Re: [PATCH RFC 4/8] fs: Introduce FALLOC_FL_PROVISION

Mike Snitzer
2022-09-21T15:21:46Z

On Wed, Sep 21 2022 at 1:54P -0400,
Sarthak Kukreti <sarthakkukreti@chromium.org> wrote:

> On Tue, Sep 20, 2022 at 12:49 AM Christoph Hellwig <hch@infradead.org> wrote:
> >
> > On Thu, Sep 15, 2022 at 09:48:22AM -0700, Sarthak Kukreti wrote:
> > > From: Sarthak Kukreti <sarthakkukreti@chromium.org>
> > >
> > > FALLOC_FL_PROVISION is a new fallocate() allocation mode that
> > > sends a hint to (supported) thinly provisioned block devices to
> > > allocate space for the given range of sectors via REQ_OP_PROVISION.
> >
> > So, how does that "provisioning" actually work in todays world where
> > storage is usually doing out of place writes in one or more layers,
> > including the flash storage everyone is using. Does it give you one
> > write? And unlimited number? Some undecided number inbetween?
>
> Apologies, the patchset was a bit short on describing the semantics so
> I'll expand more in the next revision; I'd say that it's the minimum
> of regular mode fallocate() guarantees at each allocation layer. For
> example, the guarantees from a contrived storage stack like (left to
> right is bottom to top):
>
> [ mmc0blkp1 | ext4(1) | sparse file | loop | dm-thinp | dm-thin | ext4(2) ]
>
> would be predicated on the guarantees of fallocate() per allocation
> layer; if ext4(1) was replaced by a filesystem that did not support
> fallocate(), then there would be no guarantee that a write to a file
> on ext4(2) succeeds.
>
> For dm-thinp, in the current implementation, the provision request
> allocates blocks for the range specified and adds the mapping to the
> thinpool metadata. All subsequent writes are to the same block, so
> you'll be able to write to the same block inifinitely. Brian mentioned
> this above, one case it doesn't cover is if provision is called on a
> shared block, but the natural extension would be to allocate and
> assign a new block and copy the contents of the shared block (kind of
> like copy-on-provision).

It follows that ChromiumOS isn't using dm-thinp's snapshot support?

But please do fold in incremental dm-thinp support to properly handle
shared blocks (dm-thinp already handles breaking sharing, etc.. so
I'll need to see where you're hooking into that you don't get this
"for free").

Mike