Running Linux from a ramdisk (root)

I have been pondering about this for a while now...

Is it possible to run Linux entirely from RAM (ramdisk)?

Actually, I am pretty sure it's possible (everything is possible in Linux :D), since we already have initrd. So let's change the question to... is it sensible?

A script can be written to copy the content of the ramdisk to harddrive on shutdown, and from harddrive on boot, essentially making shutdown into hibernate.

I am thinking about doing this on a laptop

advantages -
SPEED! (~2GB/s read/write)
low power consumption if we can turn off the harddrive (RAM is powered all the time anyways)
weight (if we can take out the harddrive, but I don't think we can, read on)

disadvantages -
small capacity - most laptops only have 2 SODIMM slots, and 4GB DDR2 modules are insanely expensive/not available right now, which means we can get a maximum of 2x2GB. Potentially 2x4GB in a year or 2.
volatile - data loss on power loss, and makes complete shutdowns longer (copying ramdisk to HD)

The small capacity problem can be elevated by using tmpfs (like ramdisk, except it only uses as much memory as it needs to store the files, and CAN be swapped), and a large swap partition. This is why I said it's probably not possible to remove the harddrive entirely.

Volatility is not so much of a problem on laptops, since they have batteries, and can be configured to hibernate automatically when battery is critically low. Most of the time it can be put to suspend/standby (powering down everything except RAM) when not in use. On my laptop, for example, suspend takes about 1%/hr, which I think is acceptable, as long as it's charged everyday.

Obviously important files shouldn't be saved on the ramdisk. They can be saved to either harddrive or over network.

So... what do you think? Anything I overlooked?

EDIT:
Looks like someone thought about this already =D.
http://www.rikiji.it/index.php?action=archive&nid=7

Okay, getting it down to < 2GB is hard...

I am at 1.9GB right now. Already deleted all the docs and my virtual machines. Time to start deleting programs and libraries...

Need to get more RAM for sure if this works out :).

After hours of head scratching... I did it!

Code:

yberfish@cyberfish-tablet:/$ sudo dd if=/dev/zero of=test bs=4M count=100 100+0 records in 100+0 records out 419430400 bytes (419 MB) copied, 1.35247 s, 310 MB/s cyberfish@cyberfish-tablet:/$ df -h Filesystem Size Used Avail Use% Mounted on tmpfs 975M 0 975M 0% /lib/init/rw varrun 975M 108K 975M 1% /var/run varlock 975M 0 975M 0% /var/lock udev 975M 164K 975M 1% /dev tmpfs 975M 88K 975M 1% /dev/shm rootfs 10G 2.2G 7.9G 22% / lrm 975M 2.4M 973M 1% /lib/modules/2.6.28-11-generic/volatile tmpfs 975M 12K 975M 1% /tmp cyberfish@cyberfish-tablet:/$ free -m total used free shared buffers cached Mem: 1949 1357 592 0 0 838 -/+ buffers/cache: 518 1431 Swap: 4094 1421 2673

Had to modify the patch on that page a bit, added tar to initrd (doesn't come with the ubuntu one for some reason, and changed fstab to remove the root.

For the dd test I cheated by creating and deleting a huge file before the dd command to swap other files and make room for the test file. That would be the performance if I had enough RAM, though.

The system felt noticeably faster... firefox takes about 3 seconds to start (CPU-bound!), which is about how fast it is usually when it's already in disk cache. That's reasonable since tmpfs uses the kernel disk caching system (without backing store, so pages are always marked dirty).

There's still disk activity (swapping) once in a while since I don't have enough RAM.

Reading the "root" archive on boot is not too bad at all. My 2GB root partition can be gzipped to about 500MB. Uncompressing and copying it to the new tmpfs takes about 20 seconds (essentially the time to read 500MB from harddrive, since gzip is so fast that it's bottlenecked by the drive. Wouldn't be the case for bzip2. And writing to RAM is of course also very fast).