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Date: Fri, 1 Dec 2023 12:02:12 +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  1 11:22, Cedric Blancher via Cygwin wrote:
> I see the value of 128k (128*1024 bytes) quite often in your sparse
> file commits.

Yes, but they have been removed.  Read the patches again, especially commit
65831f88d6c4.

> Can you please make this value a per filesystem tunable?

No, because we don't have a facility for that.

> 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

I don't know how Windows stores short files, maybe they are part of the
metadata up to a point or some such.  However, as soon as you raise the
size over a given point, the *allocation size* will be rounded up to 64K
and from that point on, it will be in 64K chunks.  Also sparsifying and
desparsifying of blocks only works in 64K chunks.


Corinna

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