# Re: \[PATCH 1/2\] man2/fallocate.2: document behavior with shared blocks

- From: Michael Kerrisk (man-pages) \<hidden\>
- Date: 2016-10-18 06:06:14 
- List: linux-api
- Page: https://ratatoskr.run/linux-api/2016/10/113367
- Thread: https://ratatoskr.run/linux-fsdevel/2016/10/113368/t.md

```text
Hi Darrick,

On 10/18/2016 03:54 AM, Darrick J. Wong wrote:
> Add a blurb to the fallocate manpage explaining that the fallocate
> command with the UNSHARE mode flag may use CoW to unshare blocks to
> guarantee that a disk write won't fail with ENOSPC.

Thanks! Applied.

Cheers,

Michael


> 
> Signed-off-by: Darrick J. Wong <redacted>
> ---
>  man2/fallocate.2 |   10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> 
> diff --git a/man2/fallocate.2 b/man2/fallocate.2
> index 54d6340..3ed1460 100644
> --- a/man2/fallocate.2
> +++ b/man2/fallocate.2
> @@ -79,6 +79,16 @@ but the file size will not be changed even if
>  is greater than the file size.
>  Preallocating zeroed blocks beyond the end of the file in this manner
>  is useful for optimizing append workloads.
> +
> +If the
> +.B FALLOC_FL_UNSHARE
> +flag is specified in
> +.IR mode ,
> +shared file data extents will be made private to the file to guarantee
> +that a subsequent write will not fail due to lack of space.
> +Typically, this will be done by performing a copy-on-write operation on
> +all shared data in the file.
> +This flag may not be supported by all filesystems.
>  .PP
>  Because allocation is done in block size chunks,
>  .BR fallocate ()
> 
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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```