Block Device arrays Relation?

[Xiangfei Jia <xjianz@xxxxxxxxx>]

For block devices, the "blkdevs" array is used to hold all the
registered block device drivers, and the maximum number of allowed
block drivers is 255. There is another hash table array called
"bdev_hashtable", which is array of struct "block_device".

I'm quite confused with these two arrays. (1)  what is the relationship between the "blkdevs" and the "bdev_hashtable"?? (2)
  Why a extra sruct, called "block_device" is necessary to be used?

Thanks in advance.

Fei