Re: [PATCH 3/3] block/mq-deadline: Disable I/O prioritization in certain cases

[Jaegeuk Kim <jaegeuk@kernel.org>]

On 12/13, Damien Le Moal wrote:
> On 12/13/23 04:03, Jaegeuk Kim wrote:
> > On 12/12, Christoph Hellwig wrote:
> >> On Tue, Dec 12, 2023 at 10:19:31AM -0800, Bart Van Assche wrote:
> >>> "Fundamentally broken model" is your personal opinion. I don't know anyone
> >>> else than you who considers zoned writes as a broken model.
> >>
> >> No Bart, it is not.  Talk to Damien, talk to Martin, to Jens.  Or just
> >> look at all the patches you're sending to the list that play a never
> >> ending hac-a-mole trying to bandaid over reordering that should be
> >> perfectly fine.  You're playing a long term losing game by trying to
> >> prevent reordering that you can't win.
> > 
> > As one of users of zoned devices, I disagree this is a broken model, but even
> > better than the zone append model. When considering the filesystem performance,
> > it is essential to place the data per file to get better bandwidth. And for
> > NAND-based storage, filesystem is the right place to deal with the more efficient
> > garbage collecion based on the known data locations. That's why all the flash
> > storage vendors adopted it in the JEDEC. Agreed that zone append is nice, but
> > IMO, it's not practical for production.
> 
> The work on btrfs is a counter argument to this statement. The initial zone
> support based on regular writes was going nowhere as trying to maintain ordering
> was too complex and/or too invasive. Using zone append for the data path solved
> and simplified many things.

We're in supporting zoned writes, and we don't see huge problem of reordering
issues like you mention. I do agree there're pros and cons between the two, but
I believe using which one depends on user behaviors. If there's a user, why it
should be blocked? IOWs, why not just trying to support both?

> 
> I do think that zone append has a narrower use case spectrum for applications
> relying on the raw block device directly. But for file systems, it definitely is
> an easier to use writing model for zoned storage.
> 
> -- 
> Damien Le Moal
> Western Digital Research