ramdisk size clarification

ramdisk size clarification

[Padraig Brady]

It's not at all obvious to me what the {ramdisk,ramdisk_size,rd_size}
parameters for ramdisks do from reading ramdisk.txt (note rd_size
is used when using the ramdisk as a module).

I think they only set the size to be reported, for e.g. mke2fs does
the following to determine the size of a ramdisk:

open("/dev/ram0", O_RDONLY|O_LARGEFILE) = 3
ioctl(3, BLKGETSIZE, 0xbffff81c)        = 0
close(3)                                = 0

However there is no actual RAM allocated until it's required,
and also there is no upper limit on the amount of RAM used,
so the following will kill your system (well it did for me):
dd if=/dev/zero of=/dev/ram0

can this be clarified in the ramdisk.txt file please.

As a side note there is a lovely "trivial ramdisk" module
written by Andrw tridgell @:
http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/gkernel/ext3/tools/trd/
that does static allocation and (hence) doesn't grow beyond the
specified size.

thanks,
Padraig.

Re: ramdisk size clarification

[Mike Castle]

On Mon, Dec 17, 2001 at 08:41:32PM +0000, Padraig Brady wrote:
> and also there is no upper limit on the amount of RAM used,
> so the following will kill your system (well it did for me):
> dd if=/dev/zero of=/dev/ram0

Bug in recent 2.4.*.  It *should* stop.  One line patch has been
posted to the linux-kernel list a couple of times.  Check the archive.
It's probably in the recent pre-release kernels as well.

As a work around, try the following:

count=`blockdev --getsize /dev/ram0`
bsize=`blockdev --getss /dev/ram0`
dd if=/dev/zero of=/dev/ram0 count=${count} bs=${bsize}

-- 
     Mike Castle      dalgoda@ix.netcom.com      www.netcom.com/~dalgoda/
    We are all of us living in the shadow of Manhattan.  -- Watchmen
fatal ("You are in a maze of twisty compiler features, all different"); -- gcc

Re: ramdisk size clarification

[Padraig Brady]

Mike Castle wrote:

> On Mon, Dec 17, 2001 at 08:41:32PM +0000, Padraig Brady wrote:
> 
>>and also there is no upper limit on the amount of RAM used,
>>so the following will kill your system (well it did for me):
>>dd if=/dev/zero of=/dev/ram0
>>
> 
> Bug in recent 2.4.*.  It *should* stop.  One line patch has been
> posted to the linux-kernel list a couple of times.  Check the archive.
> It's probably in the recent pre-release kernels as well.
> 

Great thanks!
http://marc.theaimsgroup.com/?l=linux-kernel&m=100797607432139&w=2

Padraig.