@@ -28,7 +28,17 @@ void LMDB::Open(const string& source, Mode mode) {
   }
   int flags = 0;
   if (mode == READ) {
-    flags = MDB_RDONLY | MDB_NOTLS;
+    // No locking, assume DB is not written to at the same time, otherwise
+    // LMDB tries to lock the file, which fails if filesystem is read-only
+    flags = MDB_RDONLY | MDB_NOTLS | MDB_NOLOCK;
+  }
+  // Allow DB to be stand-alone file
+  {
+    struct stat st_buf;
+    stat(source.c_str(), &st_buf);
+    if (S_ISREG(st_buf.st_mode)) {
+      flags |= MDB_NOSUBDIR;
+    }
   }
   MDB_CHECK(mdb_env_open(mdb_env_, source.c_str(), flags, 0664));
   LOG(INFO) << "Opened lmdb " << source;
#include "caffe/util/db.hpp"

#include <sys/stat.h>
#include <string>

namespace caffe { namespace db {

const size_t LMDB_MAP_SIZE = 1099511627776;  // 1 TB

void LevelDB::Open(const string& source, Mode mode) {
  leveldb::Options options;
  options.block_size = 65536;
  options.write_buffer_size = 268435456;
  options.max_open_files = 100;
  options.error_if_exists = mode == NEW;
  options.create_if_missing = mode != READ;
  leveldb::Status status = leveldb::DB::Open(options, source, &db_);
  CHECK(status.ok()) << "Failed to open leveldb " << source
                     << std::endl << status.ToString();
  LOG(INFO) << "Opened leveldb " << source;
}

void LMDB::Open(const string& source, Mode mode) {
  MDB_CHECK(mdb_env_create(&mdb_env_));
  MDB_CHECK(mdb_env_set_mapsize(mdb_env_, LMDB_MAP_SIZE));
  if (mode == NEW) {
    CHECK_EQ(mkdir(source.c_str(), 0744), 0) << "mkdir " << source << "failed";
  }
  int flags = 0;
  if (mode == READ) {
    flags = MDB_RDONLY | MDB_NOTLS;
  }
  MDB_CHECK(mdb_env_open(mdb_env_, source.c_str(), flags, 0664));
  LOG(INFO) << "Opened lmdb " << source;
}

LMDBCursor* LMDB::NewCursor() {
  MDB_txn* mdb_txn;
  MDB_cursor* mdb_cursor;
  MDB_CHECK(mdb_txn_begin(mdb_env_, NULL, MDB_RDONLY, &mdb_txn));
  MDB_CHECK(mdb_dbi_open(mdb_txn, NULL, 0, &mdb_dbi_));
  MDB_CHECK(mdb_cursor_open(mdb_txn, mdb_dbi_, &mdb_cursor));
  return new LMDBCursor(mdb_txn, mdb_cursor);
}

LMDBTransaction* LMDB::NewTransaction() {
  MDB_txn* mdb_txn;
  MDB_CHECK(mdb_txn_begin(mdb_env_, NULL, 0, &mdb_txn));
  MDB_CHECK(mdb_dbi_open(mdb_txn, NULL, 0, &mdb_dbi_));
  return new LMDBTransaction(&mdb_dbi_, mdb_txn);
}

void LMDBTransaction::Put(const string& key, const string& value) {
  MDB_val mdb_key, mdb_value;
  mdb_key.mv_data = const_cast<char*>(key.data());
  mdb_key.mv_size = key.size();
  mdb_value.mv_data = const_cast<char*>(value.data());
  mdb_value.mv_size = value.size();
  MDB_CHECK(mdb_put(mdb_txn_, *mdb_dbi_, &mdb_key, &mdb_value, 0));
}

DB* GetDB(DataParameter::DB backend) {
  switch (backend) {
  case DataParameter_DB_LEVELDB:
    return new LevelDB();
  case DataParameter_DB_LMDB:
    return new LMDB();
  default:
    LOG(FATAL) << "Unknown database backend";
  }
}

DB* GetDB(const string& backend) {
  if (backend == "leveldb") {
    return new LevelDB();
  } else if (backend == "lmdb") {
    return new LMDB();
  } else {
    LOG(FATAL) << "Unknown database backend";
  }
}

}  // namespace db
}  // namespace caffe

You should check the return value of stat(2).