Ubuntu 12.10 and tmpfs?

Question

ubuntu

Hello!
I did not think that I would come to you with such a question.
Ubuntu server 12.10 is installed on the web server.
4 SCSI disks in raid 5, 64G RAM.
With the growth of the database, I decided to simplify some tasks a little, moved the temporary directory of the muscle to / tmp and mounted part of the RAM (15G) in the same directory.
After mounting, I was a little surprised - nothing was reserved from the RAM. Suppose we have learned to determine a place by its employment.
It’s good that I didn’t believe it, I decided to copy large files there ... the speed is 150 Mbps.

df -h
Файл.система                                      Размер Использовано  Дост Использовано% Cмонтировано в
/dev/sda2                                           8,1T         1,2T  6,6T           16% /
udev                                                 32G         4,0K   32G            1% /dev
tmpfs                                                13G         328K   13G            1% /run
none                                                5,0M            0  5,0M            0% /run/lock
none                                                 32G            0   32G            0% /run/shm
none                                                100M            0  100M            0% /run/user
/dev/sda1                                            94M         126K   94M            1% /boot/efi
/run/shm                                             15G          12K   15G            1% /tmp

I mount this line:
mount -t tmpfs /dev/shm /tmp -o size=15G
Please comment on this situation.

Answer 1

I have it in fstab
Everything works. I copy files to / tmp - the memory is filled.

Answer 2

Found such data
Tmpfs has been supported by Linux since version 2.4. Tmpfs (also known as shmfs) differs from the Linux RAM disk in that it dynamically allocates memory and moves unused pages to swap. RAMfs, on the other hand, does not use swap (this can be both an advantage and a disadvantage). In addition, MFS, and some older versions of RAMfs, did not change their size dynamically, but remained the same size as they were mounted.
The question persists, why the speed is low and why the parameters of the used RAM do not change