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On 02/16/2012 05:58 PM, Ted Ts'o wrote:

I don't see the bug here.  If there are no leftover sparse bytes,
there's no need to write the last zero byte.  The whole point was to
make sure i_size was set correctly, and if sparse==0, then i_size is
correct.

- From what I can see, when sparse == 0, the last write does a seek to move
the file pointer, but doesn't write anything beyond the last hole, so i_size
is not updated.  This resulted in an image file I took of a 20gb fs being 124
MiB too small.  I can only assume that this is to be expected, and is the
reason for passing one byte of zero_buff to write_block instead of not giving
it any bytes to write, and just asking it to do the seek the way the loop does.

ftruncate() happens to work today for Linux, but it's not guaranteed
to do anything on all operating systems or even all file systems.  Per
the standards spec:

	If the file previously was smaller than this size, ftruncate()
	shall either increase the size of the file or fail.

Speaking of which, you're not checking the return value from ftruncate
in your patch.  So I'd be happy if you checked ftruncate, and if it
failed, falling back to the

	if (sparse)
		write_block(fd, zero_buf, sparse-1, 1, -1);

code path.  That way, if ftruncate() happens to fail on NFS, or ceph,
or some random file system that chose to meet the standards spec, but
by failing if someone tries to increase the size of the file using
ftruncate.  (Or some other OS; there are other operating systems,
including GNU Hurd and BSD, and I don't know for sure how ftruncate
behaves on all of those other OS's and file systems.)

Good idea.

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