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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root directory).
//
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.
#pragma once
#include <atomic>
#include <sstream>
#include <string>
#include "file/random_access_file_reader.h"
#include "port/port.h"
#include "rocksdb/env.h"
#include "util/aligned_buffer.h"
namespace ROCKSDB_NAMESPACE {
// FilePrefetchBuffer is a smart buffer to store and read data from a file.
class FilePrefetchBuffer {
public:
// Constructor.
//
// All arguments are optional.
// file_reader : the file reader to use. Can be a nullptr.
// readahead_size : the initial readahead size.
// max_readahead_size : the maximum readahead size.
// If max_readahead_size > readahead_size, the readahead size will be
// doubled on every IO until max_readahead_size is hit.
// Typically this is set as a multiple of readahead_size.
// max_readahead_size should be greater than equal to readahead_size.
// enable : controls whether reading from the buffer is enabled.
// If false, TryReadFromCache() always return false, and we only take stats
// for the minimum offset if track_min_offset = true.
// track_min_offset : Track the minimum offset ever read and collect stats on
// it. Used for adaptable readahead of the file footer/metadata.
//
// Automatic readhead is enabled for a file if file_reader, readahead_size,
// and max_readahead_size are passed in.
// If file_reader is a nullptr, setting readadhead_size and max_readahead_size
// does not make any sense. So it does nothing.
// A user can construct a FilePrefetchBuffer without any arguments, but use
// `Prefetch` to load data into the buffer.
FilePrefetchBuffer(RandomAccessFileReader* file_reader = nullptr,
size_t readadhead_size = 0, size_t max_readahead_size = 0,
bool enable = true, bool track_min_offset = false)
: buffer_offset_(0),
file_reader_(file_reader),
readahead_size_(readadhead_size),
max_readahead_size_(max_readahead_size),
min_offset_read_(port::kMaxSizet),
enable_(enable),
track_min_offset_(track_min_offset) {}
// Load data into the buffer from a file.
// reader : the file reader.
// offset : the file offset to start reading from.
// n : the number of bytes to read.
// for_compaction : if prefetch is done for compaction read.
Status Prefetch(RandomAccessFileReader* reader, uint64_t offset, size_t n,
bool for_compaction = false);
// Tries returning the data for a file raed from this buffer, if that data is
// in the buffer.
// It handles tracking the minimum read offset if track_min_offset = true.
// It also does the exponential readahead when readadhead_size is set as part
// of the constructor.
//
// offset : the file offset.
// n : the number of bytes.
// result : output buffer to put the data into.
// for_compaction : if cache read is done for compaction read.
bool TryReadFromCache(uint64_t offset, size_t n, Slice* result,
bool for_compaction = false);
// The minimum `offset` ever passed to TryReadFromCache(). This will nly be
// tracked if track_min_offset = true.
size_t min_offset_read() const { return min_offset_read_; }
private:
AlignedBuffer buffer_;
uint64_t buffer_offset_;
RandomAccessFileReader* file_reader_;
size_t readahead_size_;
size_t max_readahead_size_;
// The minimum `offset` ever passed to TryReadFromCache().
size_t min_offset_read_;
// if false, TryReadFromCache() always return false, and we only take stats
// for track_min_offset_ if track_min_offset_ = true
bool enable_;
// If true, track minimum `offset` ever passed to TryReadFromCache(), which
// can be fetched from min_offset_read().
bool track_min_offset_;
};
} // namespace ROCKSDB_NAMESPACE
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