How to create software raid 10 with mdadm

RAID 10, also called as RAID 1+0 is a stripe of mirrors. It require  four disks at least. It stripes data across mirrored pairs. So, as long as one disk in each mirrored pair is functional, data can be retrieved. If two disks in the same mirrored pair fail, all data will be lost, because there is no parity.

Raid 10 provides redundancy and performance despite of 50% capacity of disks.
Note on why to use different manufacturers disks: Disks will fail, this is not a matter of a “if” but a “when”. Disks of the same manufacturer and the same model have similar properties, and so, higher chances of failing together under the same conditions and time of use. The suggestion so is to use disks from different manufacturers, different models and, in special, that do not belong to the same batch (consider buying from different stores if you are buying disks of the same manufacturer and model). This is not uncommon that a second disk fail happen during a resotre after a disk replacement when disks of the same batch are used. You certainly don’t want this to happen to you.
So we have four disk fo this: /dev/sdc, /dev/sdd, /dev/sde, /dev/sdf. At first, we check, if there is any previous md superblock. So we examine this disks:

 mdadm -E /dev/sd[c-f]
/dev/sdc:
 MBR Magic : aa55
/dev/sdd:
 MBR Magic : aa55
/dev/sde:
 MBR Magic : aa55
/dev/sdf:
 MBR Magic : aa55

Now, we must clear this mbr (512b):

dd if=/dev/zero of=/dev/sdc bs=512 count=1
512 bytes copied, 0.000379187 s, 1.4 MB/s
dd if=/dev/zero of=/dev/sdd bs=512 count=1
512 bytes copied, 0.000251414 s, 2.0 MB/s
dd if=/dev/zero of=/dev/sde bs=512 count=1
512 bytes copied, 0.000487665 s, 1.0 MB/s
dd if=/dev/zero of=/dev/sdf bs=512 count=1
512 bytes copied, 0.000436107 s, 1.2 MB/s

And now, we can see, that there is no superblock:

mdadm -E /dev/sd[c-f]
mdadm: No md superblock detected on /dev/sdc.
mdadm: No md superblock detected on /dev/sdd.
mdadm: No md superblock detected on /dev/sde.
mdadm: No md superblock detected on /dev/sdf.

Now, we must create a partitions with the same size. Disks from different manufacturers (or even different models of the “same” capacity from the same manufacturer) don’t necessarily have the exact same disk size. And in future, we can replace failed disk with another disk (maybe a bigger), but we must create partition with the same size.
So, list the disks size:

fdisk -l /dev/sd[c-f]
Disk /dev/sdc: 465.8 GiB, 500107862016 bytes, 976773168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk /dev/sdd: 465.8 GiB, 500107862016 bytes, 976773168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk /dev/sde: 465.8 GiB, 500107862016 bytes, 976773168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk /dev/sdf: 465.8 GiB, 500107862016 bytes, 976773168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

We can create partitions with fdisk command. Create a new primary partition with the same sectors:

fdisk -l /dev/sd[c-f]
Disk /dev/sdc: 465.8 GiB, 500107862016 bytes, 976773168 sectors
/dev/sdc1 2048 976773167 976771120 465.8G 83 Linux
Disk /dev/sdd: 465.8 GiB, 500107862016 bytes, 976773168 sectors
/dev/sdd1 2048 976773167 976771120 465.8G 83 Linux
Disk /dev/sde: 465.8 GiB, 500107862016 bytes, 976773168 sectors
/dev/sde1 2048 976773167 976771120 465.8G 83 Linux
Disk /dev/sdf: 465.8 GiB, 500107862016 bytes, 976773168 sectors
/dev/sdf1 2048 976773167 976771120 465.8G 83 Linux

For sure, check, if there is no magic block in partitions:

mdadm -E /dev/sd[c-f]1
mdadm: No md superblock detected on /dev/sdc1.
/dev/sdd1:
 MBR Magic : aa55
Partition[0] : 1836016416 sectors at 1936269394 (type 4f)
Partition[1] : 544437093 sectors at 1917848077 (type 73)
Partition[2] : 544175136 sectors at 1818575915 (type 2b)
Partition[3] : 54974 sectors at 2844524554 (type 61)
mdadm: No md superblock detected on /dev/sde1.
mdadm: No md superblock detected on /dev/sdf1.

So, clear this superblock:

dd if=/dev/zero of=/dev/sdd1 bs=512 count=1
512 bytes copied, 0.000261033 s, 2.0 MB/s

And check for the last time:

mdadm -E /dev/sd[c-f]1
mdadm: No md superblock detected on /dev/sdc1.
mdadm: No md superblock detected on /dev/sdd1.
mdadm: No md superblock detected on /dev/sde1.
mdadm: No md superblock detected on /dev/sdf1.

And finally we create a raid array:

mdadm --create /dev/md1 --level=10 --raid-devices=4 /dev/sd[c-f]1
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md1 started.

Check the status of initial synchronization:

cat /proc/mdstat
Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10]
md1 : active raid10 sdf1[3] sde1[2] sdd1[1] sdc1[0]
 976508928 blocks super 1.2 512K chunks 2 near-copies [4/4] [UUUU]
 [>....................] resync = 0.2% (2810176/976508928) finish=138.5min speed=117090K/sec
 bitmap: 8/8 pages [32KB], 65536KB chunk

 

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