When write()ing to a raw device, what is known on return?

[Doug Gwyn <gwyn>]

> When a write to a raw disk partition returns, is anything known
> concerning whether data is actually on the disk or not?

Yes, the transfer is by DMA from user address space and it is
complete when the write() system call returns.  Block buffers
are not used for raw disk I/O.  Further, EIO is supposed to
be returned if the write fails; this is much better than in
the case of block I/O if you are concerned with reliability.