Linux – How to make a three disk software RAID by adding the third disk

centos-7hard drivelinuxsoftware-raid

The server is running CentOS and it has been successfully running a two disk software RAID. I wish to add a third disk to this for redundancy, so when one disk fails, the system still has two disks operating until a replacement for the failed disk arrives.

When installing CentOS from a DVD-ROM, that's where I have set-up the software two disk RAID in the past. But how do you add a third disk to the existing two disk software RAID in a running system?

Best Answer

2 disks and redundancy suggest RAID1 is already used. To increase redundancy I do not think RAID5 is an option as it offers more space but - with 3 disks it is the same as RAID1 with 2 and allows losing one disk.

So presuming RAID1:

mdadm --add /dev/mdX /dev/sdY
mdadm --grow --raid-devices=3 /dev/mdX

Using lvm2 (either mirror or dm-raid) it would be:

vgextend VG /dev/sdY
lvconvert -m2 VG/LV

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Linux – How to associate linux scsi device names with disk devices

You can look in /sys/block/sda/device, which will have a symlink starting with scsi_device that identifies the device:



$ ls -d /sys/block/sda/device/scsi_device*
/sys/block/sda/device/scsi_device:0:0:0:0


Or you can look in /sys/bus/scsi/devices:


$ ls /sys/bus/scsi/devices/
0:0:0:0  1:0:0:0  2:0:0:0  3:0:0:0  7:0:0:0  8:0:0:0


Each entry here represents a SCSI device.  Inside each directory is a symlink starting with block: that identifies the corresponding block device:

$ ls -d /sys/bus/scsi/devices/0\:0\:0\:0/block*
/sys/bus/scsi/devices/0:0:0:0/block:sda

Several questions about software and onboard RAID 5

Is there any way to add a disk to an existing RAID 5 array using either software or onboard RAID?

Depends on the OS version and onboard RAID vendor/version. For example, onboard RAID using Intel Rapid Storage Technology (any recent Intel chipset uses it), you can even convert RAID1 to RAID5 online. You can also add a disk to an existing RAID5 online too.

All motherboards I'm interested in come with both SATA2 and SATA3 ports. Using either software or onboard RAID, is it possible to combine disks connected to different ports in the same array

Most probably not. Usually, one chipset is RAID-based, and the other is not. For example, my motherboard has Intel ICH10 chipset and Marvell chipset. Intel RAID sees only disks connected to the Intel chipset.

Update: According to Intel, on the PCH controller hub (used in Intel P67), RAID5 with 5 or 6 drives is supported. However, be aware that all RAID features can be disabled by the board manufacturer.

If I have two operating systems installed (on a disk that does not belong to the array), can still use software RAID?

Depends on the OS. You cannot use Windows RAID in Linux and vice versa, but if these are two installations of Windows, why not?

Best Answer

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Linux – How to associate linux scsi device names with disk devices

Several questions about software and onboard RAID 5

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