Hard Disk device IDs in Linux

Nicholas Leippe nick at leippe.com
Fri Mar 8 00:18:56 MST 2013


Historically it was sd[a-z][a-z][0-14]

The number of devices is determined by (number of major numbers) x
(number of minor numbers) ÷ (number of partitions).
I think there's 12 bits for major number and 20 bits for minor
numbers, and a max of 15 partitions. Newer kernels added a means to do
dynamic device number allocation, so i don't know what the actual
limits are.

I don't know what the kernel built-in default naming scheme for the
higher drive numbers is currently. Nowadays that's often left up to
udev rules--some distros override the kernel defaults to provide a
persistent naming convention.

As for mdadm, that depends on the superblock format chosen. v 0.90
limits it to 28 components. Version 1.0 supports at least 384, to a
theoretical max of 64k component devices. Also, the superblock version
determines limits on both maximum component size and total volume size
of the combined array.


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