block size vs bvec length

I created a simple block device driver with a logical queue block size of 512 bytes. $ cat /sys/block/myblock/queue/physical_block_size 512 $ cat /sys/block/myblock/queue/logical_block_size 512 I used rq_for_each_segment() to print bvec.bv_len of the segments and it appears to be 4096. Why is it 4096 rather than 512? Also writing a block of 4096 bytes with dd to /dev/myblock will result in a single write request, while writing a block of 512 bytes will result in a read request followed by a write request. Can someone explain this behavior? Thanks, Michele.