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zfsconcepts: add description of block cloning

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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 <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #15362
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Rob N 2023-10-07 03:06:29 +11:00 committed by Brian Behlendorf
parent bcd010d3a5
commit 8495536f7f
1 changed files with 39 additions and 1 deletions
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40
man/man7/zfsconcepts.7
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@ -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.