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Re: Upcomming trouble with Sid kernel and TAO CDs

On Thu, Feb 18, 2016 at 03:43:21PM +0100, Thomas Schmitt wrote:
>how much is debian-cd concerned/interested that CDs written by write type
>TAO stay readable for e.g. blkid ?
>I ask because Archlinux ISOs already do not boot properly from such CDs
>as they rely on /dev/disk/by-label/ links to mount the ISO.
>  https://bugs.archlinux.org/task/48234
>The current amd64 mini.iso would suffer the same fate, if it did rely
>on blkid for booting. I burned it to a CD-RW with option -tao and
>booted it on real iron to rescue mode (without mounted root).
>Same symptoms as with Archlinux:
>- blkid does not work with /dev/sr0. dmesg shows i/o errors,
>  both at the first TAO run-out block.
>- dd reports one i/o error at some random sector near the end,
>  and one i/o error at the first TAO run-out block.
>On Archlinux uname says 4.3.3-3, on Debian it says 4.3.0-1.
>So we are up to reports about strange behavior of some netinst CDs
>as soon as kernel 4.3 hits the users.

Oh, ugh. :-(

>If there is interest i could need help in understanding the block
>device procedures which distinguish 4.3 from 3.16. A few months ago
>i was glad to read in /usr/src/linux-{3.16,4.1.6}/drivers/scsi/sr.c
>a quite mislead comment and remedy, which nevertheless causes nearly
>flawless behavior in 3.16.
>         * The SCSI specification allows for the value
>         * returned by READ CAPACITY to be up to 75 2K
>         * sectors past the last readable block.
>         * Therefore, if we hit a medium error within the
>         * last 75 2K sectors, we decrease the saved size
>         * value.
>It is actually about the last 2 sectors. The number 75 is unrelated
>and probably a schily rumor. (I dare the world to show me that specs part.)
>I also wonder where the SCSI specs promise this info:
>       error_sector = (SCpnt->sense_buffer[3] << 24) |
>                (SCpnt->sense_buffer[4] << 16) |
>                (SCpnt->sense_buffer[5] << 8) |
>                SCpnt->sense_buffer[6];
>which seems to work the miracle of exact read amount in 3.16.
>Is this artificial info from the drivers ?
>Something must have changed with buffering and reading-ahead which
>now devalues this effort. Maybe it's even about the miracle sense data.

Have you mentioned this on the linux kernel list?

Steve McIntyre, Cambridge, UK.                                steve@einval.com
"I can't ever sleep on planes ... call it irrational if you like, but I'm
 afraid I'll miss my stop" -- Vivek Das Mohapatra

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