Naming of USB mass storage devices

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04-14-2011
schnitten

Naming of USB mass storage devices

Hi,

I am searching for a way to change the ordering of loading for USB mass storage devices.

I have a small embedded board with a USB stick containing root and /boot partitions and a big USB hdd for data. My bootloader is uBoot.

When booting with only the USB stick attached everything works as expected. Stick is detected as sda by the kernel.
When I additionally attach my hdd at boot time, my stick is detected as sdb and the hdd claims sda which gives me troubles defining "root" kernel parameter.

As this is a headless machine there is no way to define multiple boot targets or whatsoever.

Is there a way to force the kernel to assign sda to my USB stick?

thanks
Christian
04-21-2011
Rubberman

The order devices are discovered is indeterminate, by design. Your mount order in /etc/fstab can be configured to use /dev identifiers, partition UID's, or file system label names. Myself, I prefer label names. You can label your devices, and specify the label in /etc/fstab, and it will mount stuff accordingly, otherwise, every time you add a new HD device, things go wonky (highly technical term implying "doesn't work any more")... :-)

Example (from my system's /etc/fstab) illustrating both methods:

Code:

UUID=0e9c3025-59d3-424e-818f-c305c819b4e7 / ext4 defaults 1 1 UUID=7819b966-a8e6-4d61-87c1-d377e3074bd8 /boot ext4 defaults 1 2 UUID=b2703a11-9292-4d2f-afe6-665c46b8fa60 swap swap defaults 0 0 /dev/mapper/VolGroup00-LogVol00 /home ext3 defaults 1 2 LABEL=afs-esata-a1 /mnt/array1 ext4 defaults 1 2 LABEL=afs-esata-1 /mnt/esata1 ext4 defaults 0 0

Hope this helps.
04-21-2011
schnitten

Thanks for pointing me to use LABEL in fstab. Works like acharm there.

So I see there is no way to influence kernel usb detection. Which makes me change to plan B, use an initrd to mount root by label...

thanks for your reply!
04-22-2011
Rubberman

Happy to provide what little help I can! :-)