what's extsort.* (8 - 11 MB) in MongoDB dbpath? Directory _tmp I never saw this in my dbpath and didn't found any explanation with a search engine.

We run MongoDB 3.2.8

# ls -alh; pwd
total 151M
drwxr-xr-x. 2 mongod mongod 4.0K Sep 12 22:18 .
drwxr-xr-x. 6 mongod mongod  12K Sep 12 22:16 ..
-rw-r--r--. 1 mongod mongod 8.0M Sep 12 21:54 extsort.0
-rw-r--r--. 1 mongod mongod  10M Sep 12 21:54 extsort.1
-rw-r--r--. 1 mongod mongod 7.9M Sep 12 22:02 extsort.10
-rw-r--r--. 1 mongod mongod 7.9M Sep 12 22:07 extsort.11
-rw-r--r--. 1 mongod mongod 9.9M Sep 12 22:07 extsort.12
-rw-r--r--. 1 mongod mongod 7.9M Sep 12 22:11 extsort.13
-rw-r--r--. 1 mongod mongod 9.9M Sep 12 22:12 extsort.14
-rw-r--r--. 1 mongod mongod 8.0M Sep 12 22:16 extsort.15
-rw-r--r--. 1 mongod mongod  11M Sep 12 22:18 extsort.16
-rw-r--r--. 1 mongod mongod 7.9M Sep 12 21:55 extsort.2
-rw-r--r--. 1 mongod mongod 9.9M Sep 12 21:55 extsort.3
-rw-r--r--. 1 mongod mongod 7.9M Sep 12 21:55 extsort.4
-rw-r--r--. 1 mongod mongod 9.8M Sep 12 21:55 extsort.5
-rw-r--r--. 1 mongod mongod 7.9M Sep 12 21:56 extsort.6
-rw-r--r--. 1 mongod mongod  11M Sep 12 21:57 extsort.7
-rw-r--r--. 1 mongod mongod 8.0M Sep 12 21:58 extsort.8
-rw-r--r--. 1 mongod mongod 9.9M Sep 12 22:01 extsort.9
/data/name-replicaset/name-replicaset_0/_tmp
These files are used when the sort you are attempting in an aggregation exceeds the maximum allowed memory and you have allowDiskUse : true specified.

The relevant pieces of the code are here (where the SortedFileWriter is called) and here (where the extsorter.# files are written to temp by SortedFileWriter). Basically, this is the number of files required by MongoDB to perform the sorting that is being specified in your aggregations.