LVM SCSI dev "Dance"

[commonloon]

I've got a couple of usb external drives that I am trying to use with LVM. My problem is that I also have 2 raid arrays on this same machine. In old school terms I have /dev/sda,b,c,d,.... When I unplug a drive and then plug it back in the kernel assigns it the next device (say /dev/sde). This is compounded by the fact that if the machine should be rebooted that the devices names are all different again.

I found out the hard way the LVM has issue with devfs, i.e., the open issue with too many symlinks.

My question is there a way to trick the kernel/fs into using the same scsi device names somehow, for lack of a better way of describing it. LVM seems ok with the dev names changing, so I guess I could write a little bash script to figure out what drive is plugged in where, but it just doesn't seem like a good solution.

Any suggestions greatly appreciated.

[garlicbread]

One solution would be to look into using udev
instead of devfs (which has to be disabled for udev to work)

the new 2.6 kernel now creates a block device within several different places within the /sys directory (virtual filesystem similar to /proc)
the idea is, is that when something is removed or added, the device node is created within /sys and a userspace tool called udev is called to create a symlink from /sys to /dev automaticaly

the advantage of this is that you can specify in the udev config file how you want it to respond to devices appearing within /sys
since the device appears in several different places within /sys it's possible for udev to look it up in different ways

One example of this is that /sys can give a unique ID for the device just connected based on the chipset and other features of the hardware
If udev recognises this unique ID in it's config file, it can create a symlink with a specific name

Disadvantages are
It can be difficult to setup at the moment
not all hardware is supported to appear under /sys yet
I think you'll need to unmask some of the more experimental ebuilds relating to the startup scripts for udev to work

see here http://www.linuxjournal.com/article.php?sid=7316

in order to turn off devfs for udev to work
I believe there is a kernel parameter you can pass at boot time
something like devfs=nomount or something (check the Kernel Documentation)

Also one thing to be aware of, I think when emerging udev
it will automaticly look at all the device nodes located within /dev that have been setup by devfs that your using at the moment
it'l then copy these device nodes into a compressed archive as a backup
and upon bootup extract the archive into /dev (the same as using mknod)
I think this is just a temporary fix to the problem of udev not supporting all the hardware yet

[commonloon]

Thanks I'll check it out.