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Date: Tue, 5 Dec 2023 16:15:14 +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 15:47, Thomas Wolff via Cygwin wrote:
> 
> Am 05/12/2023 um 15:40 schrieb Thomas Wolff via Cygwin:
> > Am 05/12/2023 um 14:53 schrieb Corinna Vinschen via Cygwin:
> > > 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?
> > Ups, sorry, I was just throwing my 2p into something. Yes, on a local
> > NTFS; setting chattr +s does not change it but lsattr says:
> > ---a-------- .ls-s
> > 
> > So why does chattr not seem to work?
> Sorry again, I failed to test properly (I had copied your `chattr +S x`
> which is not the filename I tested... blush).
> With chattr +S, I reproduce your observation.

No worries. Thanks for testing and confirming!


Corinna

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