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'''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.
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While ploop is never creating sparse image files, they can be accidentally made so when tools such as <code>rsync --sparse</code> or <code>cp --sparse=always</code> are used to copy images. So, one should not use backup or copy tools that create sparse files out of non-sparse ones.
== Kernel warning ==<!--T:3-->
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In case sparse file is detected by the kernel, it gives a warning in dmesg, for example:
<!--T:5-->Nov 21 20:37:27 sun kernel: [43451.057489] ploop(24002): a hole in image file detected (0)
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In case there are no other ploop-related warnings or errors immediately after, this one is harmless ({{B|2825}}).
== Userspace warnings ==<!--T:7-->
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You might see something like this (reported by ploop library) during container start or mount:
<!--T:9-->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
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The last line tells that ploop library is going to fix the problem.
== How to fix ==<!--T:11-->
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Since [[Download/ploop/1.10|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 <code>ploop check --repair-sparse</code> to check for and repair such images.
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Alternatelely, you can just do something like this to get rid of holes. Make sure ploop file is not used!
<!--T:14-->cat root.hdd > root.hdd2 && mv root.hdd2 root.hdd
== See also ==<!--T:15-->
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* [[Ploop]]
* [[w:Sparse file]]