[Beowulf] cloning issue, hidden module dependency

Bogdan Costescu Bogdan.Costescu at iwr.uni-heidelberg.de
Mon Dec 8 17:40:35 EST 2008

On Mon, 8 Dec 2008, David Mathog wrote:

> The straw that broke this particular camel's back was a decision 
> (presumably by Mandriva, maybe by RedHat) to change in the kernel 
> config BLK_DEV_IDE and BLK_DEV_IDEDISK from y to m, similarly, 
> DEV_AMD74XX (and etc.) also changed from y to m.

You were just lucky previously that Red Hat engineers found a good 
idea to put those into the kernel. How would you have felt if you were 
booting an all-SCSI (to stay with old tech) system, where the IDE 
drivers present in the kernel would not have helped ?

> As a consequence, they went from a system where a simple initrd 
> would boot anywhere (as all the needed drivers were built into the 
> kernel) to one where a much more complex initrd ended up being 
> highly machine specific.

Sorry to disapoint you... the initrd was always machine specific. All 
Red Hat docs specify that after modifying /etc/modules.conf or 
/etc/modprobe.conf the initrd should be regenerated via mkinitrd so 
that the next boot will use the proper drivers/settings.

As to the complexity of initrd: my current method choice for setting 
up compute nodes is to sync a root FS from the master server during 
the initrd, which means that I have to build an initrd. As I already 
know what hardware components are in the node (which is also the case 
f.e. when I run mkinitrd), it's easy to just add these modules to the 
initrd archive and insert a few 'insmod module.ko' in the proper order 
in the init script.

Having a monolithic kernel that "just works" on a large variety of 
hardware means answering "y" to most drivers; the kernel itself would 
then grow as large as the "immense initrd" that you mention. How would 
that be better ?

Bogdan Costescu

IWR, University of Heidelberg, INF 368, D-69120 Heidelberg, Germany
Phone: +49 6221 54 8240, Fax: +49 6221 54 8850
E-mail: bogdan.costescu at iwr.uni-heidelberg.de
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