What is a raw device?

What is a raw device?

Generally when you access(read or write) a device without going through the usual layers, it is considered a "raw" access. For a harddrive, the usual layer would be a filesystem.

When you click "save" to save a file, the writing is handled/organized by the filesystem.

If you do this (as root), 'less /dev/sda', then you will view the raw output of what your harddrive disk looks like.

Raw access to devices is a feature in Linux that can be very powerful, but must be used with care.

Raw by definition means you are not mounting it. It may or may not also mean that the kernel won't buffer/cache/etc data to and from the hardware.

If you mounted /dev/sda or /dev/hda or whatever, then you would see the filesystem on it, like '/' or '/home' or whatever.
If you don't mount it, then you (or a program) can see the bytes that make up the filesystem directly, like the previous poster said.

There is a further (stricter) meaning of 'raw device', too. When you access /dev/sda etc, you're bypassing the kernel's filesystem layers, but you're not bypassing its buffered I/O layers.

Some software (such as I believe Oracle, optionally) thinks it knows better than the kernel how to buffer/optimise disk I/O and lay out data, so it bypasses both layers, and uses other 'raw' devices. These 'raw' devices used to be called /dev/raw[1-8] (see man MAKEDEV), but they're obsolete in kernel 2.6.22.1 according to its kconfig help.