From f6e7c471f5a6d9fb1c9defc514d3b668a9cb9f8f Mon Sep 17 00:00:00 2001
From: Andrew Cady <d@jerkface.net>
Date: Mon, 12 Oct 2020 14:59:59 -0400
Subject: btrfs-shrink

---
 btrfs-shrink | 101 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 101 insertions(+)
 create mode 100755 btrfs-shrink

diff --git a/btrfs-shrink b/btrfs-shrink
new file mode 100755
index 0000000..1d059ff
--- /dev/null
+++ b/btrfs-shrink
@@ -0,0 +1,101 @@
+#!/bin/sh
+
+die() { printf "%s: Error: %s\n" "$0" "$*" >&2; exit 1; }
+
+shrink()
+{
+    local shrinkmegs=100 mountpoint="$1"
+    while true; do
+        while ! btrfs filesystem resize -${shrinkmegs}M "$mountpoint"/; do
+            shrinkmegs=$((shrinkmegs - 10 ))
+            if [ $shrinkmegs -lt 10 ]; then
+                return
+            fi
+        done
+    done
+}
+
+smart_shrink()
+{
+    local min_dev_size mountpoint="$1"
+    if min_dev_size=$(btrfs inspect-internal min-dev-size --id 1 "$mountpoint") && [ "$min_dev_size" ]
+    then
+        set -- $min_dev_size
+        [ "$2" = bytes ] || die "Unexpected output from btrfs inspect-internal min-dev-size: '$min_dev_size'"
+        btrfs filesystem resize "$1" "$mountpoint" && return
+    fi
+    shrink "$mountpoint"
+}
+
+btrfs_truncate()
+{
+    local img="$1" bytes
+#   548044800/1176715264 bytes used
+    bytes=$(file "$img" | sed -ne 's?.*/\([0-9]*\) bytes used.*?\1?p')
+    if [ "$bytes" ]; then
+        truncate -s "$bytes" "$img"
+    fi
+}
+
+parse_btrfs_dev_size()
+{
+    local dev="$1"
+    sed -n 's?.* size \([0-9]*\) .* path \([^ ]*\)$?\1 \2? p' |
+        while read size name
+        do [ "$name" = "$dev" ] || continue
+           echo $size
+           break
+        done
+}
+
+get_truncate_size()
+{
+    local imgfile="$1" mountpoint="$2" loop_dev dev_size
+    loop_dev=$(losetup -ONAME --raw -nj "$imgfile")
+    btrfs fi sh --raw "$mountpoint" | parse_btrfs_dev_size "$loop_dev"
+}
+
+main()
+{
+    [ "$(id -u)" = 0 ] || die 'you are not root'
+    if [ -d "$1" ]; then
+        mountpoint=$1
+        mountpoint -q "$mountpoint" || die "not a mountpoint: $1"
+        smart_shrink "$mountpoint"
+    elif [ -f "$1" ]; then
+        truncate=
+        mountpoint="$1".mnt.tmp
+        mkdir -p "$mountpoint"
+        mount -t btrfs "$1" "$mountpoint"
+        result=$?
+
+        if [ $result = 0 ]; then
+            smart_shrink "$mountpoint"
+            result=$?
+            if [ $result = 0 ]
+            then
+                truncate=$(get_truncate_size "$1" "$mountpoint")
+            fi
+            umount "$mountpoint"
+        fi
+        rmdir "$mountpoint"
+        if [ "$truncate" ]
+        then
+            truncate -s "$truncate" "$1"
+        fi
+        return $result
+    else
+        die "not a file or directory: $1"
+    fi
+}
+
+usage()
+{
+    echo "Usage: $0 <mountpoint>" >&2
+}
+
+case $# in
+    0) usage ;;
+    1) main "$1" ;;
+    *) usage; exit 1 ;;
+esac
-- 
cgit v1.2.3