Compressing VCF files

Compressing a VCF or BCF file

$ genozip myfile.vcf.gz
genozip myfile.vcf.gz : Done (2 seconds, VCF compression ratio: 21.9 - better than .vcf.gz by a factor of 2.2)

$ ls -lh myfile.vcf*
-rwxrwxrwx 1 divon divon 1.9M Aug 22 00:15 myfile.vcf.genozip
-rwxrwxrwx 1 divon divon 4.0M Aug 22 00:14 myfile.vcf.gz

This creates a compressed file, without modifying the original file. This also works with .vcf.gz, .vcf.bz2, .vcf.xz and .bcf.

Some useful command line options (for a full list, see genozip manual):

genozip --test myfile.vcf: after completing the compression, the file is uncompressed in memory, and its MD5 is compared to that of the original file.

genozip --replace myfile.vcf: the original file is removed after successful compression

Compressing multiple files into a tar archive

genozip *.vcf --tar mydata.tar. See details: Archiving.

Best compression

Using the --best option causes Genozip to use more aggressive compression methods, at the expense of higher CPU and memory usage, resulting in better compression.

genozip --best myfile.vcf.gz

Fast compression

Using the --fast option causes Genozip compress faster, at the expense of a lower compression ratio. This option also usually results in faster decompression and lower memory consumption.

genozip --fast myfile.vcf.gz

Optimizing compression

These are options that modify the file in ways that improve compression. --optimize is an umbrella option that activates all optimization options.

genozip --optimize-sort myfile.vcf.gz

Sorts INFO subfields alphabetically.

genozip --optimize-phred myfile.vcf.gz

Applied to FORMAT/PL FORMAT/PRI FORMAT/PP and (VCF v4.2 or earlier) FORMAT/GL - Phred scores are rounded to the nearest integer and capped at 60.

genozip --GL-to-PL myfile.vcf.gz

The FORMAT/GL field is converted to PL and Phred values are capped at 60.

genozip --GP-to-PP myfile.vcf.gz

Applicable to VCF v4.3 and later: The FORMAT/GP field is converted to PP and Phred values are capped at 60.

genozip --optimize-VQSLOD myfile.vcf.gz

The VQSLOD value is rounded to 2 significant digits.

genozip --reference reference-file.ref.genozip myfile.vcf.gz

Compresses against a reference. This option is not included in --optimize. It improves compression in files in which the “REF+ALT” field consumes significant part of the GENOZIP content (see --stats below).

The option --stats can be used in genozip, genounzip or genocat to get a better understanding of the information content of the file. For example:

$ genocat --stats myfile.vcf.genozip

VCF file: myfile.vcf.gz
Samples: 1211   Variants: 1,500   Dictionaries: 93   Vblocks: 3 x 16 MB  Sections: 210
Genozip version: 12.0.30 github
Date compressed: 2021-08-22 00:15:08 ACDT
License v12.0.11 granted to: ***** accepted by: ***** on 2021-07-23 14:33:51 ACDT from IP=*****

Sections (sorted by % of genozip file):
NAME                   GENOZIP      %      TXT       %   RATIO
PL                      1.1 MB  59.9%   10.7 MB  26.8%    9.8X
GQ                    288.4 KB  15.4%    2.3 MB   5.6%    8.0X
AD                    281.7 KB  15.1%    5.5 MB  13.6%   19.9X
GT                     70.7 KB   3.8%    5.2 MB  13.0%   75.3X
TXT_HEADER             21.0 KB   1.1%  145.6 KB   0.4%    6.9X
PID                    17.4 KB   0.9%    1.4 MB   3.4%   80.6X
PGT                    10.3 KB   0.6%    1.3 MB   3.3%  130.4X
QUAL                    4.4 KB   0.2%    9.6 KB   0.0%    2.2X
InbreedingCoeff         4.2 KB   0.2%    9.7 KB   0.0%    2.3X
AF                      4.2 KB   0.2%   11.0 KB   0.0%    2.6X
MQ                      4.0 KB   0.2%    7.0 KB   0.0%    1.8X
MLEAF                   3.9 KB   0.2%   10.0 KB   0.0%    2.6X
QD                      3.8 KB   0.2%    6.4 KB   0.0%    1.7X
ExcessHet               3.7 KB   0.2%    7.8 KB   0.0%    2.1X
DP                      3.6 KB   0.2%    5.9 KB   0.0%    1.6X
AN                      3.5 KB   0.2%    5.6 KB   0.0%    1.6X
SOR                     3.4 KB   0.2%    7.2 KB   0.0%    2.1X
FS                      2.9 KB   0.2%    4.8 KB   0.0%    1.6X
MQRankSum               2.8 KB   0.2%    5.8 KB   0.0%    2.1X
BaseQRankSum            2.8 KB   0.1%    5.5 KB   0.0%    2.0X
ReadPosRankSum          2.7 KB   0.1%    5.6 KB   0.0%    2.0X
MLEAC                   1.9 KB   0.1%    2.1 KB   0.0%    1.1X
POS                     1.6 KB   0.1%    8.7 KB   0.0%    5.4X
DP                      1.5 KB   0.1%    2.0 MB   5.0% 1338.7X
Other                   1.2 KB   0.1%   11.3 MB  28.1% 9658.3X
REF+ALT                 1.1 KB   0.1%    5.9 KB   0.0%    5.3X
CHROM                    915 B   0.0%    2.9 KB   0.0%    3.3X
INFO                     729 B   0.0%  179.4 KB   0.4%  252.0X
AC                       555 B   0.0%    1.9 KB   0.0%    3.4X
FORMAT                   526 B   0.0%   30.7 KB   0.1%   59.9X
COORDS                   391 B   0.0%         -   0.0%    0.0X
BGZF                      56 B   0.0%         -   0.0%    0.0X
oXSTRAND                  44 B   0.0%         -   0.0%    0.0X
ID                        42 B   0.0%    2.9 KB   0.0%   71.4X
FILTER                    42 B   0.0%    2.9 KB   0.0%   71.4X
ClippingRankSum           42 B   0.0%    1.2 KB   0.0%   29.4X
GENOZIP vs BGZF         1.8 MB 100.0%    4.0 MB 100.0%    2.2X
GENOZIP vs TXT          1.8 MB 100.0%   40.1 MB 100.0%   21.9X

In this paritcular example, we observe that the PL field consumes a whopping 59.9% of the total compressed file size. Therefore, we can expect that --optimize-phred will significantly reduce the compressed file size. In contrast, the REF+ALT field, in this case, consumes only 0.1% of the compressed file size. Therefore, we can expect that using --reference will not significantly reduce the compressed file size.


genounzip myfile.vcf.genozip

Uncompresses a file

genocat myfile.vcf.genozip

Uncompresses a file into stdout (i.e. the terminal).

genounzip --index myfile.vcf.genozip

Uncompresses a file and also generates a CSI index file, using bcftools index. bcftools needs to be installed for this option to work.

genocat --bgzf 6 myfile.vcf.genozip
genounzip --bgzf 6 myfile.vcf.genozip

Sets the level BGZF compression (for .vcf.gz output format) - from 0 (no compression) to 12 (best yet slowest compression). Absent this option, genounzip attemps to recover the BGZF compression level of the original file, while genocat uncompresses without BGZF compression.

Using in a pipeline

Compressing piped input:
my-pipeline | genozip - --input vcf --output myfile.vcf.genozip
Uncompressing to a pipe:
genocat myfile.vcf.genozip | my-pipeline


genocat --downsample 10,0 myfile.vcf.genozip

Displays only the first (#0) variant in every 10 variants.


genocat --grep-w AC=2 myfile.vcf.genozip

Displays the variants containing “AC=2” (strings that match exactly).

genocat --grep ACCTTAAT myfile.vcf.genozip

Displays the variants containing “ACCTTAAT” (possibly a substring of a longer string).

Selecting samples

genocat myfile.vcf.genozip --samples HG00255,HG00256

Shows two samples.

genocat myfile.vcf.genozip --samples ^HG00255,HG00256

Shows all samples except these two.

genocat myfile.vcf.genozip --samples 5

Shows the first 5 samples.

genocat myfile.vcf.genozip --drop-genotypes

Drops all samples and the FORMAT columns. --drop-genotypes is the same as --samples 0, -s 0 and -G.

genocat myfile.vcf.genozip --GT-only

Within samples, outputs only genotype (GT) data - dropping the other subfields.

SNPs or indels only

genocat myfile.vcf.genozip --snps-only

Drops variants that are not a Single Nucleotide Polymorphism (SNP).

genocat myfile.vcf.genozip --indels-only

Drops variants that are not Insertions or Deletions (indel).

The VCF header

genocat --header-only myfile.vcf.genozip

Displays only the VCF header.

genocat --no-header myfile.vcf.genozip

Displays the file without the VCF header.

genocat --header-one myfile.vcf.genozip

Displays the file without the VCF header, except for the #CHROM line.

genocat --no-PG myfile.vcf.genozip

When modifying the data in a file using genocat, Genozip normally adds a “##genozip_command” line to the VCF header. With this option it doesn’t.

Filtering specific regions of the genome

Examples of using --regions (or its shortcut -r):

genocat myfile.vcf.genozip -r 22:1000-2000

Positions 1000 to 2000 on contig 22

genocat myfile.vcf.genozip -r 22:1000+151

151 bases, starting pos 1000, on contig 22

genocat myfile.vcf.genozip -r -2000,2500-

Two ranges on all contigs

genocat myfile.vcf.genozip -r chr21,chr22

Contigs chr21 and chr22 in their entirety

genocat myfile.vcf.genozip -r ^MT,Y

All contigs, excluding MT and Y

genocat myfile.vcf.genozip -r ^-1000

All contigs, excluding positions up to 1000

genocat myfile.vcf.genozip -r chrM

Contig chrM

genocat --regions-file <filename> myfile.vcf.genozip

Get regions from a tab-separated file. An example of a valid file:

chr22        17000000        17000099
chr22        17000000        +100
chr22        17000000


genozip --sort myfile.vcf

Variants are sorted by CHROM and POS. This works for “mildly unsorted” files. This is the default with --chain is used, unless --unsorted is specified.

genocat --unsorted myfile.vcf.genozip

Shows the variants in their original order.

Adding line numbers

genozip --add-line-numbers myfile.vcf

Replaces the ID field in each variant with a sequential line number starting from 1.

Flat coordinates (GPOS)

genocat --gpos --reference reference-file.ref.genozip myfile.vcf.genozip

Replaces (CHROM,POS) with a coordinate in GPOS (Global POSition) terms. GPOS is a single genome-wide coordinate defined by a reference file, in which contigs appear in the order of the original FASTA data used to generate the reference file.

genocat --show-ref-contigs reference-file.ref.genozip

Shows the mapping of CHROM to GPOS.

BCF files

Genozip does not support BCF natively - it uses bcftools to convert BCF files to/from the VCF format, and as such it requires bcftools to be installed for the BCF features to work.

genozip myfile.bcf

Compresses a BCF file.

genocat --bcf myfile.vcf.genozip

Outputs the file in BCF format.

Dual-coordinate VCF files

Genozip has the unique ability to represent a VCF file with coordinates in two different reference genomes concurrently. See Dual-coordinate VCF files.

genozip --chain mychainfile.chain.genozip myfile.vcf

Lifts a VCF file to a dual-coordinate VCF (DVCF) - this generates myfile.d.vcf.genozip.

When using --chain, additional options may be combined:
- --dvcf-rename, --dvcf-drop - specify annotations that should be renamed or dropped when cross rendering Primary➝Luft or Luft➝Primary. See Renaming and dropping annotations in a DVCF.
- --show-rename-tags - shows tags that are to be renamed. Used when compressing a DVCF or in combination with –chain.
- --show-lifts - output successful lifts to the rejects file too, not only rejected lifts.
- --show-counts=o\$TATUS, --show-counts=COORDS - see below
- --show-chain - displays all chain file alignments.

genocat myfile.d.vcf.genozip

Displays the file in the Primary coordinates.

genocat --luft myfile.d.vcf.genozip

Displays the file in the Luft coordinates.

genocat --single-coord myfile.d.vcf.genozip
genocat --single-coord --luft myfile.d.vcf.genozip

Removes all DVCF-specific lines from the VCF header, and removes the DVCF INFO annotations, leaving the file as a normal VCF file in single coordinates - either the Primary coordinates, or Luft coordinates (when combined with --luft).

genocat --show-ostatus myfile.d.vcf.genozip

Adds oSTATUS to the INFO field - the status of the variant relative to the lift process.

genocat --show-counts=o\$TATUS myfile.d.vcf.genozip

Shows summary statistics of variant lift outcome (also works with genozip --chain).

genocat --show-counts=COORDS myfile.d.vcf.genozip

Shows summary statistics of variant coordinates (also works with genozip --chain).

genocat --show-dvcf myfile.d.vcf.genozip

For each variant, shows its coordinate system (Primary or Luft or Both) and its oStatus. May be used with or without –luft.


By default, Genozip attempts to utilize as many cores as available. For that, it sets the number of threads to be a bit more than the number of cores (a practice known as “over-subscription”), as at any given moment some threads might be idle, waiting for a resource to become available. The --threads <number> option allows explicit specification of the number of “compute threads” to be used (in addition a small number of I/O threads is used too, usually 1 or 2).

Memory (RAM) consumption

In genozip, each compute thread is assigned a segment of the input file, known as a VBlock. By default, the size of the VBlock is set automatically to balance memory consumption and compression ratio for the particular input file, however it may be set explicitly with genozip --vblock <megabytes> (<megabytes> is an integer between 1 and 2048). A larger VBlock usually results in better compression while a smaller VBlock causes genozip to consume less RAM. The VBlock size can be observed at the top of the --stats report. genozip’s memory consumption is linear with (VBlock-size X number-of-threads).

genocat and genounzip also consume memory linearly with (VBlock-size X number-of-threads), where VBlock-size is the value used by genozip of the particular file (it cannot be modified genocat or genounzip). Usually, genocat and genounzip consume significantly less memory compared to genozip.