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mash on Biowulf

Mash uses MinHash hashing to reduce large sequences to a representative sketch. Distances between sketches of sequences can be calculated very rapidly and can provide an estimate of average nucleotide identity. Sketches of all the genomes in RefSeq 70 are only ~90MB (at a kmer size of 16 using 400 hashes).


Important Notes

Interactive job
Interactive jobs should be used for debugging, graphics, or applications that cannot be run as batch jobs.

Allocate an interactive session and run the program. Sample session:

[user@biowulf]$ sinteractive --gres=lscratch:20
salloc.exe: Pending job allocation 46116226
salloc.exe: job 46116226 queued and waiting for resources
salloc.exe: job 46116226 has been allocated resources
salloc.exe: Granted job allocation 46116226
salloc.exe: Waiting for resource configuration
salloc.exe: Nodes cn3144 are ready for job

[user@cn3144 ~]$ module load mash
[user@cn3144 ~]$ cd /lscratch/$SLURM_JOB_ID

Some example data is included in the TEST_DATA directory inside the application directory.

[user@cn3144 ~]$ ls $MASH_TEST_DATA
BA000007.2.fna  CP009273.1.fna  NC_000913.2.fna  RefSeqGenomes_V70.msh  SRR292770_1.fastq.gz
[user@cn3144 ~]$ cp $MASH_TEST_DATA/* .

Estimate the distance between two E. coli genomes

[user@cn3144 ~]$ mash dist BA000007.2.fna NC_000913.2.fna
/usr/local/apps/mash/TEST_DATA/BA000007.2.fna   /usr/local/apps/mash/TEST_DATA/NC_000913.2.fna
0.0222766      0       456/1000

The result shows the reference sequence, the query sequence, the distance estimate, the p value, and the number of matching hashes.

Instead of calculating sketches each time they can be precalculated. For example, we can sketch the two genomes from above

[user@cn3144 ~]$ mash sketch -o 2coli.msh BA000007.2.fna NC_000913.2.fna
Sketching /usr/local/apps/mash/TEST_DATA/BA000007.2.fna...
Sketching /usr/local/apps/mash/TEST_DATA/NC_000913.2.fna...
Writing to 2coli.msh...
[user@cn3144 ~]$ ls -lh 2coli.msh
-rw-r--r-- 1 user group  16K Dec  4 09:56 2coli.msh
[user@cn3144 ~]$ mash info 2coli.msh
  Hash function (seed):          MurmurHash3_x64_128 (42)
  K-mer size:                    21 (64-bit hashes)
  Alphabet:                      ACGT (canonical)
  Target min-hashes per sketch:  1000
  Sketches:                      2

  [Hashes]  [Length]  [ID]             [Comment]
  1000      5498450   BA000007.2.fna   -
  1000      4639675   NC_000913.2.fna  -

So the sketch for the 10MB genomes takes up 16kB. Now we can compare our query against the sketches with

[user@cn3144 ~]$ mash dist 2coli.msh CP009273.1.fna
BA000007.2.fna   CP009273.1.fna   0.0222766      0       456/1000
NC_000913.2.fna  CP009273.1.fna   0              0      1000/1000

[user@cn3144 ~]$ exit
salloc.exe: Relinquishing job allocation 46116226
[user@biowulf ~]$

Batch job
Most jobs should be run as batch jobs.

Create a batch input file (e.g. mash.sh), which uses the input file 'mash.in'. For example:

#! /bin/bash

function fail {
  echo "$@" >&2
  exit 1

module load mash/2.0 || fail "could not load mash module"
mash sketch -i -k 21 -s 1000 -o coli.msh -l list_of_ecoli_genomes

Submit this job using the Slurm sbatch command.

sbatch [--cpus-per-task=#] [--mem=#] mash.sh
Swarm of Jobs
A swarm of jobs is an easy way to submit a set of independent commands requiring identical resources.

Create a swarmfile (e.g. mash.swarm). For example:

mash sketch -o NC_000913.2.msh /usr/local/apps/mash/TEST_DATA/NC_000913.2.fna
mash sketch -o BA000007.2.msh /usr/local/apps/mash/TEST_DATA/BA000007.2.fna
mash sketch -o CP009273.1.msh /usr/local/apps/mash/TEST_DATA/CP009273.1.fna

Submit this job using the swarm command.

swarm -f mash.swarm [-g #] [-t #] --module mash
-g # Number of Gigabytes of memory required for each process (1 line in the swarm command file)
-t # Number of threads/CPUs required for each process (1 line in the swarm command file).
--module mash Loads the mash module for each subjob in the swarm