more tmpfs info

[rbr28]

I'm using tmpfs for some stuff on a machine with an SSD and I have some questions I haven't seen answered in the documentation I've found.

1. If I have three partitions mounted w/ tmpfs and set a limit of 1GB for each, does that mean that each individually could use 1GB for a total of 3GB? If that's the case, is there a way to set the maximum total amount that can be used by all tmpfs mounts. As an example, I may want all three partitions to be able to use up to 1GB each, but to limit the combined total to 2GB.

2. I've seen reference to the fact that a tmpfs mount does not use much memory if no files are there. Anyone have any idea what the resource usage is for an empty tmpfs mount? Is there some amount of space reserved for any mount, like a small percentage of the limit or something? I'm just wondering if there's any advantage at all to setting the limit on the low side. Am I saving any resources whatsoever if I set the limit to 512M instead of 2GB for example?

3. Right now I have /var/run, /var/lock, /var/tmp/portage, and /tmp using tmpfs, and I'm wondering if there are any other reasonable possibilities.

Thanks,
Vern

[BradN]

1. The limits specified are per each mount, so 3 1GB mounts could at most hold 3GB. You could of course limit the total to 2GB if your system only has 2GB, but this is probably not what you want I don't know of a good way to set a total limit - even disk quotas I believe are be per-filesystem. Newer kernels have various cpu/process/etc group features that I don't understand in the least, maybe it could be worth looking into them.

2. The overhead for an empty tmpfs is constant and not related to the maximum limit as far as I know. It's small enough to be ignored. The limits are there mostly to prevent a tmpfs from running out of control and consuming all the RAM (and swap?).

3. This depends on your usage and amount of RAM available. You could load your entire system into tmpfs if you have enough memory and don't mind waiting for it to fill at boot. Chances are /var/tmp/portage is the only one of your examples that will give a meaningful performance difference, but in the case of an SSD system, /tmp will slightly reduce write count (/var/run is usually only written to when services start, and on the kubuntu machine I'm on at the moment, /var/lock isn't used at all).

[Section_8]

As BradN said, I think each tmpfs mount limit in fstab applies to each mount individually. tmpfs is just a ramdisk in virtual storage instead of physical ram, so the total max tmpfs size is the amount of virtual storage (ram + swap) you have free. However, if you fill up physical ram with tmpfs and it starts swapping, that kind of defeats the purpose.

If you have a 1 gig limit on /var/tmp/portage, that's not enough for some packages. Openoffice takes about 7 gig, I think. If you don't want to make /var/tmp/portage that big, you can just umount /var/tmp/portage before emerging openoffice (assuming you /var partition has enough free space).