[linux-lvm] Re: LVM2 on RAID

[Måns Rullgård]

cpurn at cpurn.net (Christian Purnomo) writes:

> Thanks,
>
> I have tried changing the partitions type to 0xFD and the auto
> detects kicked in and LVM still worked like a charm.
>
> However I found this from mailling list archive
>
> /* start quote */
>
> From: Luca Berra <bluca comedia it> 
> To: linux-lvm redhat com 
> Subject: Re: [linux-lvm] Re: putting lvm autodetect into the kernel ala md 
> Date: Fri, 1 Apr 2005 19:41:22 +0200 
>
> quote Neil Brown back in july 2001
>
> "autorun/autodetect just doesn't belong in the kernel.  It should be
> done in user space.  The only time the kernel should assemble a raid
> array itself is for the root device, and this is best done with
> md=0,/dev/whatever,etc
>
> If I could start with a clean slate, I would rip out the autodetect
> stuff completely.  But lots of people are depending on it so I cannot."
>
> anyway the major issues are:
> 1) raid autodetect uses only the minor number stored in the superblock
> to identify the array components, move a disk from a different machine
> to yours and reboot it to enjoy the show

Is this really true?  My kernel log has messages like these:

md: Autodetecting RAID arrays.
md: autorun ...
md: considering sdb3 ...
md:  adding sdb3 ...
md: sdb1 has different UUID to sdb3
md:  adding sda3 ...
md: sda1 has different UUID to sdb3
md: created md2

If it uses the UUID, there should be little risk of misdetection.

A problem I have run into with LVM, was disks from different machines,
using the same VG names.

> 2) raid autodetect will try to start everything it finds, it will not
> scale with shared storage, or other complex configurations
>
> /* end quote */
>
> Can anyone conclude that passing the parameter md=0,/dev/* to kernel
> parameter is a better approach than using auto detect? I don't have
> a problem with having a rather long parameters in my
> grub.conf/lilo.conf.

As usual, both ways have their own advantages and disadvantages.  What
I like about autodetection, is that things just work, even if I
shuffle my disks for some reason.  

-- 
Måns Rullgård
mru at inprovide.com