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SAS RAID and SATA drives assigned wrong address  ie. /dev/sda

Posted on 2008-10-17
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Last Modified: 2012-06-21
I am installing CentOS 5.2 on a Dell Power Edge 2950 with a SAS 6/ir controller (not a PERC card.)
I have 3 hard dives.  I wish to mirror the  first two drives and leave the third un-mirrored.  The OS will be on the mirror.

I can do all of the above fine, but CentOS is detecting the stand alone drive before the mirrored array.

Example:

Configuration with just the first two drives installed and morred:

mirror = /dev/sda


configuration with all three drives installed:

third drive - /dev/sda
mirror = /dev/sdb


Can anybody help me configure CentOS to assign the mirror before the stand-alone drive?
I called Dell support, thinking I was somehow missing an option in the SAS controller, but they weren't much help in solving the issue.  They recommending buying another drive or creating  a software mirror.
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Question by:kelntaylor
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8 Comments
 
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Expert Comment

by:Duncan Roe
ID: 22747627
Look at the address jumpers on each disk unit (i.e. the mirrored ones and the standalone). What do you see? How have you told the system which 2 disks to mirror?
Basically I think you want the standalone drive to have the lowest SCSI id, then the 2 mirrored disks. But somehow you have to configure that sdb mirrors sdc
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by:kelntaylor
ID: 22748221
Duncan Roe,


All three drives are SATA drives, and have no jumpers for SCSI ID, I can configure the mirror in the bios of the SAS controller.  
I think you telling me to mirror the second and third drive instead of the first and second drive?  That would give expected behavior for now, but not when I add another disk in the future.

After updating the firmware and drivers for the controller, I noticed in this in Dell README.txt From Dell's driver readme:
5.3 SLES10/RHEL5 disk enumeration
---------------------------------
Both SLES10 and RHEL5 mptlinux device drivers enumerate unconfigured disks
ahead of virtual disks regardless of slot ordering. Additionally, under certain
circumstances virtual disks can be presented in reverse order of scan order.
While installing your operating system please pay close attention to which disk
is selected by the installeir as the target of your installation and which disk
is selected as the disk to install the boot loader and MBR onto.

So there may not be any hope. :(
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by:kelntaylor
ID: 22748456
After more research it is looking like I shouldn't be worried about what order my hard drives are detected and enumerated.  As long as I use partition labels, it shouldn't matter. hmm.
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Expert Comment

by:Duncan Roe
ID: 22750642
I'm not sure how that will go with booting either - it will be fine for you fstab though. You can configure LILO to boot off a RAID1 device (e.g. /dev/md0) - would that be any help?
LILO from 22.6 on can accept UUID= or LABEL= for the root partition, but not for boot = (current LILO is 22.8)
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Expert Comment

by:Duncan Roe
ID: 22751504
Please post what you did to get it working, especially as regards the boot configuration
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kelntaylor earned 0 total points
ID: 22751942
I didn't do much to get it working, other than get a better understanding of how it works.  The BIOS has no problem booting off my hardware raid device.  (which is not is not /dev/md0) the OS does not not know about the mirrored drives,  it only sees  them as one drive.  CentOS installed fine, as long as I was able to tell it which "drive" to install to.   I've never had a problem booting, I was more concerned with how things would misbehave when I installed more drives in the future.  I can see now that as long as I label partitions and don't reference device id's  (ie /dev/sda1) in /etc/fstab I shouldn't have any issues installing more drives.

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by:Duncan Roe
ID: 22753634
Ok - guess I was on the wrong track - go ahead and close
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