/dev/raw support?

[PhiloVivero]

Alan Shutko wrote:

> > I don't really know what a raw device is, but vlc uses the block device
> > designing your DVD (often /dev/dvd beign a ling to /dev/hdc).
>
> Basically, it skips the overhead of buffered disk devices, and sicne
> it isn't buffered, streaming the DVD won't purge everything else from
> the buffer cache.

Often raw devices are considered in respect to RDBMS performance (I'm a DBA). But you don't need raw devices to
get unbuffered disk access. Raw disk device and buffered disk device and, indeed, synchronous vs. asynchronous
access are all unrelated concepts.

That is to say, it's three different attributes. (Raw/Filesystem, Buffered/Unbuffered, Synch/Asynch).

There's some chmod you can do on your DVD device to turn off buffering for the device. When you're accessing
/dev/dvd you're using the raw device whereas if you somehow mounted /dev/dvd to the filesystem, then used files
in that directory, you'd be using 'cooked.'

The reason there's so much confusion on this subject is that on many OSen, raw device access and OS buffering
*ARE* linked. In fact, until recently, you didn't have a choice to not buffer disk I/O to raw devices under Linux
-- this is why Sybase/Oracle et al were slow to support Linux.

Sorry for the mostly RDBMS-related discussion, but it gives a little background to understand the confusion and
misinformation about this topic.

--
PhiloVivero
ps -- there are a lot of smart people who will disagree with me in this post -- so I'm not surprised if you've
heard otherwise. If you want a very in-depth technical discussion, I suggest reading the Sybase ASE for Linux
mailing list archives on isug.com -- look for postings by a guy named Wim Ten Have (wtenhave at sybase.com) on the
subject of raw device support.

http://forum.swarthmore.edu/epigone/ase-linux-list