Re: [PATCH v6 00/10] block atomic writes

Kent Overstreet
2024-04-05T06:14:10Z

On Wed, Mar 27, 2024 at 03:50:07AM +0000, Matthew Wilcox wrote:
> On Tue, Mar 26, 2024 at 01:38:03PM +0000, John Garry wrote:
> > The goal here is to provide an interface that allows applications use
> > application-specific block sizes larger than logical block size
> > reported by the storage device or larger than filesystem block size as
> > reported by stat().
> >
> > With this new interface, application blocks will never be torn or
> > fractured when written. For a power fail, for each individual application
> > block, all or none of the data to be written. A racing atomic write and
> > read will mean that the read sees all the old data or all the new data,
> > but never a mix of old and new.
> >
> > Three new fields are added to struct statx - atomic_write_unit_min,
> > atomic_write_unit_max, and atomic_write_segments_max. For each atomic
> > individual write, the total length of a write must be a between
> > atomic_write_unit_min and atomic_write_unit_max, inclusive, and a
> > power-of-2. The write must also be at a natural offset in the file
> > wrt the write length. For pwritev2, iovcnt is limited by
> > atomic_write_segments_max.
> >
> > There has been some discussion on supporting buffered IO and whether the
> > API is suitable, like:
> > https://lore.kernel.org/linux-nvme/ZeembVG-ygFal6Eb@casper.infradead.org/
> >
> > Specifically the concern is that supporting a range of sizes of atomic IO
> > in the pagecache is complex to support. For this, my idea is that FSes can
> > fix atomic_write_unit_min and atomic_write_unit_max at the same size, the
> > extent alignment size, which should be easier to support. We may need to
> > implement O_ATOMIC to avoid mixing atomic and non-atomic IOs for this. I
> > have no proposed solution for atomic write buffered IO for bdev file
> > operations, but I know of no requirement for this.
>
> The thing is that there's no requirement for an interface as complex as
> the one you're proposing here. I've talked to a few database people
> and all they want is to increase the untorn write boundary from "one
> disc block" to one database block, typically 8kB or 16kB.
>
> So they would be quite happy with a much simpler interface where they
> set the inode block size at inode creation time, and then all writes to
> that inode were guaranteed to be untorn. This would also be simpler to
> implement for buffered writes.
>
> Who's asking for this more complex interface?

I get the impression the atomic writes stuff has suffered from too
/much/ review and too many maintainers asking for and demanding all
their different must-haves.