Re: Healthy amount of free space?

[Chris Murphy <lists@colorremedies.com>]

On Wed, Jul 18, 2018 at 6:35 AM, Austin S. Hemmelgarn
<ahferroin7@gmail.com> wrote:

> If you're doing a training presentation, it may be worth mentioning that
> preallocation with fallocate() does not behave the same on BTRFS as it does
> on other filesystems.  For example, the following sequence of commands:
>
>     fallocate -l X ./tmp
>     dd if=/dev/zero of=./tmp bs=1 count=X
>
> Will always work on ext4, XFS, and most other filesystems, for any value of
> X between zero and just below the total amount of free space on the
> filesystem.  On BTRFS though, it will reliably fail with ENOSPC for values
> of X that are greater than _half_ of the total amount of free space on the
> filesystem (actually, greater than just short of half).  In essence,
> preallocating space does not prevent COW semantics for the first write
> unless the file is marked NOCOW.

Is this a bug, or is it suboptimal behavior, or is it intentional?

And then I wonder what happens with XFS COW:

     fallocate -l X ./tmp
     cp --reflink ./tmp ./tmp2
     dd if=/dev/zero of=./tmp bs=1 count=X



-- 
Chris Murphy