zfsconcepts: add description of block cloning

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.

Phew.

Sponsored-By: Klara, Inc.
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Rob Norris <[email protected]>
Closes openzfs#15362

man/man7/zfsconcepts.7

@@ -28,8 +28,9 @@
 .\" Copyright 2019 Richard Laager. All rights reserved.
 .\" Copyright 2018 Nexenta Systems, Inc.
 .\" Copyright 2019 Joyent, Inc.
+.\" Copyright 2023 Klara, Inc.
 .\"
-.Dd June 30, 2019
+.Dd October 6, 2023
 .Dt ZFSCONCEPTS 7
 .Os
 .
@@ -205,3 +206,40 @@ practices, such as regular backups.
 Consider using the
 .Sy compression
 property as a less resource-intensive alternative.
+.Ss Block cloning
+Block cloning is a facility that allows a file (or parts of a file) to be
+.Qq 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
+.Qq reflinks
+or
+.Qq file-level copy-on-write .
+.Pp
+Cloned blocks are tracked in a special on-disk structure called the Block
+Reference Table
+.Po BRT
+.Pc .
+Unlike deduplication, this table has minimal overhead, so can be enabled at all
+times.
+.Pp
+Also unlike deduplication, cloning must be requested by a user program.
+Many common file copying programs, including newer versions of
+.Nm /bin/cp ,
+will try to create clones automatically.
+Look for
+.Qq clone ,
+.Qq dedupe
+or
+.Qq reflink
+in the documentation for more information.
+.Pp
+There are some limitations to block 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
+.Sy recordsize
+properties differ.
+The OS may add additional restrictions;
+for example, most versions of Linux will not allow clones across datasets.