/tmp, /var/log, /var/tmp as /dev/md - why?

[Anton Shterenlikht]

Why is it a good idea to mount /tmp and some var dirs on memory disks:

root at raspberry-pi:/usr/ports # df -m
Filesystem     1M-blocks Used Avail Capacity  Mounted on
/dev/mmcsd0s2a     14694  777 12742     6%    /
devfs                  0    0     0   100%    /dev
/dev/mmcsd0s1         16    3    13    20%    /boot/msdos
/dev/md0              28    4    22    16%    /tmp
/dev/md1              14    0    12     0%    /var/log
/dev/md2               4    0     4     0%    /var/tmp
root at raspberry-pi:/usr/ports # 

Is this about speed or power, or maybe space?

Can I not put all these dirs on sd card?

I'm new to arm, so maybe things are different
to other arches.


Thanks

Anton