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Re: disk moves from /dev/sdd to /dev/sde

Wolfgang S. Rupprecht wrote:
I'm seeing something strange where a disk appears to change from
/dev/sdd to /dev/sde under f12.  I have a motherboard (Asus M3A78T) that
appears to have multiple onboard disk controllers.  When I boot with no
external storage plugged into the USB, my hard disks are assigned sda,
sdb, sdc sdd.  When I boot with, say, a flash drive, camera or cell
phone attached the external device gets the "sdd" name and my last disk
gets the name sde.  Now, that in itself doesn't cause any problems
because I don't have the disk sdX names wired into anything.

What is a problem is that after booting, something unknown (perhaps an
ATA reset?) causes the disk letters to be re-assigned just as if it was
at boot time.  If I have some flash-like external storage plugged in my
last disk gets shifted to /dev/sde.  At that point programs like
smartmon that are looking at the disk under the old name fail to find it
and generate an error.  "smartmon -a /dev/sde" does show the disk under
it's new name, but even the kernel appears to look for the disk under
its old name. I see lots of the following mailed to me by chron:

    /dev/dm-0: read failed after 0 of 4096 at 0: Input/output error
    /dev/dm-0: read failed after 0 of 4096 at 0: Input/output error

How do I nail down the disk numbering a bit tighter so that things don't
move around after boot-time?

This is a new one on me,m as I've never seen a device name change except a boot time. Obviously when a new device comes on line, USB or similar, it will get a name, but I've never seen the name change on a continuously connected device.

Therefore, it would be worth investigating carefully to see how and when this happens. Might I suggest running a script every ten minutes or so, so you can see what changes, and look at /var/log/messages for the time when the change happened.

Maybe something like this, so you are sure which device is which:
#   blkdevtrc 1.1 2009-12-20 12:53:07-05 root Exp
# trace which devices are connected to which names
cd /sys/block || exit 1

# NOTE: works for real devices, not member of fakeraid
# controllers.

# Typical values for this define are "Serial" or "Capacity"

echo "block device trace for host $(hostname) $(date +%Y%m%d-%H%M)"

for dev in sd*; do
  echo "${SepLine}"
  echo "Device name /dev/${dev}"
  echo -n "Device connection: "
  cat ${dev}/dev; echo
  smartctl --all /dev/$dev | sed -n "1,/INFORMATION/d;1,/${LastLine}/p"

I swiped tis from my tool kit, later versions are more complex, but this will show when a device moves, and where.

Bill Davidsen <davidsen tmr com>
  "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot

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