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yesterday I installed CentOS on my machine.There are two hard disk(SCSI) in my machine,but after installing,I used "fdisk -l"command to check disks only find one disk has been mounted.I also have check out the /dev directory,only find sda.Theorectically,there should exist sda and sdb under /dev.I have found some materials on the internet,maybe I must re-scan my disk,but how to re-scan? Anybody has encountered the same problem before?Or anybody could help me to use the second hard disk?

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  • What type of disk controller are they attached to? (Don't say "SCSI." I mean brand and model.) Does the disk controller show these disks during the text phase of the boot process?
    – Warren Young
    Nov 7, 2014 at 14:31
  • Yes,the boot process has detect both two hard disk.I don't know what type of disk controller are they attached to,where can I find this information?
    – seabiscuitxf
    Nov 7, 2014 at 14:34
  • Follow the disk control cable inside the computer from the disk drive to the board it attaches to. Read the brand and model off the board. If you trace it to the motherboard, this information will be in the motherboard's manual, but you could just give the motherboard brand and model instead.
    – Warren Young
    Nov 7, 2014 at 14:43
  • The machine I used is a server,not a normal computer,so it may not be easy to open the machine and check the motherboard.Can I use some command to show this information?
    – seabiscuitxf
    Nov 7, 2014 at 14:48
  • You need special server screwdrivers, then? :) Well, as you wish. Try dmesg | less.
    – Warren Young
    Nov 7, 2014 at 14:49

2 Answers 2

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You have a RAID controller there, not a bare-disk SCSI controller. If you have it configured for a single RAID, it is going to present just a single SCSI device to the host system.

If your RAID controller is configured as RAID-0, the host OS will see a single SCSI device whose size is equal to twice the size of the smallest disk in the RAID. That is, if you have two 1 TB disks in the RAID, it will appear to be 2 TB to the host OS.

If it's configured as RAID-1 instead, it will appear to be the size of the smallest disk in the array, or 1 TB in our example configuration.

If that's what you want — that is, you want the disks to be in a hardware RAID — just set the RAID up in the BIOS, and format it in Linux as if it were a single disk. If you want to be able to configure the RAID from Linux, simply go to LSI's download page, tell it what kind of disk controller you have, and download the MegaRAID utility, megacli.

If you were hoping to be able to treat this RAID controller as a dumb SCSI disk controller, you're going to have to hack it.

Few LSI SAS RAID controllers support JBOD mode, which is what you'd need to get a RAID controller to export its connected disks individually as raw disks. (JBOD = Just a Bunch of Disks.) The best you can do with that type of disk controller is to create a single-disk RAID for each disk, which the controller will dutifully export as multiple /dev/sd[a-z] devices. This is not the same thing as JBOD, but from the host OS's perspective, it's close enough.

At the very low end of LSI's SAS RAID range, there are a few controllers like the 9201 that will run in so-called IT mode, or integrator mode. You have to reflash their firmware to do this; they will not do it out of the box. Having done so, your RAID controller appear to the host OS as a dumb SCSI controller, so your host OS will see the connected disks individually.

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  • Thank you for your help.I have installed megacli,and used "MegaCli64 -pdlist -aALL"command to display disk information.I found one disk's Firmware state is online,but the other's Firmware state is unconfigured.Is that mean I need add the unconfigured disk to RAID,but how to add?Before yesterday,I have no idea about RAID.Could you give me more details?
    – seabiscuitxf
    Nov 8, 2014 at 5:38
  • @seabiscuitxf: Stack Exchange isn't an open-ended technical support forum. Ask one question, get one answer, accept one answer. If you have another question, ask it separately. RAID card configuration isn't on-topic here anyway.
    – Warren Young
    Nov 8, 2014 at 5:49
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You can try to run the command :

/usr/bin/scsi-rescan

(This tool is provided by the package : sg3_utils)

In order to scan the SCSI bus to see the storage

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  • Running this command doesn't work,I still can not find the other hard disk.Any other way can I try?
    – seabiscuitxf
    Nov 7, 2014 at 14:43

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