David A. Roth wrote:
Hi Alan.
Thanks very much for your kind and helpful reply. I wasn't aware of
this mailing list you mentioned, thanks. Please see my comments
below.
On Thu, May 21, 2009 at 3:46 AM, Alan Jenkins
<alan-jenkins@xxxxxxxxxxxxxx> wrote:
David A. Roth wrote:
Hello Alan.
I hope I have the right Alan Jenkins. I've seen your name while
Googling regarding udev. If you aren't the right person, please
forgive the error.
Well, you're not very specific about who you're looking for :-). I'm not
the maintainer, if that's what you mean.
You certainly have the wrong email address. I've CC'd linux-hotplug, which
is the mailing list for Udev.
No, I saw a bunch of postings of yours with udev in them, figured
you'd know more than most. :-)
I have some udev questions and was wondering if you could shed any
light on the problems I'm having. Here is a posting of mine to the
CentOS forum concerning udev, udevinfo and udevtest.
Thanks in advance,
David A. Roth
davidalanroth@xxxxxxxxx
Background:
I have two external USB drives (Maxtor and LaCie), and I want them to
be assigned the same device at boot. /dev/sda for the Maxtor and
/dev/sdb for the LaCie.
With a default rules installation of CentOS 5.3 the LaCie gets
assigned /dev/sda and the Maxtor /dev/sdb, which is the reverse order
of what I want.
That sounds like a really bad idea.
So it was suggested to me that I look into udev and write some rules.
I looked at two web pages for help with udev being:
http://www.axllent.org/docs/data_storage/udev_and_external_devices
http://reactivated.net/writing_udev_rules.html
I added the file 10-local.rules to /etc/udev/rules.d with these two lines:
KERNEL=="sd*",SYSFS{manufacturer}=="Maxtor",NAME:="sda%n",SYMLINK+="usbmaxtor%n"
KERNEL=="sd*",SYSFS{manufacturer}=="LaCie",NAME:="sdb%n",SYMLINK+="usblacie%n"
After booting, a 'df' shows that sda and /dev/usbmaxtor is indeed
assigned to the Maxtor.
Problem:
While the 'df' shows that the sda is assigned to the Maxtor
(/dev/usbmaxtor), udevinfo -a -p /sys/block/sda indicated that sda is
the LaCie drive. When doing dmesg, it has a remark that fsck should be
run on sdb3, but sdb only has one partition of sdb1. It's the Maxtor
which has sda3.
Questions:
Is the problem here with my rules statement above? If so, what do I
need to change?
There's no way change the name printed in kernel messages (dmesg) or under
/sys. Udev only controls the name in /dev. (The exception is networking
devices, which don't appear in /dev; I think they can have their kernel
names changed). I hope that clears things up.
If you only use the links, it should be fine. Btw, if you don't mind long
link names, you may find udev already creates useful links under
/dev/disk/by-id/.
I see, wow, that's great! I wasn't aware of that.
I constructed the above from examples, but wasn't able to find
documentation that goes into detail. For example, what is the
different between NAME:= vs. NAME= ?
I think you're right that udev isn't comprehensively documented. But I this
particular one is covered in the manual page ("man udev"). ":=" assignment
is final, and cannot be overridden by an assignment in a subsequent rule.
Thanks again, Alan.
After reviewing your reply, I believe this makes more sense to me. I
shouldn't care if the drive is sda or sdb, since I should be
referencing the symlink? In the past, I am use to doing backups and
with rsync, for example and would mount /dev/sda3 to /mnt/maxtor, for
example. After I adding the second USB external drive, sda was no
longer the Maxtor drive but the Lacie. So I felt I needed to force the
maxtor to always be sda in order for my scripts to work. My real goal,
is that the correct drive and partition is being reference to it's
ultimate mount point after boot time regardless of which drive is
assigned to sda or sdb.
Sounds like I should remove NAME:= altogether from the rules here so I
shouldn't be forcing sda or sdb on the drives?
Yes, remove NAME:= and use symlinks.
I mentioned "by-id" since that was closest to what you were trying.
This is not word of god, but I would recommend trying "by-uuid" instead.
Modern distributions use links under "/dev/disk/by-uuid", which are
generated based on filesystem serial numbers. This has the advantage of
not hard-coding the partition number (or the device). The filesystem
UUID will remain stable even if you resize it or re-order the partition
table for some reason. You can generally expect UUIDs to be unique, so
long as you aren't copying raw parition images around.
You can use "ls -l /dev/disk/by-uuid" to see which UUID link corresponds
to the desired partition.
Thanks in advance for allow me pester you. I will check out the
mailing list you mentioned as well.
Best regards,
David
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