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Date: Tue, 5 Dec 2023 15:47:58 +0100
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Subject: Re: Cygwin api to punch a hole into a file?
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From: Thomas Wolff via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Thomas Wolff <towo AT towo DOT net>
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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.

> Thomas
>
>> Corinna
>>
>
>


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