From: Cedric Blancher <cedric.blancher@gmail.com>
To: cygwin-developers@cygwin.com
Subject: Re: SEEK_HOLE Support in Cygwin?
Date: Mon, 21 Aug 2023 06:33:00 +0200 [thread overview]
Message-ID: <CALXu0Uc7CYc4LQZCw_3zumgxHbmE+RTcy9P26FdvSKvCfc-exQ@mail.gmail.com> (raw)
In-Reply-To: <ZNSfn5vA+UQKn0fF@calimero.vinschen.de>
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
next prev parent reply other threads:[~2023-08-21 4:33 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-08-06 17:14 Cedric Blancher
2023-08-07 8:33 ` Corinna Vinschen
2023-08-09 15:30 ` Corinna Vinschen
2023-08-10 2:43 ` Cedric Blancher
2023-08-10 8:28 ` Corinna Vinschen
2023-08-21 4:33 ` Cedric Blancher [this message]
2023-08-21 8:32 ` Corinna Vinschen
2023-08-24 12:03 ` Roland Mainz
2023-08-25 9:05 ` Corinna Vinschen
2023-08-25 11:09 ` Roland Mainz
2023-08-25 11:23 ` Corinna Vinschen
2023-08-25 11:43 ` Corinna Vinschen
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