How can I set RAID 0 using Ubuntu Server 12.04

Question

I have a server which installed 2 HDD (500 GB each).

I would like to install Ubuntu Server 12.04 on it but when I partition my disks like :

"Configure software RAID" > YES > "Create MD device" > RAID0 > and check both "/dev/sda1" and "/dev/sdb1" > continue > Finish. Now I got

RAID0 device #0 - 1.0TB Software RAID device
#1 1.0 TB
512.0 B unusale
SCSI1 (0,0,0) (sda) - 500.1 GB ATA MB0500EBNCR
#1 primary 500.1 GB K raid
SCSI1 (0,0,0) (sdb) - 500.1 GB ATA MB0500EBZQA
#1 primary 500.1 GB K raid

Then I select "Finish partitioning and write changes to disk".
After that, I got an error message

No root file system
No root file system is defined.
Please correct this from the partitioning menu

Could you please tell me, how can I solve this problem?

Thank you for every answer.

PS. This Machine will be used as File Server

Answer 1

Did you configure the RAID before you installed anything to /dev/sda?

If you did not then sda is already in use and you can not convert it to part of a RAID array. In the case of RAID 0 (striping) you need all disks available for the array to work. If one is in use it will fail.

This means you will have to create the stripe (RAID 0 array) before you install anything to it. In other words, boot from a liveCD (or pendrive), configure the RAID (e.g. to add both disks completely to an array: mdadm --create /dev/md0 --level=linear --raid-devices=2 /dev/sda /dev/sdb).

I strongly recommend not to do this though. Use partitions instead and only create a stripe for the data.

Example

disk 0  | sda1: boot | sda2: root     | sda3: part of md0 |
disk 1  | sdb1: boot | sdb2: root bkp | sdb3: part of md0 |

This way you you recover to a working system if the array fails.

The commands you want for this is:
mdadm --create /dev/md0 --level=linear --raid-devices=2 /dev/sda3 /dev/sdb3
Format as a regular drive after the array is created. Optionally add --verbose

I hope you plan on having backups. RAID 0 (striping) is good when you need speed at all costs, but one of the costs is redundancy. If either of the two drives fails then you will loose all data from that array.