About borg create --chunker-params
==================================

--chunker-params CHUNK_MIN_EXP,CHUNK_MAX_EXP,HASH_MASK_BITS,HASH_WINDOW_SIZE

CHUNK_MIN_EXP and CHUNK_MAX_EXP give the exponent N of the 2^N minimum and
maximum chunk size. Required: CHUNK_MIN_EXP < CHUNK_MAX_EXP.

Defaults: 19 (2^19 == 512KiB) minimum, 23 (2^23 == 8MiB) maximum.
Currently it is not supported to give more than 23 as maximum.

HASH_MASK_BITS is the number of least-significant bits of the rolling hash
that need to be zero to trigger a chunk cut.
Recommended: CHUNK_MIN_EXP + X <= HASH_MASK_BITS <= CHUNK_MAX_EXP - X, X >= 2
(this allows the rolling hash some freedom to make its cut at a place
determined by the windows contents rather than the min/max. chunk size).

Default: 21 (statistically, chunks will be about 2^21 == 2MiB in size)

HASH_WINDOW_SIZE: the size of the window used for the rolling hash computation.
Default: 4095B


Trying it out
=============

I backed up a VM directory to demonstrate how different chunker parameters
influence repo size, index size / chunk count, compression, deduplication.

repo-sm: ~64kiB chunks (16 bits chunk mask), min chunk size 1kiB (2^10B)
         (these are attic / borg 0.23 internal defaults)

repo-lg: ~1MiB chunks (20 bits chunk mask), min chunk size 64kiB (2^16B)

repo-xl: 8MiB chunks (2^23B max chunk size), min chunk size 64kiB (2^16B).
         The chunk mask bits was set to 31, so it (almost) never triggers.
         This degrades the rolling hash based dedup to a fixed-offset dedup
         as the cutting point is now (almost) always the end of the buffer
         (at 2^23B == 8MiB).

The repo index size is an indicator for the RAM needs of Borg.
In this special case, the total RAM needs are about 2.1x the repo index size.
You see index size of repo-sm is 16x larger than of repo-lg, which corresponds
to the ratio of the different target chunk sizes.

Note: RAM needs were not a problem in this specific case (37GB data size).
      But just imagine, you have 37TB of such data and much less than 42GB RAM,
      then you'ld definitely want the "lg" chunker params so you only need
      2.6GB RAM. Or even bigger chunks than shown for "lg" (see "xl").

You also see compression works better for larger chunks, as expected.
Duplication works worse for larger chunks, also as expected.

small chunks
============

$ borg info /extra/repo-sm::1

Command line: /home/tw/w/borg-env/bin/borg create --chunker-params 10,23,16,4095 /extra/repo-sm::1 /home/tw/win
Number of files: 3

                       Original size      Compressed size    Deduplicated size
This archive:               37.12 GB             14.81 GB             12.18 GB
All archives:               37.12 GB             14.81 GB             12.18 GB

                       Unique chunks         Total chunks
Chunk index:                  378374               487316

$ ls -l /extra/repo-sm/index*

-rw-rw-r-- 1 tw tw 20971538 Jun 20 23:39 index.2308

$ du -sk /extra/repo-sm
11930840    /extra/repo-sm

large chunks
============

$ borg info /extra/repo-lg::1

Command line: /home/tw/w/borg-env/bin/borg create --chunker-params 16,23,20,4095 /extra/repo-lg::1 /home/tw/win
Number of files: 3

                       Original size      Compressed size    Deduplicated size
This archive:               37.10 GB             14.60 GB             13.38 GB
All archives:               37.10 GB             14.60 GB             13.38 GB

                       Unique chunks         Total chunks
Chunk index:                   25889                29349

$ ls -l /extra/repo-lg/index*

-rw-rw-r-- 1 tw tw 1310738 Jun 20 23:10 index.2264

$ du -sk /extra/repo-lg
13073928    /extra/repo-lg

xl chunks
=========

(borg-env)tw@tux:~/w/borg$ borg info /extra/repo-xl::1
Command line: /home/tw/w/borg-env/bin/borg create --chunker-params 16,23,31,4095 /extra/repo-xl::1 /home/tw/win
Number of files: 3

                       Original size      Compressed size    Deduplicated size
This archive:               37.10 GB             14.59 GB             14.59 GB
All archives:               37.10 GB             14.59 GB             14.59 GB

                       Unique chunks         Total chunks
Chunk index:                    4319                 4434

$ ls -l /extra/repo-xl/index*
-rw-rw-r-- 1 tw tw 327698 Jun 21 00:52 index.2011

$ du -sk /extra/repo-xl/
14253464    /extra/repo-xl/

