boot time autoassembly: how come it works?

boot time autoassembly: how come it works?

[Jules Bean]

I'm trying to understand the boot sequence of new kernels (2.6.24, 
initramfs) better and I really don't understand how the RAID assembly works.

The docs (md.txt) say that auto-assembly only occurs if the RAID arrays 
are marked as 'RAID autodetect' aka partition type 0xfd. On the other 
hand mdadm needs mdadm.conf to get going.

My RAID arrays (including the one which contains the root file system, 
/dev/md2) are not marked as 0xfd, and they are not correctly listed in 
/etc/mdadm/mdadm.conf on the initramfs filesystem either. (There are 
some listings in there but they are out of date : UUIDs wrong and number 
of devices wrong).

So how come it works?

Jules

Re: boot time autoassembly: how come it works?

[Bryan Mesich]

On Wed, Jun 04, 2008 at 11:05:16PM +0100, Jules Bean wrote:

> The docs (md.txt) say that auto-assembly only occurs if the RAID arrays are 
> marked as 'RAID autodetect' aka partition type 0xfd. On the other hand 
> mdadm needs mdadm.conf to get going.


You may to pull apart your initrd image and look at your init
script (or whatever your startup script is called) to see how it
is assembling your arrays.  If you're using user space to assemble
your arrays at boot time (i.e. mdadm via initrd) it is most likely
using the mdadm.conf file located in the initrd.  Typically,
mkinitrd is called when a new kernel is installed.  The mkinitrd
script will grab the necessary files (i.e. mdadm.conf) needed to
mount the root filesystem. 

I'm not sure I've seen a system auto assemble an array when the
partitions were not set to 0xfd.  My bet is that you have a
mdadm.conf file in your initrd and that the init script is
starting the array in user space via mdadm.

Though, I could be wrong :-)

Bryan 

Re: boot time autoassembly: how come it works?

[Jules Bean]

Bryan Mesich wrote:
> I'm not sure I've seen a system auto assemble an array when the
> partitions were not set to 0xfd.  My bet is that you have a
> mdadm.conf file in your initrd and that the init script is
> starting the array in user space via mdadm.

Yes, there is an mdadm.conf and a full copy of mdadm in my initramfs.

However the mdadm.conf is wrong. weird.

It has the wrong number of devices listed for both arrays, and the wrong 
UUIDs. Somehow that must be close enough to work.

Jules

Re: boot time autoassembly: how come it works?

[Michal Soltys]

Jules Bean wrote:
> I'm trying to understand the boot sequence of new kernels (2.6.24, 
> initramfs) better and I really don't understand how the RAID assembly 
> works.
> 
> The docs (md.txt) say that auto-assembly only occurs if the RAID arrays 
> are marked as 'RAID autodetect' aka partition type 0xfd. On the other 
> hand mdadm needs mdadm.conf to get going.
> 

Initramfs is a tiny linux. It (usually) either has simple mdassamble or 
mdassemble.auto, or full-blown mdadm - and when you use those, partition 
type is ignored.

Partitions with 0xfd relate to [deprecated in times of early userpsace] 
method of assembling the arrays by the kernel itself, before any 
userspace code (initramfs/rd, your system) takes action.