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? 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
