Dear Chris!
Am 25.06.21 um 15:20 schrieb Chris Hofstaedtler:
> * Kevin Price <k...@osnanet.de> [210625 11:00]:
>> fallocate -d seems to attempt to unallocate that last block if it
>> contains only zeros, and it even updates the file's mtime. The
>> unallocation is reported, but doesn't effectively happen.
>
> Did you verify this is a bug in fallocate and not a bug or
> side-effect of the kernel-side implementation, possibly in the used
> filesystem?
I'm rather convinced the culprit is fallocate from util-linux, because
using the same kernel and filesystem:
1. With coreutils (dd, truncate) I'm able to create sparse files with
zero blocks and sizes other than multiples of ${block size}.
2. With coreutils (truncate) I'm able to truncate the last block of
zeros and re-add it, becoming an unallocated block. That's how I
successfully sparsified the last block (containing 2048 zeros) of the
debian iso, btw.
Is that convincing, or do you request any additional verification? Some
strace maybe?
> Can the effect still be seen with Linux and util-linux from
> bullseye?
IDK. I'd need to set up a bullseye machine iot test that.
> BTW, which filesystem are you trying this on?
I'm using ext4 with block size=4096, running on buster's stock
linux-image-4.19.0-17-amd64, version 4.19.194-2.
Best
Kevin
