On Tue, Feb 21, 2012 at 12:55:36PM -0500, Ted Unangst wrote: > On Tue, Feb 21, 2012, Kenneth R Westerback wrote: > > On Tue, Feb 21, 2012 at 03:15:29PM +0200, Alexey Vatchenko wrote: > >> Reading 512 bytes from raw device with 2048 sector size fails. If i read > >> 512 bytes from block device the problem is not reproduced. > > > This is intentional. You cannot read from a raw device fewer bytes > > at a time than the minimum the device provides per i/o. On block > > devices the buffer cache does the correct size i/o and extracts > > just the number of bytes you requested. > > That's backwards from what I thought. The raw device should let you > read byte by byte, the block device only lets you read block by block, > as it were.
Block/Sector based devices can only provide entire blocks/sectors, at block/sector addresses. The buffer cache and standard i/o routines provide the abstraction that you can start and stop at any byte. Doing I/O to raw devices means you are taking full responsibility for paying attention to the boundaries and sizes of the i/o. This is how OpenBSD works. Whether that is 'correct' or not, I don't know. :-) Hence the mad dancing over the last few years to make more devices with non-DEV_BSIZE sectors work with various bits of software that do 'raw' i/o and have always assumed DEV_BSIZE is and always will be 512 bytes. .... Ken
