Re: [PATCH 00/14] Pramfs: Persistent and protected ram filesystem

Bryan Henderson
2009-06-15T15:51:39Z

> Marco wrote:
> > To enable direct
> > I/O at all times for all regular files requires either that
> > applications be modified to include the O_DIRECT flag on all file
> > opens, or that a new filesystem be used that always performs direct
> > I/O by default."
>
> This could be done as well by just introducing a "direct_io_only"
> mount option to a file-system which would need this feature.

A mount option would not be the right way. Mount options are for things
that are characteristic of the way you're going to access the files.
_This_ is a characteristic of the block device. So if one were to make
this memory accessible with a block device, it would make more sense to
have a block device ioctl. And it wouldn't ask the question, "should I
use direct I/O only," but "does this device have the performance
characteristics of a classic disk drive?"

But it's possible that there's just no advantage to having a block device
in the stack here. When unix block devices were invented, their main
purpose was that they could reorder reads and writes and do buffering and
caching -- all things essential for disk drives. We don't want to stretch
the concept too far.

--
Bryan Henderson IBM Almaden Research Center
San Jose CA Storage Systems