Re: [RFC] mount flag "direct"

["Peter T. Breuer" <ptb@it.uc3m.es>]

"A month of sundays ago David Lang wrote:"
> Peter, the thing that you seem to be missing is that direct mode only
> works for writes, it doesn't force a filesystem to go to the hardware for
> reads.

Yes it does. I've checked! Well, at least I've checked that writing
then reading causes the reads to get to the device driver. I haven't
checked what reading twice does.

If it doesn't cause the data to be read twice, then it ought to, and
I'll fix it (given half a clue as extra pay ..:-)

> for many filesystems you cannot turn off their internal caching of data
> (metadata for some, all data for others)

Well, let's take things one at a time. Put in a VFS mechanism and then
convert some FSs to use it.

> so to implement what you are after you will have to modify the filesystem
> to not cache anything, since you aren't going to do this for every

Yes.

> filesystem you end up only haivng this option on the one(s) that you
> modify.

I intend to make the generic mechanism attractive.

> if you have a single (or even just a few) filesystems that have this
> option you may as well include the locking/syncing software in them rather
> then modifying the VFS layer.

Why? Are you advocating a particular approach? Yes, I agree that that
is a possible way to go - but I will want the extra VFS ops anyway, 
and will want to modify the particular fs to use them, no?

Peter