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

[Sarthak Kukreti <sarthakkukreti@chromium.org>]

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).

[reflowed]
> How is it affected by write zeroes to that range or a discard?

The current semantics of discards for dm-thinp/ext4/sparse files will
apply as they do today; discards will unmap the dm-thin block/free the
file extent. Write zeroes is more interesting; dm-thinp will treat the
command as usual. ext4_zero_range will mark the extents as unwritten,
so essentially if a user did provision + write to a block, write zeros
to the block would essentially leave it in the original provisioned
state, but ext4 would now show the contents of the block as zero on
the next read. I think, similar to above, the semantics of a request
will depend on each layer that it passes through.

Best
Sarthak