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

[Anton Shterenlikht]

>From imp at bsdimp.com Tue Jul  1 18:09:59 2014
>
>On Jul 1, 2014, at 3:25 AM, Anton Shterenlikht <mexas at bris.ac.uk> wrote:
>
>> Why is it a good idea to mount /tmp and some var dirs on memory disks:
>>=20
>> 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 #=20
>>=20
>> Is this about speed or power, or maybe space?
>>=20
>> Can I not put all these dirs on sd card?
>>=20
>> I'm new to arm, so maybe things are different
>> to other arches.
>
>It isn=92t so much about ARM as it is about SD cards. Each write to a =
>file causes wear and tear on the card. Each update of metadata likewise. =
>There are things that can be done (like enabling trim) that reduce the =
>wear and tear on the card, NAND flash only has so much life. Do you =
>really want to use it for data that=92s at best disposable? No. SD cards =
>these days are made from NAND that=92s lucky to get 3k separate writes =
>to it (or even worse: 500 in the case of TLC NAND). Given such a limited =
>resource, nanobsd, and others, use MD devices to eliminate that wear and =
>tear. It is the same rason there=92s no swap partition...
>
>Having said that, I=92ve run many development systems without doing =
>this. They work fine, but doing it in production has shown to result in =
>some SD cards (not all) breaking prematurely.
>
>Warner

Wow, thank you!

I knew nothing about sd cards, so this is very helpful.

So, if I need to build kernel, world and ports often,
sd card is a poor choice? I better use an external disk?

What I really want to find out is whether I can
have a dumb graphical terminal from Rpi-B.

Thanks

Anton