ZVault Backup solution

Goals / Features

Space-efficient storage with deduplication

The backup data is split into chunks. Fingerprints make sure that each chunk is only stored once. The chunking algorithm is designed so that small changes to a file only change a few chunks and leave most chunks unchanged.

Multiple backups of the same data set will only take up the space of one copy.

The chunks are combined into bundles. Each bundle holds chunks up to a maximum data size and is compressed as a whole to save space ("solid archive").

Independent backups

All backups share common data in form of chunks but are independent on a higher level. Backups can be deleted and chunks that are not used by any backup can be removed.

Other backup solutions use differential backups organized in chains. This makes those backups dependent on previous backups in the chain, so that those backups can not be deleted. Also, restoring chained backups is much less efficient.

Fast backup runs

• Only adding changed files
• In-Memory Hashtable

Backup verification

• Bundles verification
• Index verification
• File structure verification

Configuration options

There are several configuration options with trade-offs attached so these are exposed to users.

Chunker algorithm

The chunker algorithm is responsible for splitting files into chunks in a way that survives small changes to the file so that small changes still yield many matching chunks. The quality of the algorithm affects the deduplication rate and its speed affects the backup speed.

There are 3 algorithms to choose from:

The Rabin chunker is a very common algorithm with a good quality but a mediocre speed (about 350 MB/s). The AE chunker is a novel approach that can reach very high speeds (over 750 MB/s) but at a cost of quality. The FastCDC algorithm has a slightly higher quality than the Rabin chunker and is quite fast (about 550 MB/s).

The recommendation is FastCDC.

Chunk size

The chunk size determines the memory usage during backup runs. For every chunk in the backup repository, 24 bytes of memory are needed. That means that for every GiB stored in the repository the following amount of memory is needed:

• 8 KiB chunks => 3 MiB / GiB
• 16 KiB chunks => 1.5 MiB / GiB
• 32 KiB chunks => 750 KiB / GiB
• 64 KiB chunks => 375 KiB / GiB

On the other hand, bigger chunks reduce the deduplication efficiency. Even small changes of only one byte will result in at least one complete chunk changing.

Hash algorithm

Blake2 Murmur3

Recommended: Blake2

Bundle size

10 M 25 M 100 M

Recommended: 25 MiB

Compression

Recommended: Brotli/2-7

Design

Semantic Versioning

zVault sticks to the semantic versioning scheme. In its current pre-1.0 stage this has the following implications:

• Even now the repository format is considered pretty stable. All future versions will be able to read the current repository format. Maybe conversions might be necessary but the backups should always be forward-compatible.
• The CLI might see breaking changes but at least it is guaranteed that calls that are currently non-destructive will not become destructive in the future. Running todays commands on a future version will not cause any harm.

TODO

Packaging

• Included works
• Proper manpage

Core functionality

• Recompress & combine bundles

CLI functionality

• list --tree

Other

• Stability
• Tests & benchmarks
  • Chunker
  • Index
  • BundleDB
  • Bundle map
  • Config files
  • Backup files
  • Backup
  • Prune
  • Vacuum
• Documentation
  • All file formats
  • Design