USB HDD boot panic (2.6.31)

USB HDD boot panic (2.6.31)

[Ben Ryan]

I've a question regarding boot from USB-attached IDE HDD under 2.6.31
(slackware/lilo).
Searches haven't shed much light on status or resolutions.
Booting from this device throws:


VFS: Cannot open root device "sda1" or unknown-block(0,0)
Please append a correct "root=" boot option; here are the available
partitions:
.
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown
block(0,)


I notice that there's no /dev/sd(x) devices listed as possible roots in the
error.
I've tried a kernel build with all relevant drivers inbuilt (not modules).
Tested on multiple machines with same result.
This issue seems to have some history, earlier kernels had patches written
for genhd.c (but I lack the skills to port them forward).
Discussion here:
http://lkml.indiana.edu/hypermail/linux/kernel/0405.0/0224.html

>From what I can gather the USB drivers aren't finished initialising before
the disks are enumerated and mapped to /dev.

Is this a known, current issue?
Does anyone have a patch that can help? If so, why isn't it mainlined?
(Please CC responses!)

Thanks
Ben

Re: USB HDD boot panic (2.6.31)

[Robert Hancock]

On 09/24/2009 10:47 PM, Ben Ryan wrote:
> I've a question regarding boot from USB-attached IDE HDD under 2.6.31
> (slackware/lilo).
> Searches haven't shed much light on status or resolutions.
> Booting from this device throws:
>
>
> VFS: Cannot open root device "sda1" or unknown-block(0,0)
> Please append a correct "root=" boot option; here are the available
> partitions:
> .
> Kernel panic - not syncing: VFS: Unable to mount root fs on unknown
> block(0,)
>
>
> I notice that there's no /dev/sd(x) devices listed as possible roots in the
> error.
> I've tried a kernel build with all relevant drivers inbuilt (not modules).
> Tested on multiple machines with same result.
> This issue seems to have some history, earlier kernels had patches written
> for genhd.c (but I lack the skills to port them forward).
> Discussion here:
> http://lkml.indiana.edu/hypermail/linux/kernel/0405.0/0224.html
>
>  From what I can gather the USB drivers aren't finished initialising before
> the disks are enumerated and mapped to /dev.
>
> Is this a known, current issue?
> Does anyone have a patch that can help? If so, why isn't it mainlined?
> (Please CC responses!)

For this sort of setup you really need some sort of initrd that will 
wait until the devices are detected before trying to mount the root file 
system, rather than trying to have the kernel mount the root file system 
on the USB HD directly. The problem is that the USB device detection is 
asynchronous and the kernel doesn't really know when all possible USB 
devices have "shown up".

Re: USB HDD boot panic (2.6.31)

[Calvin Walton]

On Fri, 2009-09-25 at 14:47 +1000, Ben Ryan wrote:
> I've a question regarding boot from USB-attached IDE HDD under 2.6.31
> (slackware/lilo).

> From what I can gather the USB drivers aren't finished initialising before
> the disks are enumerated and mapped to /dev.
> 
> Is this a known, current issue?
> Does anyone have a patch that can help? If so, why isn't it mainlined?
> (Please CC responses!)

This is a bit of a known issue, because USB simply doesn't have a way of
letting you know when scanning for devices has been completed. Current
versions of linux have a simple workaround: add "rootwait" to your
kernel command line in LILO.

(from Documentation/kernel-parameters.txt:)
        rootwait        [KNL] Wait (indefinitely) for root device to show up.
                        Useful for devices that are detected asynchronously
                        (e.g. USB and MMC devices).

Alternately, you can use an initramfs which is set up to wait for the
device to appear. The rootwait option is off by default in order to
speed up booting under normal circumstances.

> Thanks
> Ben

-- 
Calvin Walton <calvin.walton@gmail.com>