CRAM benchmarking

(Updated Sep 2019)

Benchmarks of CRAM 2.1 and 3.0 are using the faster CRAM codecs; primarily deflate and rANS.

Also included is the performance of the proposed CRAM v3.1 standard. This is not yet a ratified GA4GH standard, but these figures give indicative results.

The options listed below also include the new proposed compression profiles (fast, normal (default), small and archive) that ease the trade-off between speed vs size vs random access. The profiles are synonyms for a collection of existing options. At the time of writing, these profiles are:

Profile CRAM versions options
fast 3.0, 3.1 seqs_per_slice=1000, level=1
normal (default) 3.0, 3.1 seqs_per_slice=10000
small 3.0 seqs_per_slice=25000, level=6,use_bzip2
small 3.1 seqs_per_slice=25000, level=6,use_bzip2,use_fqz
archive 3.0 seqs_per_slice=100000,level=7,use_bzip2
archive 3.1 seqs_per_slice=100000,level=7,use_bzip2,use_fqz,use_arith

To demonstrate the absolute smallest size we use add option “use_lzma” to the archive profile tests. This adds considerable encode cost, but minimal decode.

Coordinate sorted human data

This test set is chr1 of NA12878_S1, downloaded from

Conversion from BAM to is achieved via htslib test_view -b or -C using 8 threads on a multi-core Intel Xeon E5-2660 system. Decoding times are computed using test_view -B (benchmarking mode) with performs input, uncompression and decoding only. Encoding time includes decoding of the input BAM, so subtract the BAM decoding time to get encode-only, although the difference is only minor.

All times are reported as wall-clock, although some I/O time will impact this test as the file is large, particularly decode times. In this first test we also show both vanilla Zlib and Libdeflate implementations of the gzip standard.

Format Options Size(Mb) Encoding(s) Decoding(s)
BAM (zlib)   121710 3101 438
BAM (libdeflate)   122405 2192 357
CRAM v2.1   78259 2121 407
CRAM v3.0   66494 1094 407
CRAM v3.0 small 64682 2122 573
CRAM v3.0 archive,use_lzma 63629 6654 394
CRAM v3.1 (proposed)   62150 1324 417
CRAM v3.1 (proposed) small 56204 2465 1405
CRAM v3.1 (proposed) archive,use_lzma 54237 3048 1395

E.Coli: sorted, unsorted, with and without references

To compare CRAM efficiency in a variety of circumstances we chose a smaller dataset to more completely explore the parameter space. MiSeq_Ecoli_DH10B_110721_PF.bam is the smallest example data taken from the Deez paper, so we also include Deez itself here for comparison too.

The BAM/SAM.gz implementation here uses libdeflate. Again we use 8 threads, but note deez was only able to use around 2.

Position sorted

Format Options Size(Mb) Encoding(s) Decoding(s)
BAM   1420 20.4 1.8
SAM.gz   1387 22.4 2.9
CRAM v2.1   1048 24.6 3.5
CRAM v3.0   868 10.6 3.8
CRAM v3.0 small 844 23.6 4.8
CRAM v3.0 archive,use_lzma 838 94.9 4.5
CRAM v3.1   830 13.2 4.2
CRAM v3.1 small 771 35.4 17.3
CRAM v3.1 archive,use_lzma 747 81.7 16.9
Deez   842 134.6 62.8

With light-weight level 1 compression and uncompressed level 0 files we see CRAM 3 being slower for uncompressed data than CRAM 2. This is due to the additional CRC integrity checks.

Note the slower time for BAM level 0 than level 1 is purely down to increased disk I/O costs; CPU times double for level 1. Why SAM does not pay this penalty is unknown, but it is likely this picture would change given a large enough file.

Format Options Size(Mb) Encoding(s) Decoding(s)
SAM.gz level=1 1830 22.7 3.2
BAM level=1 1531 17.8 1.9
CRAM v2.1 level=1 1126 12.2 3.5
CRAM v3.0 level=1 889 9.5 3.6
CRAM v3.1 level=1 858 10.3 3.7
SAM level=0 4463 7.8 1.5
BAM level=0 4310 19.6 2.1
CRAM v2.1 level=0 2632 10.1 4.1
CRAM v3.0 level=0 2632 11.4 4.9
CRAM v3.1 level=0 2632 10.9 4.8

Embeded reference & referenceless encoding (position sorted alignments)

By default aligned CRAM uses an external reference file. Portions of that reference can be embedded within each slice to remove this external file dependency. On deep data this has minimal impact as the reference is small in comparison to the alignments.

CRAM can also do non reference-based compression, storing the sequence as-is (like BAM). This leads to larger files.

Format Options Size(Mb) Encoding(s) Decoding(s)
CRAM v2.1   1048 24.6 3.5
CRAM v2.1 embed_ref 1049 25.2 3.8
CRAM v2.1 no_ref 1089 53.7 13.9
CRAM v3.0   868 10.6 3.8
CRAM v3.0 embed_ref 869 10.6 3.7
CRAM v3.0 no_ref 905 13.4 3.8
CRAM v3.1   830 13.2 4.2
CRAM v3.1 embed_ref 831 13.4 4.0
CRAM v3.1 no_ref 866 15.9 4.6

The significant speed difference of no_ref between version 2.1 and 3.0 is due to improved ways of storing multi-base differences instead of requiring one CRAM feature for each base call.

Name sorted alignments

This is the same file above, with aligned sequencing data, but sorted into name order using “samtools sort -n”. BAM is significantly larger as the sequences are no longer in sorted order, harming gzip, but CRAM does not change size considerably. This is due to the use of reference based compression. With referenceless compression CRAM will grow in size, similar to BAM, as is visible with the “no_ref” option. Although “no_ref” it makes minimal difference here, with a very large reference it may be preferable to use this on name sorted data to reduce memory usage.

Format Options Size(Mb) Encoding(s) Decoding(s)
BAM   2016 26.2 2.3
SAM.gz   2005 30.8 3.4
CRAM v2.1   1034 26.3 5.1
CRAM v3.0 no_ref 1327 12.7 4.0
CRAM v3.0   846 12.5 4.7
CRAM v3.0 small 832 22.9 4.4
CRAM v3.0 archive,use_lzma 818 84.7 4.8
CRAM v3.1 no_ref 1302 15.5 4.2
CRAM v3.1   825 14.6 5.9
CRAM v3.1 small 768 37.4 18.1
CRAM v3.1 archive,use_lzma 741 98.3 17.5

Unmapped (name order)

This is the name sorted data above, but with alignments and all auxiliary tags stripped out. This was achieved by converting back to FASTQ via “samtools fastq” and from there back to unaligned BAM. As expected the CRAMs are broadly similar in size to the no_ref mapped name sorted files.

Format Options Size(Mb) Encoding(s) Decoding(s)
FASTQ.pigz   1681 58.0 21.4
FASTQ.bgzf   1710 30.0 7.9
BAM   1686 21.7 2.0
SAM.gz   1722 28.3 2.6
CRAM v2.1   1476 28.3 3.6
CRAM v3.0   1236 12.0 3.7
CRAM v3.0 small 1231 19.5 3.5
CRAM v3.0 archive,use_lzma 1041 378.5 5.2
CRAM v3.1   1208 14.6 5.9
CRAM v3.1 small 1139 73.0 16.8
CRAM v3.1 archive,use_lzma 967 423.3 18.2

Note the fastq was compressed with pigz and bgzip both using 8 threads. Pigz is smaller, but bgzip’s use of libdeflate greatly improves the speed. Both are significantly behind unaligned CRAM though so our recommendation is against storing data in native FASTQ format.

Unmapped (Minhash order)

As above, but passed through the experimental “samtools sort -M” command first. This clusters reads by a hash of their sequence, having the effect of grouping similar looking data together which helps LZ compression algorithms. Note the data is still unaligned. It is a quick alternative to (a better) full genome assembly. With 8 threads this sort process took 21 seconds real time, 170 seconds CPU, although expect this to be less performant on a very large file as would spill temporary files to disk and require a large merge sort.

Note some aligners will need these files sorting (or collating) back to name order prior to converting back to FASTQ.

Format Options Size(Mb) Encoding(s) Decoding(s)
BAM   1207 17.0 1.5
CRAM v3.0   871 13.3 3.6
CRAM v3.0 archive,use_lzma 831 125.8 4.9
CRAM v3.1   842 16.3 3.6
CRAM v3.1 archive,use_lzma 748 163.5 16.0

The effect of “sort -M” on a small deeply sequenced genome is profound, giving file sizes comparable to the aligned position sorted data and around half the size of the name sorted compressed FASTQ file. Expect this effect to be less pronounced on shallow data sets or much larger genomes.