When I installed XenServer 7.1, I had selected the RAID array during installation, however it does not show up after installation. This is what I did to fix it.

First, find the disk: cat /proc/partitions

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In this case it is md126. You can check its status with cat /proc/mdstat:

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In my case, it did not have a file system, so I did mkfs -t ext4 -L bigdisk /dev/md126

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Find the RAID device ID: ll /dev/disk/by-id:

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Find the host ID in XenCenter:

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Then create the Storage Repository:

xe sr-create content-type=user device-config:device=/dev/disk/by-id/<RAID ID> host-uuid=<HOST ID> name-label="Local Storage RAID" shared=false type=lvm