SEEK_HOLE Support in Cygwin?

[Cedric Blancher]

On Thu, 10 Aug 2023 at 10:28, Corinna Vinschen
<corinna-cygwin@cygwin.com> wrote:
>
> On Aug 10 04:43, Cedric Blancher wrote:
> > On Wed, 9 Aug 2023 at 17:30, Corinna Vinschen <corinna-cygwin@cygwin.com> wrote:
> > >
> > > On Aug  7 10:33, Corinna Vinschen wrote:
> > > > Wrong mailing list, use cygwin AT cygwin DOT com for user questions.
> > > >
> > > > On Aug  6 19:14, Cedric Blancher wrote:
> > > > > Good evening!
> > > > >
> > > > > Are there plans for a SEEK_HOLE support to Cygwin, e.g. on top of the
> > > > > FSCTL_QUERY_ALLOCATED_RANGES api (if appropriate)?
> > > >
> > > > No plans, but patches are welcome.  This looks like a fun task for a
> > > > rainy weekend.
> > >
> > > Today was a really rainy day and I had to wait for a test system to
> > > come up, so I hacked on SEEK_DATA/SEEK_HOLE support.
> > >
> > > Check out the next Cygwin test release 3.5.0-0.384.g9939aa7d0945.
> >
> > Thank you!
> > Or should I thank the weather gods? ;)
> >
> > How can I download this version?
>
> Cygwin setup, as usual.  Just install the above version of the packages
> cygwin and cygwin-devel.

Corinna, Thank you!

>
> > > Note that this defaults to the default behaviour described in the
> > > Linux man page, see https://man7.org/linux/man-pages/man2/lseek.2.html
> > >
> > > I.e., "In the simplest implementation, a filesystem can support the
> > > operations by making SEEK_HOLE always return the offset of the end of
> > > the file, and making SEEK_DATA always return offset"
> >
> > So this supports multiple holes in one file, right?
>
> Curious question.  Cygwin supports sparse files for ages.  Only the
> lseek options SEEK_HOLE and SEEK_DATA are new.

Is SEEK_HOLE support enabled for MSFT NFSv3 client?

>
> But, yes, it supports what Windows supports.  But note that Cygwin
> *only* switches files to sparse, if you seek by more than 128K beyond
> current EOF and then wrote something, or if you call truncate/ftruncate,
> raising the filesize by more than 128K.

Where does the value of 128K come from? Is this a Windows limit,
kernel tunable, or something from Cygwin?

>
> In contrast to Linux, one has to sparsify a file explicitely on WIndows,
> and that's Cygwin's strategy.

Maybe add a O_SPARSEFILE to open/openat()?

>
> > > The real deal obviously requires a filesystem supporting sparse files,
> > > *and* it requires that you access the file via a mount point with the
> > > "sparse" option set!  The mount option "sparse" is not set by default.
> >
> > Why is "sparse" not the default mount option?
>
> It was, at one point long ago.  People complained that sparse file
> access is slower than standard file access, so the "sparse" mount option
> was added.

Do you have any refs in the mailing list?

Ced