Comment 2 for bug 271772

My mistake, iostat is right. What iostat calls a block is a _physical sector_ and this is not the same as a filesystem block.

Also from the FAQ:
3.4. I don't understand the output of iostat. It doesn't match what I expect it
to be...

By default iostat displays I/O activity in blocks per second. With old
kernels (i.e. older than 2.4.x) a block is of indeterminate size and therefore
the displayed values are not useful.
With recent kernels (kernels 2.4 and later), iostat is now able to get disk
activities from the kernel expressed in a number of sectors. If you take a
look at the kernel code, the sector size is actually allowed to vary although
I have never seen anything other than 512 bytes.

In my case hdparm returns
LBA48 user addressable sectors: 488397168
device size with M = 1024*1024: 238475 MBytes (ie 250059161600 bytes)

sector size = 250059161600 / 488397168 = 511.9996