob files.");
 
-DEFINE_uint64(blob_db_min_blob_size, 0,
+DEFINE_uint64(blob_db_min_blob_size,
+              rocksdb::blob_db::BlobDBOptions().min_blob_size,
               "Smallest blob to store in a file. Blobs smaller than this "
               "will be inlined with the key in the LSM tree.");
 
-DEFINE_uint64(blob_db_bytes_per_sync, 0, "Bytes to sync blob file at.");
+DEFINE_uint64(blob_db_bytes_per_sync,
+              rocksdb::blob_db::BlobDBOptions().bytes_per_sync,
+              "Bytes to sync blob file at.");
 
-DEFINE_uint64(blob_db_file_size, 256 * 1024 * 1024,
+DEFINE_uint64(blob_db_file_size,
+              rocksdb::blob_db::BlobDBOptions().blob_file_size,
               "Target size of each blob file.");
 
 // Secondary DB instance Options
@@ -3994,6 +4007,7 @@ class Benchmark {
     } else if (FLAGS_use_blob_db) {
       blob_db::BlobDBOptions blob_db_options;
       blob_db_options.enable_garbage_collection = FLAGS_blob_db_enable_gc;
+      blob_db_options.garbage_collection_cutoff = FLAGS_blob_db_gc_cutoff;
       blob_db_options.is_fifo = FLAGS_blob_db_is_fifo;
       blob_db_options.max_db_size = FLAGS_blob_db_max_db_size;
       blob_db_options.ttl_range_secs = FLAGS_blob_db_ttl_range_secs;