Block a physical property?

[Artem Kachitchkine]

Block and char are not symmetrical. Block means: addressable, random 
access data blocks + buffer cache. Char means: everything else.

Here's my reading of the HAL spec, I don't know the history:

Block *capability* is interesting because OS has special use for them: 
filesystems can be mounted (only) on block devices, etc.

Physical devices can have block capabilities. We can't merge block with 
storage because block capability can be presented by non-storage 
devices. One example is partitioned disk: you can access entire disk vie 
its block capability or you can access individual partitions, if the OS 
presents partitions as separate block devices.

I was actually surprised to read that "device objects can only have the 
storage capability if they already got capability block". True, most 
storage is random access, but some is not. For instance, tapes, besides 
being sequential access, use variable block size, and are not block 
devices at the OS level.

-Artem.