I've been recently investigating booting Linux from USB drives. From past experience, this list is not the ideal place to pose USB related questions, but I think at this stage my questions concern such fundamental issues that I can pose some here just asking that others more knowledgeable about Linux and computer operation offer corrections or clarification of my understanding of what's involved. Let me start by pasting here an item on booting Linux from USB from the FAQ at http://www.linux-usb.org/ :
"Q: Is it possible to boot off a USB Device? A:There are (at least) three things you need for this to be able to happen. 1. BIOS Support to boot from USB 2. Kernel support for USB Storage (including SCSI) 3. A patch to delay boot process The first of these is something outside your control, either your BIOS supports it or not. However, you could put your kernel and initrd on a floppy and then use a USB root file system to get around this. In your boot kernel or initial RAM disk you need to include support for all the needed items to support using a USB disk. These are documented in the Linux USB User Guide. You also need to patch this kernel to delay when it tries to open the root file system, as the USB subsystem takes longer than is allowed to initialise and make the device available to the kernel. You'll find a patch suitable for 2.2 and 2.4 here (although the 2.4 patch could be put in init/do_mounts.c:mount_block_root() instead of fs/super.c which would be cleaner). A patch may be added to kernels later than 2.4.20 (latest current released version as I type this) to remove the need for this patch, but this hasn't happened yet." (hyperlinks removed in pasting) First, the "three things . . . need(ed)" are not really all needed. From the explanation following the list, I understand that if you don't have number 1, you might still be able to boot from USB with the aid of 2 and 3. Am I correct in that understanding? Doesn't the explanation seem to imply this? If so, then some further questions on that. I can't understand from the description whether the initrd is really necessary. I think I understand about the kernel patch: the kernel is written to give some error or failure message if the root filesystem cannot be found within a certain timeframe. The time it takes for USB handling components to load or initialize and for scsi emulation to start exceeds that built-in time limit. The patch causes the kernel to wait some longer period of time before giving the error or failure message regarding the root filesystem. Do I understand that part correctly? If so, then the one remaining unclarity concerns the initrd. I know vaguely what an initrd is, and why it could be helpful in booting from USB: it is some initial filesystem and files that can be loaded into a ramdisk on bootup and that contains things like loadable kernel modules that the kernel could use to get USB going. Is that right? In other words, there would be some USB module like usb-storage there, so when the kernel began loading and found a USB mass storage device, it could load the module in order to be able to use it. Does this sound correct? Assuming it is, I would just further like to ask whether, given the right circumstances, the initrd is really necessary? By "right circumstances" I mean the following: suppose I were to compile a kernel for a particular system that I wanted to boot from USB, and that I compiled it with all the necessary USB components (e.g., ohci, usb-storage, usbcore - and whatever else might be relevant) *not* as loadable modules, but as part of the kernel (i.e., select "y" instead of "m" for those items in the kernel config). Suppose I did that, as well as applying the time-delay patch to the kernel. Were I to do that, would I really still need to have an initrd? Feedback appreciated. James - To unsubscribe from this list: send the line "unsubscribe linux-newbie" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.linux-learn.org/faqs
