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

[Anton Shterenlikht]

>From r.c.ladan at gmail.com Tue Jul  1 11:37:35 2014
>
>2014-07-01 11:25 GMT+02:00 Anton Shterenlikht <mexas at bris.ac.uk>:
>
>> 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?
>>
>> Mostly write tear because you're using an SD card, and it improves speed
>too.

"write tear"?
Is this a joke, or some technical term?
I cannot find what it means.

I get these messages on the console (well, on hdmi port...):

pid ... (svnlite), uid 0 inumber 13 on /tmp: filesystem full

If I unmount /tmp from md and leave it on sd card,
then I don't see these anymore. What does this mean?

Thanks

Anton