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

Christoph Hellwig
2021-09-21T08:32:29Z

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.