[Wylug-help] USB Flash: device numbers, filenames and grub

[Dave Fisher]

Hi,

Could someone explain, or point me to some reasonable quality
documentation that would give me a chance of figuring it out, the
following:

1. I get inconsistent behaviour when I unmount/eject different usb
   sticks on the same PC (running Ubuntu 6.10 Edgy, with a 2.6.17-10-386
   kernel).

   When I unmount the single partition on one stick, the partition
   device file (/dev/sda1 on this occasion) disappears, but the device
   file for the whole device (/dev/sda) remains.

   If I then insert a different stick, it gets the device file /dev/sdb
   (presumably because /dev/sda remains in the file system) and its
   partitions get /dev/sdb1, /dev/sdb2 and /dev/sdb3.

   But if I unmount/eject any one of those partitions:

   a) They are all unmounted.

   b) All of their device files remain on the file system.
   

2. I presume that there is a clean way of removing device files from the
   system, (e.g. to ensure that if I only have one stick inserted, it
   always gets /dev/sda ... assuming no other scsi devices).
   
   Or is that wishful thinking?


3. I've been experimenting with booting and running entire systems from
   USB sticks.  However, I suspect that my success so far is partly down
   to luck, e.g. using similar PCs, none of which had other usb or scsi
   devices attached.

   I'd like to make my current stick-hosted system more robustly
   portable, but the device file naming issue is a bit of an obstacle.

   I am booting from grub, which provides command line completion for
   finding discs, partitions and even ordinary files on unmounted
   systems (cool).

   Unfortunately, grub doesn't seem to be able to do this for device
   file names, so I have to predict (i.e. guess) which device file the
   USB-based root filesystem is going to appear on, in order pass the
   kernel the correct vaue for its "root=" parameter.

   In simple cases (e.g. one HDD, one USB stick) this needn't be a
   problem, because I can accurately predict the device file name for
   the root partition of the USB-based system.

   Unfortunately, there are many common, non-simple, cases.

   For example, If the host PC contains multiple partitioned hard
   drives, grub can number them like so:

   (Hd0)   = /dev/hda
   (Hd0,0) = /dev/hda1
   (Hd0,1) = /dev/hda2
   (Hd1)   = /dev/sda
   (Hd1,0) = /dev/sda1
   (Hd2)   = /dev/hdd
   (Hd2,0) = /dev/hdd1

   I expected that grub would name these in device type order, e.g.
   all the IDE drives in channel-disc-partition order, then all the
   usb/scsi drives in channel-disc-partition order.

   Since I know that IDE and USB devices use scsi addressing, I'm not
   really surprised by the fact that grub naming doesn't follow that
   pattern, i.e, interleaving sd* devices with hd* devices.

   What concerns me is that I don't know how the order is determined,
   e.g. why an attached USB stick has a lower number than an on-board
   IDE disc.

   This is not an absolute obstacle, because one can always use the
   grub command line to figure out and set these names interactively.
   
   It would, however, be nicer to predict them, and hence offer
   end-users simple and reliable options on a menu.

   So in effect, I have two naming predicton problems:

   a) How to predict the grub name for the device.

   b) How to predict the device file name for the root file system.

   Are the two in any way related, e.g. to scsi device orders?

Dave