The only way I've found in mdadm to make 2 drives perform like a proper RAID 1 (eg. the read speed should be 2x that of a single drive) is to use the --layout=f2 (far 2).

mdadm raid10 performance issues.
Be very aware that mdadm seems to default to layout=n2 (which means near). In this scenario it means it is like mdadm RAID 1 performance (you get maximum read speeds of a single drive).

dd if=/dev/md126 of=/dev/null bs=1M count=2000
2000+0 records in
2000+0 records out
2097152000 bytes (2.1 GB) copied, 6.46397 s, 180 MB/s

Now look at the difference in the same drives in layout=f2 (which means far).

dd if=/dev/md126 of=/dev/null bs=1M count=2000
2000+0 records in
2000+0 records out
2097152000 bytes (2.1 GB) copied, 6.46397 s, 324 MB/s

As we can see the speed is nearly double (a small amount of bandwidth appears to be lost as overhead due to mdadm)

 

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