[linux-lvm] lvm and md

[William Blunn]

Peter Yao <peter at exavio.com.cn> wrote:
> I am using some hot-plugging disks and their device number may change 
> after reboot. For example, if i build md with /dev/sda and /dev/sdb, 
> after reboot md may cannot start because the two disks' device name 
> change to /dev/sdc and /dev/sdd. If i build md on top of lvm this 
> problem is avoided. Can you give me some advice for this issue?

I don't think it would be a good idea to put MD on top of LVM for
reasons of performance. Ideally you want as few layers as possible on
top of the disk.

As an alternative to Luca Berra's suggestion to use mdadm,
it might be possible to do what you want using devfs.

I'm not saying it's a better approach, just a possible alternative. The
best approach will depend on your particular situation.

(The mdadm approach looks like a good technical solution to your
situation. However, for example, you may already have devfs installed,
in which case it might be easier to set up and "good enough" to be going
on with.)

I have never used devfs myself, by from what I can tell it gives logical
names to SCSI devices, which are invariant of devices being added to
or removed from a SCSI bus.

So your HDDs would appear as something like:

  /dev/scsi/host0/bus0/target5/lun0/disc
  /dev/scsi/host0/bus0/target6/lun0/disc

You mention that /dev/sda and /dev/sdb are sometimes displaced to
/dev/sdc and /dev/sdd.

What are the two devices which are displacing /dev/sdc and /dev/sdd ?

Regards,

Bill

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