Re: Block Device Caching

[Helge Hafting]

Timothy Miller wrote:

> Markus Schaber wrote:
>
> > This lead us to the conclusion that block devices do not cache, but the
> > filesystem does. But subsequently, I ran some tests on my developer
> > machine (Pentium 4 Mobile Laptop).
>
>
>
> I had kernel experts repeatedly insist to me that block devices were 
> cached, while all of my tests (using dd to or from, say, /dev/sda1 or 
> whatever) indicated that there was absolutely no caching whatsoever.

Well, any cache is dropped when the device is closed. "dd" closes the device
when it finishes.

Try a program that reads the same two blocks (spaced videly apart)
over and over from the same open file descriptor.  With _no_ caching 
you'll see
the drive seeking all the time.  With caching, you won't.

Helge Hafting
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