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I have 4 disks: 2 x 1TB and 2 x 4TB.

In the RHEL 7.4 installation disk partitioning section, I can create a single Volume Group over all 4 disks and set RAID level: RAID 1 (Redundancy)

This theoretically gives me +- 5TB of available space in a single volume group - but I have no idea how the RAID 1 mirror is applied.

To me it makes conceptual sense to have the 2 x 1Tb drives mirror each other and and the 2 x 4Tb drives mirror each other and define a volume group over the mirrored pairs, however there doesn't seem to be any way that I can see to "tell" the RHEL GUI how I want the mirror configured within the volume group.

My question is: Is there something I'm missing withing the GUI, and should I even be concerned about pairing "like" disks; or does it truly make no difference ie. does RHEL 7 handle the RAID aspect transparently within the volume group?

thanks so much!

Pancho
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1 Answers1

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Raid 1 can be applied in a layer between LVM and the disks.

Raid 1 can also be applied to the logical volumes themselves inside LVM without the use of a separate raid layer. This is nice because it would let you make a large raid5 or raid0 LV after installation if you needed a larger amount of space available.

You could even get more usable space that way. 3TB (from the first 1TB of each drive in raid5) + 3TB (from the rest of the 4tb drives in raid1) = 6TB usable, redundant storage. And if you do it inside LVM, you can change it around while you're booted and running.

As far as your situation goes, it depends entirely on how you build it in the installer. Test in a VM with two 1GB and two 4GB virtual disks, and find out.

Billy left SE for Codidact
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  • I can't see a way via the install GUI to make a mirror across the 4 disks giving me 5Tb usable mirrored space. What I've done is create VG "centos" in the GUI of only the 2 x 1Tb drives, which allows me to install the OS. When installed my plan is to manually create a a RAID 1 device from the 4Tb drives and then add the mirrored device to the "centos" VG ...which will hopefully give me a final "centos" VG of 5Gb (all RAID1) ie. doing what I wanted in my question above. Does this sound sensible? Thanks so much – Pancho Oct 23 '17 at 20:41
  • Yes, that should work. Anaconda has never been ideal for setting up advanced partitioning. Many people recommend setting it up manually with parted, mdadm, and lvm from a live cd, then telling anaconda only to use what is already there. – Billy left SE for Codidact Oct 23 '17 at 20:46
  • One last question: am I understanding correctly from your answer that another possible approach would be to create a single VG having no RAID across all 4 disks - and then create 2 equal size LVs from the VG and place them in a RAID 1 configuration? – Pancho Oct 23 '17 at 20:56
  • No. That's not what I meant to convey. – Billy left SE for Codidact Oct 23 '17 at 20:57
  • I said you could put all four disks in one VG and then have ANY NUMBER OF LVs inside that VG. Each of them could have their own raid level. If the highest possible fault tolerance was your goal would use raid6 on my first LV, since you have four disks to work with. It could be 2TB and span the first TB of space on all four disks. Then I'd make a 3TB raid1 (also called a 'mirrored LV' in the remaining space. – Billy left SE for Codidact Oct 23 '17 at 21:00