Create mdraid with multipath disks in in RHEL7:

I am still novice in the world of storage. I am working on a project of migrating our backups infra from Solaris to Linux.

As part of that, I have rebuilt a server to Linux (RHEL7) and it has 2 root disks (300GB each) and 30 disks (of 3TB each) from 2 Xyratex shelves (24+6). One of the 2 Xyratex shelves (the one with 24 disks) has a multipath configured.

And as Linux kernel detects each shared drive once through each path, fdisk -l output is listing 102 3TB disks (instead of actual 30).

$ fdisk -l | grep '3000.6 GB' | wc -l
102

Out of which:

48 are multipath devices.

$ fdisk -l | grep '/dev/mapper/mpath*' | grep -v '3000.6' | wc -l
48

One entry for example is:

Disk /dev/mapper/mpathb: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors

And 30 are the actual disks (24+6):

Disk /dev/sdaa: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdab: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdac: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdad: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdaf: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdae: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdai: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdah: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdam: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdal: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdao: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdan: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdaj: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdav: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdaw: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdax: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdaz: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sday: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdba: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdbc: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdbb: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdaq: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdbd: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdap: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdag: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdau: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdas: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdar: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdak: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdat: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors

And 24 are redundant devices:

Disk /dev/sde: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdd: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdf: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdg: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdh: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdk: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdl: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdc: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdo: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdi: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdm: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sds: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdp: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdu: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdw: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdq: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdr: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdn: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdt: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdv: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdy: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdx: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdz: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors
Disk /dev/sdj: 3000.6 GB, 3000592982016 bytes, 5860533168 sectors

Now, I am confused what’s the right way to configure the RAID.

I was planning to have below RAID configuration (reserve 6 disks as spares):

• Create 3 RAID 6 groups with 8 disks each (6 disks and 2 spares).
• 1 RAID 0 on top of the three RAID 6s - so basically there will all look as one unit with RAID 0.
• Then create PV on top of the RAID 0 and then use LVM.

I was reading about having multipath in mdadm but it’s little unclear to me on how I should proceed further. Any thoughts on this?