Re: advanced partition format, mdadm, and raw disks

[NeilBrown <neilb@suse.de>]

On Mon, 04 Jul 2011 17:15:33 +0100 "Tyler J. Wagner" <tyler@tolaris.com>
wrote:

> Everyone,
> 
> I've created a RAID-10 array with mdadm. I created the array using
> partitions /dev/sd[abcd]1, where the partition starts at sector 2048 and
> uses the rest of the disk, as specified here:
> 
> http://www.ibm.com/developerworks/linux/library/l-4kb-sector-disks/index.html#tools
> 
> IE:
> 1. fdisk -c -u /dev/sda
> 2. create partition at 2048 to -0, of type fd
> 3. repeat for all drives or clone with sfdisk
> 4. create the array
> 
> mdadm -A /dev/md0 -v --raid-devices=4
> --level=raid10 /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1
> 
> 5. add to mdadm.conf
> 
> mdadm -Es >> /etc/mdadm/mdadm.conf
> 
> After reboot, the array worked, and I formatted it and started loading
> it with files. Only later did I realise that at some point (perhaps 2-3
> reboots), the partitions were gone, and the array is running on the raw
> devices:

It must has stopped using partitions before you ran 'mkfs' which then
destroyed the partition tables.  Probably on the first reboot.

> 
> root@venkman:/var/log/cacti# cat /proc/mdstat 
> Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5]
> [raid4] [raid10] 
> md0 : active raid10 sdc[2] sda[0] sdd[3] sdb[1]
>       3907026944 blocks 64K chunks 2 near-copies [4/4] [UUUU]

No "super" is mentioned here so it must be using 0.90 metadata.


>       
> unused devices: <none>
> 
> fdisk shows the partition tables are gone too. I found NeilBrown's
> comment ID #1857419, partway down this page:
> 
> http://www.issociate.de/board/post/463176/Superblocks.html
> 
> "If a partition starts a multiple of 64K from the start of the device,
> and ends with about 64K of the end of the device, then a superblock on
> the partition will also look like a superblock on the whole device.
> This is one of the shortcomings of v0.90 superblocks. v1.0 doesn't
> have this problem."
> 
> I used mdadm v2.6.7.1, as it comes on Ubuntu 10.10. Is this still a
> shortcoming of superblock v1.2? Obviously I want to align to the sector
> size, and use the entire disk.
> 

mdadm defaults to 0.90 until 3.0 when it starts defaulting to 1.2.


> Should I just accept this and not raw devices? Will it happen again if I
> try to start over?

I'm not really sure what "this" and "it" refer to.  If it is working then
there is no immediate need to change anything.
If you want to make a change, be sure to back up your files.
If you create an array again, probably use "--metadata=1.2".

NeilBrown



> 
> Regards,
> Tyler
>