Peter Monta wrote:
> > Okay. I've seen this dicussion many times before. I need to know now. What the
> > hell is the difference a RAW block device and a (err.... NONRAW?) regular block
> > device?
>
> A raw device doesn't go through the buffer cache.
Another critical consideration when using raw devices is that if you issue a read()
for 32K, the read is done in one 32K chunk.Same for write()s. I'm not sure how Linux
handles this, but it
would seem fairly critical for backing up to tapes...so much so, that I'm sure Linux
must handle it some how. Can someone please
inform me? Also, how does it handle read/writes of a non-block
size? When I wrote tape drivers, lots of people would like to
write wierd size records - say, 444 bytes or some such. ???
When I need to move filesystems around on other unix machines, I
typically dd the raw device, about 64MB per read/write. This goes
much faster than going through the block device. I haven't had to
do this on Linux yet, as I only use Linux for test machines, not in
a production environment. So, how does Linux handle large
read/write requests? I can't believe that it doesn't as of
2.1.77 - I'm almost so surprised to find out that it doesn't
have raw devices that I might find the time to go look at sources.
-Dan
-- Dan A. Dickey http://www.transition.com/ mailto:ddickey@transition.com
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