read/write less than sector size

[Nehal]

if i open /dev/ad1s1 for example, i try to read() or write()
not in multiples of 512, it will return an error.

how would i do this? currently, if i need to read 100 bytes, 
i read in the full sector, then memcpy the bytes i need.

is there a better way?
-- thx, Nehal