mirror of
https://github.com/borgbackup/borg.git
synced 2024-12-27 10:18:12 +00:00
169634f2ca
one of the biggest issues with borg < 1.0 was that it had a default target chunk size of 64kiB, thus it created a lot of chunks, a huge chunk management overhead (high RAM and disk usage).
116 lines
4.2 KiB
Text
116 lines
4.2 KiB
Text
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.
|
|
|
|
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/
|
|
|