os/bluestore: precondition rocksdb on mkfs

Write N bytes of garbage to the kv store on startup.  With rocksdb,
this ensures that our log files are preallocated.

This option needs to match up with the

Signed-off-by: Sage Weil <sage@redhat.com>

src/common/config_opts.h

@@ -959,6 +959,8 @@ OPTION(bluestore_block_wal_path, OPT_STR, "")
 OPTION(bluestore_block_wal_size, OPT_U64, 96 * 1024*1024) // rocksdb wal
 OPTION(bluestore_block_wal_create, OPT_BOOL, false)
 OPTION(bluestore_block_preallocate_file, OPT_BOOL, false) //whether preallocate space if block/db_path/wal_path is file rather that block device.
+OPTION(bluestore_precondition_bluefs, OPT_U64, 48*1024*1024)  // write this much data at mkfs
+OPTION(bluestore_precondition_bluefs_block, OPT_U64, 1048576)
 OPTION(bluestore_csum, OPT_BOOL, true)
 OPTION(bluestore_csum_type, OPT_STR, "crc32c")
 OPTION(bluestore_min_csum_block, OPT_U32, 4096)

src/os/bluestore/BlueStore.cc

@@ -2738,6 +2738,36 @@ int BlueStore::mkfs()
     goto out_close_alloc;
   dout(10) << __func__ << " success" << dendl;
 
+  if (bluefs &&
+      g_conf->bluestore_precondition_bluefs > 0) {
+    dout(10) << __func__ << " preconditioning with "
+	     << pretty_si_t(g_conf->bluestore_precondition_bluefs)
+	     << " in blocks of "
+	     << pretty_si_t(g_conf->bluestore_precondition_bluefs_block)
+	     << dendl;
+    unsigned n = g_conf->bluestore_precondition_bluefs /
+      g_conf->bluestore_precondition_bluefs_block;
+    bufferlist bl;
+    bufferptr bp(g_conf->bluestore_precondition_bluefs_block);
+    for (unsigned i=0; i < g_conf->bluestore_precondition_bluefs_block; ++i) {
+      bp[i] = rand();
+    }
+    bl.append(bp);
+    string key1("a");
+    string key2("b");
+    for (unsigned i=0; i < n; ++i) {
+      KeyValueDB::Transaction t = db->get_transaction();
+      t->set(PREFIX_SUPER, (i & 1) ? key1 : key2, bl);
+      t->rmkey(PREFIX_SUPER, (i & 1) ? key2 : key1);
+      db->submit_transaction_sync(t);
+    }
+    KeyValueDB::Transaction t = db->get_transaction();
+    t->rmkey(PREFIX_SUPER, key1);
+    t->rmkey(PREFIX_SUPER, key2);
+    db->submit_transaction_sync(t);
+    dout(10) << __func__ << " done preconditioning" << dendl;
+  }
+
  out_close_alloc:
   _close_alloc();
  out_close_fm: