blastn 多加上 -task blastn-short -word_size 7 -evalue 1
-task：设置比对模式，通常情况会有一个默认task，这里要设置为blastn-short
-word_size：尽可能小，这里设为7
-evalue：类似于p值的一个指标，越小说明越可靠
Tasks
| Task | Description |
| blastp | Traditional BLASTP to compare a protein query to a protein database |
| blastp-short | BLASTP optimized for queries shorter than 30 residues |
| blastn | Traditional BLASTN requiring an exact match of 11 |
| blastn-short | BLASTN program optimized for sequences shorter than 50 bases |
| megablast | Traditional megablast used to find very similar (e.g., intraspecies or closely related species) sequences |
| dc-megablast | Discontiguous megablast used to find more distant (e.g., interspecies) sequences |
REF
http://barcwiki.wi.mit.edu/wiki/blastTips
https://web.cas.org/help/BLAST/topics/shrt_seq.htm
https://www.jianshu.com/p/a6ee3d19b832?utm_campaign=maleskine