src/kudu/fs/block_manager.cc

// Licensed to the Apache Software Foundation (ASF) under one
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// distributed with this work for additional information
// regarding copyright ownership.  The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License.  You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing,
// software distributed under the License is distributed on an
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// under the License.

#include "kudu/fs/block_manager.h"

#include <gflags/gflags.h>

#include "kudu/gutil/macros.h"
#include "kudu/util/flag_tags.h"

// The default value is optimized for throughput in the case that
// there are multiple drives backing the tablet. By asynchronously
// flushing each block before issuing any fsyncs, the IO across
// disks is done in parallel.
//
// This increases throughput but can harm latency in the case that
// there are few disks and the WAL is on the same disk as the
// data blocks. The default is chosen based on the assumptions that:
// - latency is leveled across machines by Raft
// - latency-sensitive applications can devote a disk to the WAL
// - super-sensitive applications can devote an SSD to the WAL.
// - users could always change this to "never", which slows down
//   throughput but may improve write latency.
DEFINE_string(block_manager_preflush_control, "finalize",
              "Controls when to pre-flush a block. Valid values are 'finalize', "
              "'close', or 'never'. If 'finalize', blocks will be pre-flushed "
              "when writing is finished. If 'close', blocks will be pre-flushed "
              "when their transaction is committed. If 'never', blocks will "
              "never be pre-flushed but still be flushed when closed.");
TAG_FLAG(block_manager_preflush_control, experimental);

namespace kudu {
namespace fs {

BlockManagerOptions::BlockManagerOptions()
  : read_only(false) {}

} // namespace fs
} // namespace kudu