zfsconcepts: add description of block cloning

[robn]

Motivation and Context

There's almost no documentation for block cloning. People will have questions. We should at least have something to point people at.

Description

Here I'm trying to succinctly introduce the concept, the basics of its construction, how its different to dedup, how to use it, and where its limitations lie, in four paragraphs and with enough searchable terms to help the reader find more information both within OpenZFS and elsewhere.

Sponsored-By: Klara, Inc.

How Has This Been Tested?

checkstyle (mancheck) passes.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[robn]

Rendered:

Block cloning

Block cloning is a facility that allows a file (or parts of a file) to be “cloned”, that is, a shallow copy made where the existing data blocks are referenced rather than copied. Later modifications to the data will cause a copy of the data block to be taken and that copy modified. This facility is used to implement “reflinks” or “file-level copy-on-write”.

Cloned blocks are tracked in a special on-disk structure called the Block Reference Table (BRT). Unlike deduplication, this table has minimal overhead, so can be enabled at all times.

Also unlike deduplication, cloning must be requested by a user program. Many common file copying programs, including newer versions of /bin/cp, will try to create clones automatically. Look for “clone”, “dedupe” or “reflink” in the documentation for more information.

There are some limitations to cloning. Only whole blocks can be cloned, and blocks can not be cloned if they are not yet written to disk, or if they are encrypted, or the source and destination recordsize properties differ. The OS may add additional restrictions; for example, most versions of Linux will not allow clones across datasets.

[allanjude]

maybe change:

most versions of Linux will not allow clones across datasets.
to
most versions of Linux will not allow blocks to be cloned across datasets.

Just to avoid confusion with zfs clone

[allanjude]

Reviewed-by: Allan Jude <[email protected]>