Re: Streaming disk I/O kills file buffering and makes Linux unusable

[Florian Weimer]

Peter Monta <[e-mail]> writes:

> > Will future kernels ave an O_DIRECT like flag to avoid caching ?
>
> Stephen Tweedie has written a patch for unbuffered I/O;

I don't think unbuffered I/O is the solution in this case, because
you don't want to miss read ahead.  IMHO, something like O_NOCACHE
(or O_DISCARDCACHE) would be better, and looking at the 2.2.11 page
cache code, this should be that hard to implement: simply don't set the
PG_referenced bit on page access (or when the page is inserted into the
page cache).  This should cause shrink_mmap() to favor these pages when
looking for candidates to free.

(I hope I read the code correctly.  It's almost the first time I had
a look at it...)