lkml.org 
[lkml]   [1998]   [Jan]   [5]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: lack of raw disk devices
On Mon, 5 Jan 1998, Peter Monta wrote:

> A raw device doesn't go through the buffer cache. For example, if you
> issue two reads for the same block, a raw device will physically go
> out to disk each time, whereas a buffered device will satisfy the
> second request from the cache.
>
> I've read all of the discussion I can find about this, in linux-kernel
> archives and elsewhere, and while I don't want to beat a dead horse, I
> disagree with those who say raw devices are useless or intrinsically
> bad. Granted, the vast majority of the time a buffered device is the
> right thing. But I have one application in mind that cries out for a
> raw device---it involves moving digital video as fast as possible from
> disk to DRAM (and subsequently out to a PCI bus master). The system
> will never again need to refer to this data, so a buffered device both
> incurs a copying expense (from buffer cache to DRAM buffer) and
> trashes the buffer cache to no purpose.

They are not bad, however, the implimentation would be sufficently
difficult and cumbersom, to mostly out weigh the benifits.. In the future,
it would be nice if you could adjust the buffercache characteristics on a
device by device basis and potentiall on each group of block reads
(wouldn't it be neat if ext2fs could be told by the app 'this is a mpg
movie dont cache it' while it's accessed, while the rest of the device
would be cached normally)..

>
> Cheers,
> Peter Monta pmonta@imedia.com
> Imedia Corp.


\
 
 \ /
  Last update: 2005-03-22 13:41    [W:0.114 / U:0.028 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site