Recoverable RAM Disk

Recoverable RAM Disk

[jbradford]

Just wondering - has anyone ever given any thought to the idea of a RAM disk that is not erased on a warm boot?

Obviously this is a bit architechture-specific - I don't think it's easily do-able on i386, but maybe it is other architechtures?

The idea of a recoverable, (even bootable), RAM disk was common on the Amiga, and it would be useful to, E.G. quickly re-boot in to several different kernels.

John.

Re: Recoverable RAM Disk

[Chris Friesen]

jbradford@dial.pipex.com wrote:
> 
> Just wondering - has anyone ever given any thought to the idea of a RAM disk that is not erased on a warm boot?
> 
> Obviously this is a bit architechture-specific - I don't think it's easily do-able on i386, but maybe it is other architechtures?

We did it for a product using PowerPC on compactPCI.  Critical logs are stored
in ram beyond what the kernel uses, and it can be mapped in for processes to use
it.

As long as the card has power, the information remains available, including
resets of the card.

The only tricky bit is that I don't know if a warm boot on a PC wipes ram or
not...

Chris

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Re: Recoverable RAM Disk

[Roy Sigurd Karlsbakk]

> The only tricky bit is that I don't know if a warm boot on a PC wipes ram
> or not...

Normally it does, but in the old days, you could - in DOS DEBUG or somewhere 
set 0040:0072 to a given value and then jmp f000:fff0.

IIRC, the value in 0040:0072 was these:

0x0000:	cold reboot
0x1234:	warm reboot
0x5678:	warm reboot without memory clear

hæpp

roy
-- 
Roy Sigurd Karlsbakk, Datavaktmester

Computers are like air conditioners.
They stop working when you open Windows.

Re: Recoverable RAM Disk

[jbradford]

Looks interesting for saving debug info between re-boots, but I was thinking more of reserving an area at the top of system memory, (say 4 megs), and putting a root filesystem, and some kernel images there.

The theory being that if you haven't got a hard disk, (e.g. nodes of a beowulf cluster), doing a warm boot to change kernels is 'expensive', because you're either booting from a floppy over the LAN.

Alternatively, you could preserve the in-memory filesystem cache between re-boots, (although why anyone should be re-booting that much, I don't know).

> I've recently stumbled about this:
> http://oss.missioncriticallinux.com/projects/mcore/
> 
> Maybe that's about what you're looking for.

Recoverable RAM disk

[jbradford]

Hi,

I know I brought up the subject of a recoverable RAM disk some time ago, (basically, a RAM disk that survives warm boots, and that can be made bootable itself), but this might actually be a good way to implement it...

This article:

http://hedera.linuxnews.pl/_news/2002/09/03/_long/1445.html

demonstrates making a RAM disk using graphics card memory.

Now, the video ram would presumably survive a warm boot in most cases, so if we could implement this idea in kernel space, we could keep a kernel image and root filesystem in the last 12 Mb or so of graphics RAM, and warm boot very quickly.

John.