On Thu, Mar 31, 2016 at 5:31 PM, Andreas Dilger [off-list ref] wrote:

On Mar 31, 2016, at 1:55 AM, Christoph Hellwig [off-list ref] wrote:

quoted

On Wed, Mar 30, 2016 at 05:32:42PM -0700, Liu Bo wrote:

quoted

Well, btrfs fallocate doesn't allocate space if it's a shared one
because it thinks the space is already allocated.  So a later overwrite
over this shared extent may hit enospc errors.

And this makes it an incorrect implementation of posix_fallocate,
which glibcs implements using fallocate if available.

It isn't really useful for a COW filesystem to implement fallocate()
to reserve blocks.  Even if it did allocate all of the blocks on the
initial fallocate() call, when it comes time to overwrite these blocks
new blocks need to be allocated as the old ones will not be overwritten.

There are also use-cases on BTRFS with CoW disabled, like operations
on virtual machine images that aren't snapshotted.
Those files tend to be big and having fallocate() implemented and
working like for e.g. XFS, in order to achieve space and speed
efficiency, makes sense IMHO.