Linux block device with odd (not even) size

Question

Is it possible to create a Linux (2.6) block device (such as a loopback device) with an odd size? I couldn't make it happen. losetup seems to round down to 512 byte boundary. The ubd devices of User-mode Linux ubd devices seem to round up to 512 byte boundary. In struct request, we have sector_t __sector for the block offset for read/write operations.

I'm asking this question just for educational purposes. I can cope with the 512 byte boundary, but I'm still interested if it would be possible to bypass it. In this question I'm not interested in other layers of abstraction (such as using regular files or character devices).

Answer 1

No. The Linux 2.6 block layer doesn't comprehend anything smaller than 512 bytes. Anything smaller (especially not a power of 2) would require a major rewrite of an awful lot of code.

Answer 2

This is what makes a block device instead of a character device: the block granularity. The dichotomy exists because it is vastly more efficient to model real hardware that works a block at a time as an abstraction that also deals in blocks. To do otherwise would turn every operation into a much more costly computation.

The way to bypass it is, as you mention, to use a character oriented device or abstraction. This is central to the Unix device model: everything is a series of octets, except for the things that can only be virtualized as one.