[CentOS] CentOS 7: software RAID 5 array with 4 disks and no spares?

Wed Feb 18 20:21:17 UTC 2015
Niki Kovacs <info at microlinux.fr>

Le 18/02/2015 09:24, Michael Volz a écrit :
> md127 apparently only uses 81.95GB per disk. Maybe one of the partitions has the wrong size. What's the output of lsblk?

I just spent a few hours experimenting with the CentOS 7 installer in a 
VirtualBox guest with four virtual hard disks. I can now confirm this is 
a very stupid bug in the (very stupid) installer. Or at least one more 
random weirdness. Here goes.

The new installer is organized around mount points, which have to be 
defined first. OK, so I first define my mountpoint /boot, set it to 200 
MB (which is enough), define it to be RAID level 1 across four disks 
with an ext2 filesystem. So far so good.

Next step is similar, swap mountpoint is 2 GB, also RAID level 1 across 
four disks.

Finally, the / (root partition) mountpoint is supposed to take up the 
full amount of remaining disk space. In my virtual guest, I defined 4 X 
40 GB to fiddle with. The installer shows me something like 38.6 GB, 
which looks like the remaining space on each disk's partition.

Now I define RAID level 5 across four disks...

... and here it comes.

Once RAID level 5 is defined, I have to REDEFINE the maximum disk space 
by putting in a random large number, for example 4 X 40 GB = 160 GB. 
Because what is meant here is THE TOTAL RESULTING AMOUNT OF DISK SPACE 
IN THE RAID 5 ARRAY, AND NOT THE MAXIMUM SIZE OF A DISK PARTITION. So 
once I fill that field with 160 GB, the installer "automagically" sets 
it to 106.8 GB, which is in effect the maximum available disk space 
using RAID 5.

Usability anyone?

Cheers from the sunny South of France,

Niki Kovacs
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