Re: DVD blockdevice buffers

["Stephen C. Tweedie" <sct@redhat.com>]

Hi,

On Fri, May 25, 2001 at 09:09:37AM -0600, Eric W. Biederman wrote:

> The case we don't get quite right are partial reads that hit cached
> data, on a page that doesn't have PG_Uptodate set.  We don't actually
> need to do the I/O on the surrounding page to satisfy the read
> request.  But we do because generic_file_read doesn't even think about
> that case.

That's *precisely* the case in question.  The whole design of the page
cache involves reading entire pages at a time, in fact.  We _could_
read in only partial pages, but in that case we end up wasting a lot
of the page.

> For the small random read case we could use a 
> mapping->a_ops->readpartialpage 
> function that sees if a request can be satisfied entirely 
> from cached data.  But this is just to allow generic_file_read
> to handle this, case. 

Agreed.  The only case where blockdev-in-pagecache really results in
significantly more IO is partial writes followed by partial reads.
Reads from totally-uncached pages ought to just fill the entire page
from disk; it's only when there is something already present
in the cache for that page that we want to look for partial buffers.

--Stephen