Wanted: one big ramdisk. Getting: grief :-(

[afabbro]

I have a box in which I've scraped together 2GB RAM (shows up as 2032MB). I want to play with making a 1GB RAMDisk. This is with kernel 2.4.24. I'd prefer to do it with rd as a module (so I can change the size at insmod time) rather than muck with grub.conf, so that's what I'm trying.

I've run into a wee problem...

[adaptr]

Use modprobe - insmod does not load dependencies.
It's also unclear whether you mean you have recompiled the kernel or are dithering about it without doing it.

It's either a missing dependency for the rd module, or you did not enable all the required kernel options for highmem.
_________________
>>> emerge (3 of 7) mcse/70-293 to /
Essential tools: gentoolkit eix profuse screen

[afabbro]

Alas, modprobe doesn't work, either, though I thank you for the tip on dependencies:

[FormerSlacker]

Well the ramdisk module is basicly saying that it cant find the highmem functions in your kernel. Looking at your config, it seems to be set. It's possible that some object files aren't being rebuilt and its causing these problems.

Try copying your .config file to somewhere safe, and run "make distclean" and then copy back yer .config file and recompile the kernel & modules. That may do the trick.

[afabbro]

Recompiling the kernel solved the highmem problem...but I'm back to "you can mke2fs it, but you can't mount it". Doing this gives me a very weird dmesg error:

[CheshireCat]

Is there any reason not to use tmpfs for this? Unless you want to use RAM as a raw block device, tmpfs is probably a better idea than a RAM disk, in many ways - you don't have the extra layers of a filesystem designed to run on a disk being used on an emulated block device, and the tmpfs only uses as much RAM as needed for the files it contains. Try

[afabbro]

So the answer is that (at least in 2.4.x), Linux doesn't support RAMdisks beyond 512MB...the actual limit might be 525 or 526. Some deep googling found various people posting on this and they reported hitting the error around 525 or 526MB.

One article that sums it up is: http://archive.quadratic.net/docs/ramdisk500.html

CheshireCat, thanks for the tmpfs suggestion - I'll give that a shot.

[CheshireCat]

I didn't know about that limit...

Another thing you should know about tmpfs, it doesn't strictly store data in RAM - in other words, if something needs more RAM than you have free, it's possible for stuff in your tmpfs to get pushed to swap. This should only happen when swapping out the data is good for performance, though, and not to files that are being accessed frequently.