Btrfs-progs: add feature to get mininum size for resizing a fs/device

Currently there is not way for a user to know what is the minimum size a
device of a btrfs filesystem can be resized to. Sometimes the value of
total allocated space (sum of all allocated chunks/device extents), which
can be parsed from 'btrfs filesystem show' and 'btrfs filesystem usage',
works as the minimum size, but sometimes it does not, namely when device
extents have to relocated to holes (unallocated space) within the new
size of the device (the total allocated space sum).

This change adds the ability to reliably compute such minimum value and
extents 'btrfs filesystem resize' with the following syntax to get such
value:

   btrfs filesystem resize [devid:]get_min_size

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 69 insertions, 0 deletions

diff --git a/tests/misc-tests/004-shrink-fs/test.sh b/tests/misc-tests/004-shrink-fs/test.sh
new file mode 100755
index 00000000..393cccf5
--- /dev/null
+++ b/tests/misc-tests/004-shrink-fs/test.sh
@@ -0,0 +1,69 @@
+#!/bin/bash
+#
+# Test getting the minimum size a filesystem can be resized to and verify we
+# are able to resize (shrink) it to that size.
+#
+
+source $TOP/tests/common
+
+check_prereq mkfs.btrfs
+setup_root_helper
+
+shrink_test()
+{
+	min_size=$($SUDO_HELPER $TOP/btrfs filesystem resize get_min_size $TEST_MNT)
+	if [ $? != 0 ]; then
+		_fail "Failed to get minimum size"
+	fi
+	min_size=$(echo $min_size | cut -d ' ' -f 1)
+	echo "min size = ${min_size}" >> $RESULTS
+	run_check $SUDO_HELPER $TOP/btrfs filesystem resize $min_size $TEST_MNT
+}
+
+run_check truncate -s 20G $IMAGE
+run_check $TOP/mkfs.btrfs -f $IMAGE
+run_check $SUDO_HELPER mount $IMAGE $TEST_MNT
+run_check $SUDO_HELPER chmod a+rw $TEST_MNT
+
+# Create 7 data block groups, each with a size of 1Gb.
+for ((i = 1; i <= 7; i++)); do
+	run_check fallocate -l 1G $TEST_MNT/foo$i
+done
+
+# Make sure they are persisted (all the chunk, device and block group items
+# added to the chunk/dev/extent trees).
+run_check $TOP/btrfs filesystem sync $TEST_MNT
+
+# Now remove 3 of those 1G files. This will result in 3 block groups becoming
+# unused, which will be automatically deleted by the cleaner kthread, and this
+# will result in 3 holes (unallocated space) in the device (each with a size
+# of 1Gb).
+
+run_check rm -f $TEST_MNT/foo2
+run_check rm -f $TEST_MNT/foo4
+run_check rm -f $TEST_MNT/foo6
+
+# Sync once to wake up the cleaner kthread which will delete the unused block
+# groups - it could have been sleeping when they became unused. Then wait a bit
+# to allow the cleaner kthread to delete them and then finally ensure the
+# transaction started by the cleaner kthread is committed.
+run_check $TOP/btrfs filesystem sync $TEST_MNT
+sleep 3
+run_check $TOP/btrfs filesystem sync $TEST_MNT
+
+# Now attempt to get the minimum size we can resize the filesystem to and verify
+# the resize operation succeeds. This size closely matches the sum of the size
+# of all the allocated device extents.
+for ((i = 1; i <= 3; i++)); do
+	shrink_test
+done
+
+# Now convert metadata and system chunks to the single profile and check we are
+# still able to get a correct minimum size and shrink to that size.
+run_check $SUDO_HELPER $TOP/btrfs balance start -mconvert=single \
+	-sconvert=single -f $TEST_MNT
+for ((i = 1; i <= 3; i++)); do
+	shrink_test
+done
+
+run_check $SUDO_HELPER umount $TEST_MNT