Source: Easy to expand Linux software RAID 5 with XFS. Best practices?

Created a RAID5 array:

mdadm --create --verbose /dev/md0 --level=5 --raid-devices=3 /dev/sdb1 /dev/sdc1 /dev/sdd1

Now the RAID5 array is created, and it is being built already. It takes time, but you can proceed with creating a new physical LVM2 volume:

pvcreate /dev/md0

Now let’s create a new volume group:

vgcreate vd_raid /dev/md0

Then we need to create a new logical volume inside that volume group. First we need to figure out the exact size of the created volume group:

vgdisplay vd_raid

The size can be seen from the row which indicates the “Total PE” in physical extents. Let’s imagine it is 509. Now create a new logical volume which takes all available space:

lvcreate -l 509 vd_raid -n lv_raid

Finally we can create a file system on top of that logical volume:

mkfs.xfs /dev/mapper/vd_raid-lv_raid

To be able to use our newly created RAID array, we need to create a directory and mount it:

mkdir -p /mnt/raid

mount /dev/mapper/vd_raid-lv_raid /mnt/raid

Now it is ready to use. But for it to automatically mount after reboot, we need to save RAID geometry to mdadm’s configuration file:

mdadm --detail --scan >> /etc/mdadm/mdadm.conf

Then add the following line to /etc/fstab which mounts the RAID array automatically:

/dev/mapper/vd_raid-lv_raid /mnt/raid auto auto,noatime,nodiratime,logbufs=8 0 1

Now the RAID array is ready to use, and mounted automatically to /raid directory after every boot.

Let’s imagine that now you have a new drive, /dev/sde, which you want to add to the previously created array without losing any data.

Add to the RAID array:

mdadm --add /dev/md0 /dev/sde1

Now the RAID5 array includes four drives, which only three are in use currently. The array needs to be expanded to include all four drives:

mdadm --grow /dev/md0 --raid-devices=4

Then the physical LVM2 volume needs to be expanded:

pvresize /dev/md0

Now the physical volume is resized by default to cover all available space in the RAID array. We need to find out the new size in physical extents:

vgdisplay vd_raid

Let’s imagine that the new size is now 764 (can be seen from “Total PE”). Now expand the logical volume to cover this:

lvextend /dev/mapper/vd_raid-lv_raid -l 764

Then expand the XFS file system. This needs to be done during the file system is online and mounted:

xfs_growfs /mnt/raid

By default it is expanded to cover all available space. Finally the RAID array geometry needs to be updated because the array now includes a new disk. First delete the added line from /etc/mdadm/mdadm.conf and then add a new one:

mdadm --detail --scan >> /etc/mdadm/mdadm.conf