A few questions are already asking about how to fix the mongodb warning:
** WARNING: /sys/kernel/mm/transparent_hugepage/defrag is 'always.'
** We suggest setting it to 'never'
According to the MongoDB documentation, http://docs.mongodb.org/manual/reference/transparent-huge-pages/, and support, https://jira.mongodb.org/browse/DOCS-2131, transparent_hugepage (THP) is designed to create fewer large memory blocks rather than many small memory blocks in systems with a lot of memory. This is great if your software needs large contiguous memory accesses. For MongoDB, however, regardless of memory available, it requires numerous smaller memory accesses and therefore performs better with THP disabled.
That makes me think either way will work, but you'll get better mongo (or any database) performance with THP off, giving you smaller bites of memory. If you don't have much memory anyway, THP probably ought to be off no matter what you run.
Several ways to do that are outlined in the link above. The most universally applicable appears to be editing rc.local.
$ sudo nano /etc/rc.local
Insert the following lines before the "exit 0" line.
... if test -f /sys/kernel/mm/transparent_hugepage/khugepaged/defrag; then echo 0 > /sys/kernel/mm/transparent_hugepage/khugepaged/defrag fi if test -f /sys/kernel/mm/transparent_hugepage/defrag; then echo never > /sys/kernel/mm/transparent_hugepage/defrag fi if test -f /sys/kernel/mm/transparent_hugepage/enabled; then echo never > /sys/kernel/mm/transparent_hugepage/enabled fi exit 0
Note: redhat-based systems may use "redhat_transparent_hugepage" rather than "transparent_hugepage" and can be checked by:
ls /sys/kernel/mm/*transparent_hugepage*/enabled cat /sys/kernel/mm/*transparent_hugepage*/enabled
To apply the changes, reboot (which will run rc.local) or:
$ sudo su # source /etc/rc.local # service mongod restart # exit
to properly apply the changes made above