Compressing FASTQ files

Compressing a FASTQ file

While Genozip is technically capable of compressing FASTQ files without using a reference, in practice, to achieve good compression ratios, compression should always be done against a reference genome.

Preparing the reference file is a one-time step (per-species), where the input file is a FASTA file representing the reference genome.


  • If the species has multiple versions of its reference genome FASTA, any one of them will work.

  • If there is no reference genome for the target species, a reference genome of a closely related species may be used.

  • For meta-genomic applications, it is possible to use a reference FASTA that contains sequences from multiple species, up to a total of 4 Gbp.

  • For human data, a reference FASTA may be downloaded here.

$ genozip --make-reference hs37d5.fa.gz  # this generates the file hs37d5.ref.genozip

Once a reference file is prepared, we can compress the FASTQ files:

Compressing a single FASTQ file:

$ genozip --reference hs37d5.ref.genozip myfile.fq.gz
genozip myfile.fq.gz : Done (31 seconds, FASTQ compression ratio: 22.5 - better than .fq.gz by a factor of 4.5)

$ ls -l myfile*
-rwxrwxrwx 1 divon divon 1640641 Aug 21 12:28 myfile.fq.genozip
-rwxrwxrwx 1 divon divon 7338338 Aug 21 12:02 myfile.fq.gz

Note: supported input file extensions include .fq .fq.gz .fq.bz2 .fq.xz and also .fastq .fastq.gz .fastq.bz2 .fastq.xz. For FASTQ files with a different extension, use --input fastq to inform Genozip that this is FASTQ data.

Tip: Use --REFERENCE instead of --reference to store the reference data as part of the compressed file, obliviating the need for a separate reference file when uncompressing. This is in particular beneficial when binding multiple files together with –output, see Archiving - using –tar.

Compressing a paired-end FASTQ files

For paired-end FASTQ files, it is advisable to compress the two files together, as this improves the compression ratio:

$ genozip --reference hs37d5.ref.genozip --pair myfile-R1.fq.gz myfile-R2.fq.gz
genozip myfile-R1.fq.gz : Done (2 seconds)
genozip myfile-R2.fq.gz : Done (8 seconds, FASTQ compression ratio: 20.3 - better than .fq.gz by a factor of 4.3)

$ ls -l myfile*
-rwxrwxrwx 1 divon divon 3624227 Aug 21 12:50 myfile-R1+2.fastq.genozip
-rwxrwxrwx 1 divon divon 7338338 Aug 21 12:02 myfile-R1.fq.gz
-rwxrwxrwx 1 divon divon 8232187 Aug 21 12:02 myfile-R2.fq.gz
Uncompressing both files to their original file names:
genounzip myfile-R1+2.fq.genozip
Accessing R1 data: genocat myfile-R1+2.fq.genozip --R1
Accessing R2 data: genocat myfile-R1+2.fq.genozip --R2

Many downstream bioinforamtics tools can accept paired-end FASTQ data in interleaved format, for example bwa mem -p. To access the data in interleaved format: genocat myfile-R1+2.fq.genozip

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

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

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

When using genocat on a paired file, with one or more of the subsetting options, use the --interleaved=both (this is the default option) to show the pair of reads only if both reads survived the filtering, and --interleaved=either to show the pair of reads if either of them surviving the filtering.

Compressing multiple files into a tar archive

genozip *.fq.gz --reference hs37d5.ref.genozip --tar mydata.tar.

Compressing if sequences are similar (e.g. virus data)

In files where it is expected that the sequences (reads) are similar - for example in the case of long-reads of similar virus genomes, conveying that expectation to Genozip using the --multiseq option will usually improve the compression.

genozip --multiseq myfile.fq.gz

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. It is recommended to combine this with --reference and --pair if applicable for even better compression.

genozip --best myfile.fq.gz --reference hs37d5.ref.genozip

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.fq.gz --reference hs37d5.ref.genozip

Suppressing automatic testing

By default, after compressing a file, Genozip verifies the compression by decompressing the compressed file in memory and comparing the signature (Adler32 or MD5) of the original file to that of the decompressed file. Using the --no-test option suppresses this verification, saving execution time.

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 myfile.fq.gz --reference hs37d5.ref.genozip --optimize-DESC

Replaces the description line with @filename.read_number. Also - if the 3rd line (the ‘+’ line) contains a copy of the description it is shortened to just ‘+’.

genozip myfile.fq.gz --reference hs37d5.ref.genozip --optimize-QUAL

The quality data is optimized as follows:

Old values

New value
















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-R1+2.fastq.genozip

FASTQ files (paired): myfile-R1.fq.gz myfile-R2.fq.gz
Reference: hs37d5.ref.genozip
Sequences: 200,000   Dictionaries: 25   Vblocks: 6 x 16 MB  Sections: 132
Genozip version: 12.0.30 github
Date compressed: 2021-08-21 18:34:02 Cen. Australia Daylight Time
License v12.0.29 granted to: ***** accepted by:***** on 2021-08-18 20:30:56 Cen. Australia Daylight Time from IP=*****

Sections (sorted by % of genozip file):
NAME                   GENOZIP      %      TXT       %   RATIO
QUAL                    2.0 MB  58.3%   28.3 MB  40.3%   14.1X
SEQ                     1.3 MB  37.5%   28.3 MB  40.3%   21.9X
DESC                  144.9 KB   4.1%   12.5 MB  17.8%   88.2X
Other                   1.1 KB   0.0%    1.1 MB   1.6% 1097.9X
TXT_HEADER               696 B   0.0%         -   0.0%    0.0X
LINE3                    246 B   0.0%         -   0.0%    0.0X
BGZF                     112 B   0.0%         -   0.0%    0.0X
GENOZIP vs BGZF         3.5 MB 100.0%   14.8 MB 100.0%    4.3X
GENOZIP vs TXT          3.5 MB 100.0%   70.3 MB 100.0%   20.4X

In this paritcular example, we observe that the quality line consumes 58.3% of the total compressed file size. Therefore, we can expect that --optimize-QUAL will significantly reduce the compressed file size. In contrast, the description line, in this case, consumes only 4.1% of the compressed file size. Therefore, we can expect that --optimize-DESC will not significantly reduce the compressed file size.


genounzip myfile.fq.genozip

Uncompresses a file.

genocat myfile.fq.genozip

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

genounzip --index myfile.fq.genozip

Uncompresses a file and also generates a FAI index file, using samtools faidx. samtools needs to be installed for this option to work.

genounzip --output newname.fq.gz myfile.fq.genozip

Uncompressing to a particular name. Whether or not the name has a .gz extension detemines whether the output file is BGZF-compressed.

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

Sets the level BGZF compression (for .fq.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 fastq --output myfile.fq.genozip
Uncompressing to a pipe:
genocat myfile.fq.genozip | my-pipeline        # not paired-end
genocat myfile.R1+2.fq.genozip  | my-pipeline  # interleaved paired-end

Showing only the Description, Sequence or Quality line for each read

genocat --header-only myfile.fq.genozip
genocat --seq-only myfile.fq.genozip
genocat --qual-only myfile.fq.genozip


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

Displays only the first (#0) read in every 10 reads.


genocat --grep ACCTTAAT myfile.fq.genozip

Displays reads with the string “ACCTTAAT” anywhere in the read (description, seqeuence or quality lines) - possibly a substring of a longer string.

genocat --grep-w ACCTTAAT myfile.fq.genozip

Displays reads with the string “ACCTTAAT” exactly matching a component of the description, or the entire sequence line or the entire quality line.

Filtering non-ACTGN “bases”

genocat --bases ACGTN myfile.fq.genozip

Displays only reads in which all characters of the sequence are one of A,C,G,T,N

genocat --bases ^ACGTN myfile.fq.genozip

Displays only reads in which NOT all characters of the sequence are one of A,C,G,T,N

Note: The list of IUPAC chacacters can be found here: IUPAC codes

Filtering reads by species

Genozip has the ability to filter FASTQ files by species (taxonomy id). See Filtering BAM or FASTQ reads by species using kraken2.


genocat --idxstats myfile.fq.genozip

Calculates approximate idxstats, directly from the FASTQ data, expected to be outputted by samtools idxstats after this FASTQ file is mapped against the reference genome. See idxstats.

Per-contig coverage and depth

genocat --show-coverage myfile.fq.genozip

An experimental feature for calculating coverage and depth directly from a FASTQ file, see Coverage and Depth.

Sex assignment

genocat --show-sex myfile.fq.genozip

An experimental feature for determining the sex of a sample from a FASTQ file, see Sex assignment.


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 for most FASTQ files is 16MB, 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.

When using a reference file, it is loaded to memory too. If multiple genozip/ genocat / genounzip processes are running in parallel, only one copy of the reference file is loaded to memory and shared between all processes, and depending on how busy the computer is, that reference file data might persist in RAM even between consecutive runs, saving Genozip the need to load it again from disk. All this all happens behind the scenes.