Deduplication

Going by the definition in the context of filesystems, it’s a process of looking up identical data blocks tracked separately and creating a shared logical link while removing one of the copies of the data blocks. This leads to data space savings while it increases metadata consumption.

There are two main deduplication types:

  • in-band (sometimes also called on-line) -- all newly written data are considered for deduplication before writing

  • out-of-band (sometimes also called offline) -- data for deduplication have to be actively looked for and deduplicated by the user application

Both have their pros and cons. BTRFS implements only out-of-band type.

BTRFS provides the basic building blocks for deduplication allowing other tools to choose the strategy and scope of the deduplication. There are multiple tools that take different approaches to deduplication, offer additional features or make trade-offs. The following table lists tools that are known to be up-to-date, maintained and widely used.

Name

File based

Block based

Incremental

BEES

No

Yes

Yes

duperemove

Yes

No

Yes

File based deduplication

The tool takes a list of files and tries to find duplicates among data only from that files. This is suitable eg. for files that originated from the same base image, source of a reflinked file. Optionally the tools could track a database of hashes and allow to deduplicate blocks from more files, or use that for repeated runs and update the database incrementally.

Block based deduplication

The tool typically scans the filesystem and builds a database of file block hashes, then finds candidate files and deduplicates the ranges. The hash database is kept as an ordinary file and can be scaled according to the needs.

As the file changes, the hash database may get out of sync and the scan has to be done repeatedly.

Safety of block comparison

The deduplication inside the filesystem is implemented as an ioctl that takes a source file, destination file and the range. The blocks from both files are compared for exact match before merging to the same range (ie. there’s no hash based comparison). Pages representing the extents in memory are locked prior to deduplication and prevent concurrent modification by buffered writes or mmaped writes.

Limitations, compatibility

Files that are subject do deduplication must have the same status regarding COW, ie. both regular COW files with checksums, or both NOCOW, or files that are COW but don’t have checksums (NODATASUM attribute is set).

If the deduplication is in progress on any file in the filesystem, the send operation cannot be started as it relies on the extent layout being unchanged.