On Mar 31, 2016, at 1:55 AM, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
>
> On Wed, Mar 30, 2016 at 05:32:42PM -0700, Liu Bo wrote:
>> 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.
Because of snapshots that could hold references to the old blocks,
there isn't even the guarantee that the previous fallocated blocks will
be released in a reasonable time to free up an equal amount of space.
Cheers, Andreas
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