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