2016-09-21 08:25:33 +00:00
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# ZVault Backup solution
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2017-03-10 11:43:32 +00:00
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## Goals / Features
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### Space-efficient storage with deduplication
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The backup data is split into chunks. Fingerprints make sure that each chunk is
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only stored once. The chunking algorithm is designed so that small changes to a
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file only change a few chunks and leave most chunks unchanged.
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Multiple backups of the same data set will only take up the space of one copy.
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The chunks are combined into bundles. Each bundle holds chunks up to a maximum
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data size and is compressed as a whole to save space ("solid archive").
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### Independent backups
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All backups share common data in form of chunks but are independent on a higher
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2017-03-22 16:28:45 +00:00
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level. Backups can be deleted and chunks that are not used by any backup can be
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2017-03-10 11:43:32 +00:00
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removed.
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Other backup solutions use differential backups organized in chains. This makes
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those backups dependent on previous backups in the chain, so that those backups
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can not be deleted. Also, restoring chained backups is much less efficient.
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### Fast backup runs
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* Only adding changed files
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* In-Memory Hashtable
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### Backup verification
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* Bundles verification
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* Index verification
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* File structure verification
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## Configuration options
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There are several configuration options with trade-offs attached so these are
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exposed to users.
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### Chunker algorithm
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The chunker algorithm is responsible for splitting files into chunks in a way
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that survives small changes to the file so that small changes still yield
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many matching chunks. The quality of the algorithm affects the deduplication
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rate and its speed affects the backup speed.
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There are 3 algorithms to choose from:
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The **Rabin chunker** is a very common algorithm with a good quality but a
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mediocre speed (about 350 MB/s).
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The **AE chunker** is a novel approach that can reach very high speeds
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(over 750 MB/s) but at a cost of quality.
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The **FastCDC** algorithm has a slightly higher quality than the Rabin chunker
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and is quite fast (about 550 MB/s).
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The recommendation is **FastCDC**.
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### Chunk size
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The chunk size determines the memory usage during backup runs. For every chunk
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in the backup repository, 24 bytes of memory are needed. That means that for
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every GiB stored in the repository the following amount of memory is needed:
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- 8 KiB chunks => 3 MiB / GiB
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- 16 KiB chunks => 1.5 MiB / GiB
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- 32 KiB chunks => 750 KiB / GiB
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- 64 KiB chunks => 375 KiB / GiB
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On the other hand, bigger chunks reduce the deduplication efficiency. Even small
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changes of only one byte will result in at least one complete chunk changing.
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### Hash algorithm
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Blake2
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Murmur3
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Recommended: Blake2
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### Bundle size
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10 M
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25 M
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100 M
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Recommended: 25 MiB
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### Compression
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Recommended: Brotli/2-7
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2016-09-21 08:25:33 +00:00
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## Design
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## TODO
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2017-03-22 16:28:45 +00:00
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### Core functionality
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- Recompress & combine bundles
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### CLI functionality
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- list --tree
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### Other
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- Stability
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- Tests & benchmarks
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- Chunker
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- Index
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- BundleDB
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- Bundle map
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- Config files
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- Backup files
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- Backup
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- Prune
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- Vacuum
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- Documentation
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- All file formats
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- Design
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