I am setting up a new set of drives on a CentOS 7 installation (10x1TB), however am seeing something annoying during the build process.When I attempt to build the array, it is immediately recognised but indicates an error state (degraded, one disk missing.)The full build command was:mdadm --create /dev/md1 --level=5 --raid-devices=9 /dev/sde1 /dev/sdg1 /dev/sdc1 /dev/sdd1 /dev/sdf1 /dev/sdl1 /dev/sdj1 /dev/sdi1 /dev/sdm1 --spare-devices=1 /dev/sdk1However I then tried the same without the spare drive:mdadm --create /dev/md1 --level=5 --raid-devices=9 /dev/sde1 /dev/sdg1 /dev/sdc1 /dev/sdd1 /dev/sdf1 /dev/sdl1 /dev/sdj1 /dev/sdi1 /dev/sdm1then variants of a three drive array:mdadm --create /dev/md1 --level=5 --raid-devices=3 /dev/sdj1 /dev/sdi1 /dev/sdm1mdadm --create /dev/md1 --level=5 --raid-devices=3 /dev/sde1 /dev/sdg1 /dev/sdc1In all cases, the new array initialises with the expected free size (8TB and 2TB in the variants.) The array is also useable during the rebuild, I can read and write files.Curiously, when building with the spare, the spare drive shows significant read/write activity, implying it is somehow participating in the build.Basically, is this normal and if not, what am I doing wrong?Edit:This is the output of: mdadm --detail /dev/md1/dev/md1:        Version : 1.2  Creation Time : Sun Jan  4 00:00:22 2015     Raid Level : raid5     Array Size : 7813042176 (7451.10 GiB 8000.56 GB)  Used Dev Size : 976630272 (931.39 GiB 1000.07 GB)   Raid Devices : 9  Total Devices : 10    Persistence : Superblock is persistent    Update Time : Sun Jan  4 00:05:33 2015          State : clean, degraded, recovering  Active Devices : 8Working Devices : 10 Failed Devices : 0  Spare Devices : 2         Layout : left-symmetric     Chunk Size : 512K Rebuild Status : 0% complete           Name : localhost.localdomain:1  (local to host localhost.localdomain)           UUID : f2d4f470:9d5022bf:a397b5a0:45b94d28         Events : 72    Number   Major   Minor   RaidDevice State       0       8       65        0      active sync   /dev/sde1       1       8       97        1      active sync   /dev/sdg1       2       8       33        2      active sync   /dev/sdc1       3       8       49        3      active sync   /dev/sdd1       4       8       81        4      active sync   /dev/sdf1       5       8      177        5      active sync   /dev/sdl1       6       8      145        6      active sync   /dev/sdj1       7       8      129        7      active sync   /dev/sdi1      10       8      193        8      spare rebuilding   /dev/sdm1       9       8      161        -      spare   /dev/sdk1
RAID 5 built using mdadm starts in degraded mode. Is this normal?
centos;software raid
Yes, this is normal.  When you first create the array the parity blocks have to be calculated and written, which is what you are seeing.  If you know that the drives are already filled with zeros, then you can use the --assume-clean switch to direct mdadm to skip the initial resync.  If the drives aren't actually filled with zeros, then performing a parity check on the array in the future will report many errors since you never calculated the correct parity.Also FYI, it is not recommended to create such a large array using raid5 as the probability of a second drive failing before it can rebuild is getting high.  You might want to use raid6 instead.