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Date: Tue, 5 Dec 2023 14:53:01 +0100
To: cygwin AT cygwin DOT com
Subject: Re: Cygwin api to punch a hole into a file?
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From: Corinna Vinschen via Cygwin <cygwin AT cygwin DOT com>
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Cc: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
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On Dec  5 07:36, Thomas Wolff via Cygwin wrote:
> 
> 
> Am 01.12.2023 um 12:02 schrieb Corinna Vinschen via Cygwin:
> > On Dec  1 11:22, Cedric Blancher via Cygwin wrote:
> > > Not all filesystems have a 128k block/stripe size, and certainly most
> > > filesystems have smaller minimum hole sizes than 128k (e.g. 512bytes
> > > is common, ref pathconf _PC_MIN_HOLE_SIZE).
> > There's no _PC_MIN_HOLE_SIZE in Linux or POSIX.  In Windows, a sparse
> > file uses chunks of 64K.  You can see this even with a file of just
> > a single block.  Try this:
> > 
> >    $ touch x
> >    $ chattr +S x
> > 
> >    $ echo 1234567890123456789012345678901234567890123456789012345678901234567890123456789 >> x
> >    $ ls -ls x
> >    1 -rw-r--r-- 1 corinna vinschen 80 Dec  1 11:56 x
> > 
> >    [repeat echo and ls -ls until...]
> > 
> >    $ echo 1234567890123456789012345678901234567890123456789012345678901234567890123456789 >> x
> >    $ ls -ls x
> >    1 -rw-r--r-- 1 corinna vinschen 720 Dec  1 11:56 x
> >    $ echo 1234567890123456789012345678901234567890123456789012345678901234567890123456789 >> x
> >    $ ls -ls x
> >    64 -rw-r--r-- 1 corinna vinschen 800 Dec  1 11:56 x2
> >    ^^
> >    This
> For me, it goes up from 1 to 4, then in steps of 4KB.

Is that a local NTFS, did you actually call

  chattr +S x

after touching the file, and did you check with

  lsattr x

that x is actually sparse?


Corinna

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