Ubuntu – Why does Ubuntu 16.04 set all drive IO schedulers to “deadline”

Are you sure the disk is plugged into the correct (USB 3.0) port? It looks like your USB 3.0 controller is providing USB bus #12, but the disk is attached to USB bus #3.

According to your dmesg, USB Bus #3 is provided by the PCI device '06:01.2: USB Controller: VIA Technologies, Inc. USB 2.0 (rev 65)', which will only be USB 2.0.

fstrim supports the option --all on Ubuntu 16.04.

$ lsb_release -dirc
Distributor ID: Ubuntu
Description:    Ubuntu 16.04.1 LTS
Release:        16.04
Codename:       xenial

$ fstrim --help

Usage:
 fstrim [options] <mount point>

Discard unused blocks on a mounted filesystem.

Options:
 -a, --all           trim all mounted filesystems that are supported
 -o, --offset <num>  the offset in bytes to start discarding from
 -l, --length <num>  the number of bytes to discard
 -m, --minimum <num> the minimum extent length to discard
 -v, --verbose       print number of discarded bytes

 -h, --help     display this help and exit
 -V, --version  output version information and exit

For more details see fstrim(8).

By default, Ubuntu 16.04 mounts all file systems with relatime. This works much better than noatime while adding only a very small number of write operations.

Do not mess with the I/O scheduler.

For peace of mind, you can use smartctl (install it with sudo apt-get install smartmontools if needed) from time to time to check on Life_Curve_Status, SSD_Life_Left, and Lifetime_Writes_GiB:

$ sudo smartctl --all /dev/sda
...
Vendor Specific SMART Attributes with Thresholds:
ID# ATTRIBUTE_NAME      FLAG    VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
...
194 Temperature_Celsius 0x0022  047   063   000    Old_age   Always       -       47 (Min/Max 21/63)
...
230 Life_Curve_Status   0x0013  100   100   000    Pre-fail  Always       -       100
231 SSD_Life_Left       0x0013  100   100   010    Pre-fail  Always       -       0
...
241 Lifetime_Writes_GiB 0x0032  000   000   000    Old_age   Always       -       388
...

(Life_Curve_Status and SSD_Life_Left have values in percentage points: you will want to replace the SSD when they decrease below 20% or 10%. If you can, make your terminal 132 columns before running smartctl.)

Finally, please remember that the 850 EVO is a high-quality expensive SSD from a reputable manufacturer. It is resilient and will last a long time, probably longer than any spinning disc. The only thing you can do which Samsung admits that it would help is to overprovision it a little, that is, when partitioning leave about 5% of unpartitioned space. (This will let the internal remapping / garbage collection algorithms work more efficiently; it you decide to do it then do it only when the SSD is new or after a full-disc trim or a security erase so that the overprovisioned space consists entirely of blocks known to be not in use.)