Re: [PATCH 3/5] vfs: add a zero-initialization mode to fallocate

Dan Williams
2021-09-22T02:16:39Z

On Tue, Sep 21, 2021 at 1:32 AM Christoph Hellwig <hch@infradead.org> wrote:
>
> On Tue, Sep 21, 2021 at 10:44:31AM +1000, Dave Chinner wrote:
> > I think this wants to be a behavioural modifier for existing
> > operations rather than an operation unto itself. i.e. similar to how
> > KEEP_SIZE modifies ALLOC behaviour but doesn't fundamentally alter
> > the guarantees ALLOC provides userspace.
> >
> > In this case, the change of behaviour over ZERO_RANGE is that we
> > want physical zeros to be written instead of the filesystem
> > optimising away the physical zeros by manipulating the layout
> > of the file.
>
> Yes.
>
> > Then we have and API that looks like:
> >
> > ALLOC - allocate space efficiently
> > ALLOC | INIT - allocate space by writing zeros to it
> > ZERO - zero data and preallocate space efficiently
> > ZERO | INIT - zero range by writing zeros to it
> >
> > Which seems to cater for all the cases I know of where physically
> > writing zeros instead of allocating unwritten extents is the
> > preferred behaviour of fallocate()....
>
> Agreed. I'm not sure INIT is really the right name, but I can't come
> up with a better idea offhand.

FUA? As in, this is a forced-unit-access zeroing all the way to media
bypassing any mechanisms to emulate zero-filled payloads on future
reads.