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

[Sarthak Kukreti]

On Wed, Sep 21, 2022 at 8:21 AM Mike Snitzer <snitzer@xxxxxxxxxx> wrote:
>
> On Wed, Sep 21 2022 at  1:54P -0400,
> Sarthak Kukreti <sarthakkukreti@xxxxxxxxxxxx> wrote:
>
> > On Tue, Sep 20, 2022 at 12:49 AM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> > >
> > > On Thu, Sep 15, 2022 at 09:48:22AM -0700, Sarthak Kukreti wrote:
> > > > From: Sarthak Kukreti <sarthakkukreti@xxxxxxxxxxxx>
> > > >
> > > > 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?
>
Not at the moment, but we definitely have ideas to explore re:snapshot
and dm-thinp (like A-B updates with thin volume snapshots), where this
would definitely be useful!

> 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").
>
Will do in v2. Thanks for the feedback.

Best
Sarthak

> Mike
>