strange No space left on device issues

[Christoph Anton Mitterer <calestyo@scientia.net>]

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Hey.

The following happens on Debian stretch systems:
# uname -a
Linux lcg-lrz-admin 4.9.0-3-amd64 #1 SMP Debian 4.9.30-2+deb9u2 (2017-06-26) x86_64 GNU/Linux

What I have are VMs, which run with root fs as ext4 and which I want to
migrate to btrfs.
So I've added further disk images and then something like this:
- mkfs.btrfs --nodiscard --label system /dev/sdc2 (i.e. the new image)
- mounted that at /mnt
- created a subvol "root" in it
- stopped all services on the node
- remount,ro /
- mount --bind / /media
- cp -a /media/ /mnt/subvol/
- and then I'd go on move everything in place, install bootloader etc.

That used to always work, and does when I try the same with ext4
instead of btrfs on the new images.

But with btrfs I get spurious No space error like:
cp: cannot create regular file
'/mnt/root/X/media/usr/share/doc/openjdk-8-jre-
headless/api/java/security/PrivilegedExceptionAction.html': No space
left on device
cp: cannot create regular file
'/mnt/root/X/media/usr/share/doc/openjdk-8-jre-
headless/api/java/security/Provider.Service.html': No space left on
device
cp: cannot create regular file
'/mnt/root/X/media/usr/share/doc/openjdk-8-jre-
headless/api/javax/script/AbstractScriptEngine.html': No space left on
device

or:
cp: preserving permissions for
‘/mnt/root/X/usr/include/c++/6/gnu/javax/crypto/keyring/BaseKeyring.h’:
No space left on device
cp: preserving permissions for ‘/mnt/root/X/usr/share/doc/cmake-
data/html/variable/CMAKE_CXX_STANDARD_REQUIRED.html’: No space left on
device


All these happen always (when I create a fresh btrfs on the volume and
start over) with different files... and btrfs filesystem df shows
plenty of space left like in terms of >15GB left.


Any ideas?

Cheers,
Chris.

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