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Unslung 6.8 and 256MB USB sticks: RESOLVED

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  • Mike (mwester)
    I finally broke down and bought a 256MB USB memory stick, just to try to debug the issues being reported with these small devices and Unslung 6.8. Based on the
    Message 1 of 1 , Aug 3, 2006
      I finally broke down and bought a 256MB USB memory stick, just to try to
      debug the issues being reported with these small devices and Unslung 6.8.

      Based on the available information (thanks to those who provided some fdisk
      output from their 256MB devices), here's what's happening:

      The vital bit of information is the number of cylinders on the device -- the
      Linksys firmware does all its work in units of cylinders. My USB key (a
      256MB Sandisk Cruzer Mini) has 1011 cylinders.

      [editorial note: yes, the concept of cylinders and heads are totally
      obsolete and have no application to flash memory devices, but the concept
      lives on to torment all right-thinking people. This is another example of
      how the sins of the fathers are visited upon the future generations.]

      In order to figure out the partitioning, Linksys starts from the back and
      works to the front. We will too, in the following example. One other
      important bit of information: Linksys is trying to come up with the second
      and third partitions as close to 128MB as possible: 256,000 blocks to be
      exact. Whatever is left over becomes the first (the data) partition. This
      approach works well for large disks, but let's take a look at what happens
      for a 256MB device:

      So, Linksys fetches the number of blocks per cylinder from the device (you
      can read this from the output of "fdisk -l /dev/sda"). For my Cruzer, it's
      496 blocks / cylinder. Doing the division (256,000 blocks / 496 blocks per
      cylinder) yields (rounded) 516 cylinders that together can make up the
      desired 128MB partition size.

      Since the device contains 1011 cylinders, that means that partition #3
      starts at cylinder number 495, and runs for 516 cylinders, ending at
      cylinder 1011.

      Ok. Now the second partition: going back a further 516 cylinders from the
      start of the third cylinder says that partition #2 must start at
      cylinder -21 -- and run for 516 cylinders, ending at cylinder 495.

      Not looked too good, huh?

      But, it's not over yet -- we still need to allocate the left over space (-21
      cylinders) to the first partition. Since we all know that the first
      partition must start at cylinder 1 (cylinder 0 is used for the label), the
      first partition must start at cylinder 1, and run for -21 cylinders.

      In defense of Linksys, they *do* have code in the kernel that identifies
      devices smaller than 10GB as "flash" devices, and Linksys' code will not
      attempt to format them. So there really isn't any reason for their code to
      handle this extreme case any more elegantly than it does -- which is to say
      that since this can't ever happen, Linksys doesn't check for it, and the
      hacked-up Linksys fdisk utility actually attempts to write these bogus
      values to the partition table.

      Ouch. It's no wonder some of the flash devices get "messed up" -- what ends
      up written is enough to choke any OS that tries to make sense of the
      partition table.


      256MB flash devices cannot be natively formatted by Unslung, at least by
      Unslung 6.8.

      [if there are those who would dispute these findings, please let us know.
      in particular, the output of "fdisk -l /dev/sd<n>" would be very helpful in
      trying to figure out how it might be that you can get it running. the only
      scenario where I can conceive of this working is in the extreme case of very
      small numbers of cylinders -- for example if the device claims to have only
      3 cylinders, rounding might create a situation where the format actually
      comes up with non-negative numbers. if you have such a system, please
      confirm by sending along your information.]

      Mike (mwester)

      PS: Since I now have a brand new and utterly useless 256MB memory device
      lying on my desk, I shall be attempting to hack a means to override the
      Linksys formatting parameters for small devices, and make this work. As
      soon as I get some code working well, I'll check it in and send an email to
      the nslu2-linux group. Those interested can build the head, and test with
      unslung 6.9-alpha.
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