====== Easy to expand Linux software RAID 5 with XFS. ======

Source: [[https://serverfault.com/questions/114445/easy-to-expand-linux-software-raid-5-with-xfs-best-practices|Easy to expand Linux software RAID 5 with XFS. Best practices?]]

===== Creating the initial 3 drive array =====

Created a RAID5 array:

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

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:

<code>pvcreate /dev/md0</code>

Now let's create a new volume group:

<code>vgcreate vd_raid /dev/md0</code>

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:

<code>vgdisplay vd_raid</code>

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:

<code>lvcreate -l 509 vd_raid -n lv_raid</code>

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

<code>mkfs.xfs /dev/mapper/vd_raid-lv_raid</code>

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

<code>mkdir -p /mnt/raid</code>
<code>mount /dev/mapper/vd_raid-lv_raid /mnt/raid</code>

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:

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

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

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

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


===== Adding a new drive to the array =====

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:

<code>mdadm --add /dev/md0 /dev/sde1</code>

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:

<code>mdadm --grow /dev/md0 --raid-devices=4</code>

Then the physical LVM2 volume needs to be expanded:

<code>pvresize /dev/md0</code>

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:

<code>vgdisplay vd_raid</code>

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

<code>lvextend /dev/mapper/vd_raid-lv_raid -l 764</code>

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

<code>xfs_growfs /mnt/raid</code>

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:

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