I have a SATA block device that reports a capacity that is smaller than its accessible space, and I would like to read and write to it past the reported capacity using the file created by Linux for block devices. So I hope to operate using the descriptor returned from open({"/dev/sda", O_RDWR). However, when I try to use lseek to seek past the capacity of the device, I get an error and errno gets set to EINVAL (22).
Is there a way to access the data past the capacity of the device without modifying the device drivers and while still using the file descriptor returned by open()?
My Linux release is CentOS 7 with kernel 3.10.0-514.21.1.el7.x86_64, although I'd be interested in solutions even if they involve other Linux distributions.
Edit: The drive I am working with is a FLEX protocol drive that reports the conventional capacity of the drive, but also has shingled magnetic recording available at an offset above the reported capacity of the drive. If you are interested, the details of this protocol can be found on the T13 website.
If I remember correctly, that error is caused because the device itself wasn't able to read or write that cylinder, indicating it likely does not exist. Note that many manufacturers use 1000B = 1KB and the likes, and that file systems reserve their own space as well.
The short answer is, you don't. The device will only report the space you can use, and will not report cache sizes either. This misreporting isn't at the OS level, but at the device.