initial script version, updated README

README.md

@@ -1,2 +1,46 @@
 # zfs-inplace-rebalancing
 Simple bash script to rebalance pool data between all mirrors when adding vdevs to a pool.
+
+## How it works
+
+This script simply copies all files, deletes the original and renames the copy back to the original name. The given root directory is traversed recursively (using `find`) and each file is processed individually. At no point in time are both versions of the original file deleted.
+
+## Prerequisites
+
+### Balance Status
+
+To check the current balance of a pool use:
+
+```
+zpool list -v
+```
+
+### No Deduplication
+
+Due to the working principle of this script, which essentially creates a duplicate file on purpose, deduplication will most definitely prevent it from working as intended. If you use deduplication you probably have to resort to a more expensive rebalancing method that involves additional drives.
+
+### Data selection
+
+Due to the working principle of this script, it is crucial that you **only run it on data that is not actively accessed**, since the original file will be deleted.
+
+## Usage
+
+**ALWAYS HAVE A BACKUP OF YOUR DATA!**
+
+```
+chmod +x ./zfs-in-place-mirror-rebalance.sh
+./zfs-in-place-mirror-rebalance.sh /pool/path/to/rebalance
+```
+
+Note that this script does **not** have any kind of progress bar (yet), so it might be a good idea to try a small subfolder first, or process your pool folder layout in manually selected badges.
+
+When aborting the script midway through, be sure to check the last lines of its output. When cancelling before or during the renaming process a ".rebalance" file might be left and you have to rename it manually.
+
+## Attributions
+
+This script was inspired by [zfs-balancer](https://github.com/programster/zfs-balancer).
+
+## Disclaimer
+
+This software is provided "as is" and "as available", without any warranty.  
+**ALWAYS HAVE A BACKUP OF YOUR DATA!**

zfs-in-place-mirror-rebalance.sh

@@ -0,0 +1,31 @@
+#!/usr/bin/env bash
+
+# exit script on error
+set -e
+# exit on undeclared variable
+set -u
+
+function rebalance () {
+    file_path=$1
+
+    tmp_extension=".balance"
+
+    echo "${file_path}"
+    echo "Copying '${file_path}' to '${file_path}${tmp_extension}'..."
+    # -a -- keep attributes
+    # -d -- keep symlinks (dont copy target)
+    # -x -- stay on one system
+    # -p -- preserve ACLs too
+    cp -adxp "${file_path}" "${file_path}${tmp_extension}"
+
+    echo "Removing original '${file_path}'..."
+    rm "${file_path}"
+
+    echo "Renaming temporary copy to original '${file_path}'..."
+    mv "${file_path}${tmp_extension}" "${file_path}"
+}
+
+root_path=$1
+# recursively scan through files and execute "rebalance" procedure
+find "$root_path" -type f -print0 | while IFS= read -r -d '' file; do rebalance "$file"; done
+echo Done!