Applies to SUSE Linux Enterprise Server 15 SP3

10 Creating a degraded RAID array #

A degraded array is one in which some devices are missing. Degraded arrays are supported only for RAID 1, RAID 4, RAID 5, and RAID 6. These RAID types are designed to withstand some missing devices as part of their fault-tolerance features. Typically, degraded arrays occur when a device fails. It is possible to create a degraded array on purpose.

RAID Type	Allowable Number of Slots Missing
RAID 1	All but one device
RAID 4	One slot
RAID 5	One slot
RAID 6	One or two slots

To create a degraded array in which some devices are missing, simply give the word missing in place of a device name. This causes mdadm to leave the corresponding slot in the array empty.

When creating a RAID 5 array, mdadm automatically creates a degraded array with an extra spare drive. This is because building the spare into a degraded array is generally faster than resynchronizing the parity on a non-degraded, but not clean, array. You can override this feature with the --force option.

Creating a degraded array might be useful if you want create a RAID, but one of the devices you want to use already has data on it. In that case, you create a degraded array with other devices, copy data from the in-use device to the RAID that is running in degraded mode, add the device into the RAID, then wait while the RAID is rebuilt so that the data is now across all devices. An example of this process is given in the following procedure:

To create a degraded RAID 1 device /dev/md0, using one single drive /dev/sd1, enter the following at the command prompt:
```
> sudo mdadm --create /dev/md0 -l 1 -n 2 /dev/sda1 missing
```
The device should be the same size or larger than the device you plan to add to it.
If the device you want to add to the mirror contains data that you want to move to the RAID array, copy it now to the RAID array while it is running in degraded mode.
Add the device you copied the data from to the mirror. For example, to add /dev/sdb1 to the RAID, enter the following at the command prompt:
```
> sudo mdadm /dev/md0 -a /dev/sdb1
```
You can add only one device at a time. You must wait for the kernel to build the mirror and bring it fully online before you add another mirror.
Monitor the build progress by entering the following at the command prompt:
```
> sudo cat /proc/mdstat
```
To see the rebuild progress while being refreshed every second, enter
```
> sudo watch -n 1 cat /proc/mdstat
```