| // Copyright (C) 2019 The Android Open Source Project |
| // |
| // Licensed 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 "AS IS" BASIS, |
| // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| // See the License for the specific language governing permissions and |
| // limitations under the License. |
| |
| #pragma once |
| |
| #include <stdint.h> |
| |
| #include <vector> |
| |
| namespace android { |
| namespace snapshot { |
| |
| class DmSnapCowSizeCalculator { |
| public: |
| DmSnapCowSizeCalculator(unsigned int sector_bytes, unsigned int chunk_sectors) |
| : sector_bytes_(sector_bytes), |
| chunk_sectors_(chunk_sectors), |
| exceptions_per_chunk(chunk_sectors_ * sector_bytes_ / (64 * 2 / 8)) {} |
| |
| void WriteByte(uint64_t address) { WriteSector(address / sector_bytes_); } |
| void WriteSector(uint64_t sector) { WriteChunk(sector / chunk_sectors_); } |
| void WriteChunk(uint64_t chunk_id) { |
| if (modified_chunks_.size() <= chunk_id) { |
| modified_chunks_.resize(chunk_id + 1, false); |
| } |
| modified_chunks_[chunk_id] = true; |
| } |
| |
| uint64_t cow_size_bytes() const { return cow_size_sectors() * sector_bytes_; } |
| uint64_t cow_size_sectors() const { return cow_size_chunks() * chunk_sectors_; } |
| |
| /* |
| * The COW device has a precise internal structure as follows: |
| * |
| * - header (1 chunk) |
| * - #0 map and chunks |
| * - map (1 chunk) |
| * - chunks addressable by previous map (exceptions_per_chunk) |
| * - #1 map and chunks |
| * - map (1 chunk) |
| * - chunks addressable by previous map (exceptions_per_chunk) |
| * ... |
| * - #n: map and chunks |
| * - map (1 chunk) |
| * - chunks addressable by previous map (exceptions_per_chunk) |
| * - 1 extra chunk |
| */ |
| uint64_t cow_size_chunks() const { |
| uint64_t modified_chunks_count = 0; |
| uint64_t cow_chunks = 0; |
| |
| for (const auto& c : modified_chunks_) { |
| if (c) { |
| ++modified_chunks_count; |
| } |
| } |
| |
| /* disk header + padding = 1 chunk */ |
| cow_chunks += 1; |
| |
| /* snapshot modified chunks */ |
| cow_chunks += modified_chunks_count; |
| |
| /* snapshot chunks index metadata */ |
| cow_chunks += 1 + modified_chunks_count / exceptions_per_chunk; |
| |
| return cow_chunks; |
| } |
| |
| private: |
| /* |
| * Size of each sector in bytes. |
| */ |
| const uint64_t sector_bytes_; |
| |
| /* |
| * Size of each chunk in sectors. |
| */ |
| const uint64_t chunk_sectors_; |
| |
| /* |
| * The COW device stores tables to map the modified chunks. Each table |
| * has the size of exactly 1 chunk. |
| * Each row of the table (also called exception in the kernel) contains two |
| * 64 bit indices to identify the corresponding chunk, and this 128 bit row |
| * size is a constant. |
| * The number of exceptions that each table can contain determines the |
| * number of data chunks that separate two consecutive tables. This value |
| * is then fundamental to compute the space overhead introduced by the |
| * tables in COW devices. |
| */ |
| const uint64_t exceptions_per_chunk; |
| |
| /* |
| * |modified_chunks_| is a container that keeps trace of the modified |
| * chunks. |
| * Multiple options were considered when choosing the most appropriate data |
| * structure for this container. Here follows a summary of why vector<bool> |
| * has been chosen, taking as a reference a snapshot partition of 4 GiB and |
| * chunk size of 4 KiB. |
| * - std::set<uint64_t> is very space-efficient for a small number of |
| * operations, but if the whole snapshot is changed, it would need to |
| * store |
| * 4 GiB / 4 KiB * (64 bit / 8) = 8 MiB |
| * just for the data, plus the additional data overhead for the red-black |
| * tree used for data sorting (if each rb-tree element stores 3 address |
| * and the word-aligne color, the total size grows to 32 MiB). |
| * - std::bitset<N> is not a good fit because requires a priori knowledge, |
| * at compile time, of the bitset size. |
| * - std::vector<bool> is a special case of vector, which uses a data |
| * compression that allows reducing the space utilization of each element |
| * to 1 bit. In detail, this data structure is composed of a resizable |
| * array of words, each of them representing a bitmap. On a 64 bit |
| * device, modifying the whole 4 GiB snapshot grows this container up to |
| * 4 * GiB / 4 KiB / 64 = 64 KiB |
| * that, even if is the same space requirement to change a single byte at |
| * the highest address of the snapshot, is a very affordable space |
| * requirement. |
| */ |
| std::vector<bool> modified_chunks_; |
| }; |
| |
| } // namespace snapshot |
| } // namespace android |