I run a RAID10 utilizing 4x 4TB disks, created with mdadm and without a dedicated raid controller (four hard drives plugged right into the server's motherboard). When it was originally set up it utilized disks by partition numbers - /dev/sda1, /dev/sdb1, etc. Well I had a disk fail (/dev/sdd) and just swapped it out for a fresh one, and when I added it to the array I accidentally added it as /dev/sdd instead of /dev/sdd1. It's currently rebuilding and will be for a few more hours, but it doesn't seem to have caused any problems yet.

So it got me wondering, for a basic RAID without any particular special configuration, what's the difference between a raid comprised of partitions (one partition per disk) and a raid of the devices themselves? And while I'm on the topic, will my new RAID disk cause any trouble since it was added as a device instead of as a partition?

[root@fluttershy ~]# mdadm --detail /dev/md0
        Version : 1.1
  Creation Time : Wed Feb 19 05:43:49 2014
     Raid Level : raid10
     Array Size : 7813770240 (7451.79 GiB 8001.30 GB)
  Used Dev Size : 3906885120 (3725.90 GiB 4000.65 GB)
   Raid Devices : 4
  Total Devices : 4
    Persistence : Superblock is persistent

  Intent Bitmap : Internal

    Update Time : Sun Nov  8 20:32:28 2015
          State : clean, degraded, recovering 
 Active Devices : 3
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 1

         Layout : near=2
     Chunk Size : 512K

 Rebuild Status : 3% complete

           Name : fluttershy:0  (local to host fluttershy)
           UUID : 45f8673c:332503c8:a9307bae:4177f753
         Events : 115588

    Number   Major   Minor   RaidDevice State
       0       8        1        0      active sync set-A   /dev/sda1
       1       8       17        1      active sync set-B   /dev/sdb1
       2       8       33        2      active sync set-A   /dev/sdc1
       4       8       48        3      spare rebuilding   /dev/sdd
[root@fluttershy ~]# 

The difference is that one has a partition table on the disk, and the other doesn't. There's no functional difference. Having one drive as a whole disk and the others as partitions will cause no problems.

  • Thanks for the answer. I was thinking about removing and readding the other disks one at a time so they all match up. Would there be any benefit to that besides uniformity? – Kefka Nov 9 '15 at 15:58
  • You could get a partition table's worth of extra space (probably 1MB), but apart from that, there's no benefit other than uniformity. – womble Nov 9 '15 at 21:01

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