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