Sparse file is a file that contains blocks of zeroes that are not allocated on disk (see w:Sparse file). By design, ploop images can't contain such non-allocated blocks.
While ploop is never creating sparse image files, they can be accidentally made so when tools such as rsync --sparse or cp --sparse=always are used to copy images. So, one should not use backup or copy tools that create sparse files out of non-sparse ones.
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Kernel warning
In case sparse file is detected by the kernel, it gives a warning in dmesg, for example:
Nov 21 20:37:27 sun kernel: [43451.057489] ploop(24002): a hole in image file detected (0)
In case there are no other ploop-related warnings or errors immediately after, this one is harmless (#2825).
Userspace warnings
You might see something like this (reported by ploop library) during container start or mount:
Error in check_and_repair_sparse (check.c:542): Delta file /vz/private/1407/root.hdd/root.hdd contains uninitialized blocks (offset=135573536768 len=1323008) which are not aligned to cluster size Error in fill_hole (check.c:456): Warning: ploop image '/vz/private/1407/root.hdd/root.hdd' is sparse Reallocating sparse blocks back
The last line tells that ploop library is going to fix the problem.
How to fix
Since ploop-1.10, ploop images are checked for being sparse and are automatically fixed on mount, in case DiskDescriptor.xml is used. Otherwise, you can use ploop check --repair-sparse to check for and repair such images.
Alternatively, you can just do something like this to get rid of holes. Make sure ploop file is not used!
cat root.hdd > root.hdd2 && mv root.hdd2 root.hdd