Replace pigz with zstd for backup compression (#6897)

* Replace pigz with zstd for backup compression

This change replaces pigz (parallel gzip) with zstd (Zstandard) as the
compression algorithm for mailcow backups while maintaining full backward
compatibility with existing .tar.gz backups.

Benefits:
- Better compression ratios (12-37% improvement in tests)
- Improved compression speed with modern algorithm
- Maintains rsyncable functionality for incremental backups
- Full backward compatibility for restoring old .tar.gz backups
- Wide industry adoption and active development

Changes:
- Backup compression: pigz --rsyncable -p → zstd --rsyncable -T
- Backup decompression: pigz -d -p → zstd -d -T
- File extensions: .tar.gz → .tar.zst
- Added get_archive_info() function for intelligent format detection
- Updated backup Dockerfile to install zstd alongside pigz
- Restore function now auto-detects and handles both formats
- Updated FILE_SELECTION regex to recognize both .tar.zst and .tar.gz
- Updated comments to reflect new file extension

Backward Compatibility:
- Restore automatically detects .tar.zst (preferred) or .tar.gz (legacy)
- Existing .tar.gz backups can still be restored without issues
- pigz remains installed in backup image for legacy support
- Graceful fallback if backup file format not found

Testing:
- Added comprehensive test suite (test_backup_and_restore.sh)
- 12 automated tests covering all scenarios:
  * Backup creation (both formats)
  * Restore (both formats)
  * Format detection and priority
  * Error handling (missing files, empty dirs)
  * Content integrity verification
  * Multi-threading configuration
  * Large file compression (8.59 MB realistic data)

Test Results:
✓ zstd compression working
✓ pigz compression working (legacy)
✓ zstd decompression working
✓ pigz decompression working (backward compatible)
✓ Archive detection working
✓ Content integrity verified
✓ Format priority correct (.tar.zst preferred)
✓ Error handling for missing files
✓ Error handling for empty directories
✓ Multi-threading configuration verified
✓ Large file compression: 37.05% improvement
✓ Small file compression: 12.18% improvement

* move testing script into development folder

---------

Co-authored-by: DerLinkman <niklas.meyer@servercow.de>

data/Dockerfiles/backup/Dockerfile

@@ -1,3 +1,3 @@
 FROM debian:bookworm-slim
 
-RUN apt update && apt install pigz -y --no-install-recommends
+RUN apt update && apt install pigz zstd -y --no-install-recommends

helper-scripts/backup_and_restore.sh

@@ -110,32 +110,32 @@ function backup() {
       docker run --name mailcow-backup --rm \
         -v ${BACKUP_LOCATION}/mailcow-${DATE}:/backup:z \
         -v $(docker volume ls -qf name=^${CMPS_PRJ}_vmail-vol-1$):/vmail:ro,z \
-        ${DEBIAN_DOCKER_IMAGE} /bin/tar --warning='no-file-ignored' --use-compress-program="pigz --rsyncable -p ${THREADS}" -Pcvpf /backup/backup_vmail.tar.gz /vmail
+        ${DEBIAN_DOCKER_IMAGE} /bin/tar --warning='no-file-ignored' --use-compress-program="zstd --rsyncable -T${THREADS}" -Pcvpf /backup/backup_vmail.tar.zst /vmail
       ;;&
     crypt|all)
       docker run --name mailcow-backup --rm \
         -v ${BACKUP_LOCATION}/mailcow-${DATE}:/backup:z \
         -v $(docker volume ls -qf name=^${CMPS_PRJ}_crypt-vol-1$):/crypt:ro,z \
-        ${DEBIAN_DOCKER_IMAGE} /bin/tar --warning='no-file-ignored' --use-compress-program="pigz --rsyncable -p ${THREADS}" -Pcvpf /backup/backup_crypt.tar.gz /crypt
+        ${DEBIAN_DOCKER_IMAGE} /bin/tar --warning='no-file-ignored' --use-compress-program="zstd --rsyncable -T${THREADS}" -Pcvpf /backup/backup_crypt.tar.zst /crypt
       ;;&
     redis|all)
       docker exec $(docker ps -qf name=redis-mailcow) redis-cli -a ${REDISPASS} --no-auth-warning save
       docker run --name mailcow-backup --rm \
         -v ${BACKUP_LOCATION}/mailcow-${DATE}:/backup:z \
         -v $(docker volume ls -qf name=^${CMPS_PRJ}_redis-vol-1$):/redis:ro,z \
-        ${DEBIAN_DOCKER_IMAGE} /bin/tar --warning='no-file-ignored' --use-compress-program="pigz --rsyncable -p ${THREADS}" -Pcvpf /backup/backup_redis.tar.gz /redis
+        ${DEBIAN_DOCKER_IMAGE} /bin/tar --warning='no-file-ignored' --use-compress-program="zstd --rsyncable -T${THREADS}" -Pcvpf /backup/backup_redis.tar.zst /redis
       ;;&
     rspamd|all)
       docker run --name mailcow-backup --rm \
         -v ${BACKUP_LOCATION}/mailcow-${DATE}:/backup:z \
         -v $(docker volume ls -qf name=^${CMPS_PRJ}_rspamd-vol-1$):/rspamd:ro,z \
-        ${DEBIAN_DOCKER_IMAGE} /bin/tar --warning='no-file-ignored' --use-compress-program="pigz --rsyncable -p ${THREADS}" -Pcvpf /backup/backup_rspamd.tar.gz /rspamd
+        ${DEBIAN_DOCKER_IMAGE} /bin/tar --warning='no-file-ignored' --use-compress-program="zstd --rsyncable -T${THREADS}" -Pcvpf /backup/backup_rspamd.tar.zst /rspamd
       ;;&
     postfix|all)
       docker run --name mailcow-backup --rm \
         -v ${BACKUP_LOCATION}/mailcow-${DATE}:/backup:z \
         -v $(docker volume ls -qf name=^${CMPS_PRJ}_postfix-vol-1$):/postfix:ro,z \
-        ${DEBIAN_DOCKER_IMAGE} /bin/tar --warning='no-file-ignored' --use-compress-program="pigz --rsyncable -p ${THREADS}" -Pcvpf /backup/backup_postfix.tar.gz /postfix
+        ${DEBIAN_DOCKER_IMAGE} /bin/tar --warning='no-file-ignored' --use-compress-program="zstd --rsyncable -T${THREADS}" -Pcvpf /backup/backup_postfix.tar.zst /postfix
       ;;&
     mysql|all)
       SQLIMAGE=$(grep -iEo '(mysql|mariadb)\:.+' ${COMPOSE_FILE})
@@ -154,7 +154,7 @@ function backup() {
           ${SQLIMAGE} /bin/sh -c "mariabackup --host mysql --user root --password ${DBROOT} --backup --rsync --target-dir=/backup_mariadb ; \
           mariabackup --prepare --target-dir=/backup_mariadb ; \
           chown -R 999:999 /backup_mariadb ; \
-          /bin/tar --warning='no-file-ignored' --use-compress-program='gzip --rsyncable' -Pcvpf /backup/backup_mariadb.tar.gz /backup_mariadb ;"
+          /bin/tar --warning='no-file-ignored' --use-compress-program='zstd --rsyncable' -Pcvpf /backup/backup_mariadb.tar.zst /backup_mariadb ;"
       fi
       ;;&
     --delete-days)
@@ -170,6 +170,19 @@ function backup() {
   done
 }
 
+function get_archive_info() {
+  local backup_name="$1"
+  local location="$2"
+
+  if [[ -f "${location}/${backup_name}.tar.zst" ]]; then
+    echo "${backup_name}.tar.zst|zstd -d -T${THREADS}"
+  elif [[ -f "${location}/${backup_name}.tar.gz" ]]; then
+    echo "${backup_name}.tar.gz|pigz -d -p ${THREADS}"
+  else
+    echo ""
+  fi
+}
+
 function restore() {
   for bin in docker; do
   if [[ -z $(which ${bin}) ]]; then
@@ -199,10 +212,17 @@ function restore() {
     case "$1" in
     vmail)
       docker stop $(docker ps -qf name=dovecot-mailcow)
-      docker run -i --name mailcow-backup --rm \
-        -v ${RESTORE_LOCATION}:/backup:z \
-        -v $(docker volume ls -qf name=^${CMPS_PRJ}_vmail-vol-1$):/vmail:z \
-        ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="pigz -d -p ${THREADS}" -Pxvf /backup/backup_vmail.tar.gz
+      ARCHIVE_INFO=$(get_archive_info "backup_vmail" "${RESTORE_LOCATION}")
+      if [[ -z "${ARCHIVE_INFO}" ]]; then
+        echo -e "\e[31mError: No backup file found for vmail (searched for .tar.zst and .tar.gz)\e[0m"
+      else
+        ARCHIVE_FILE=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f1)
+        DECOMPRESS_PROG=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f2)
+        docker run -i --name mailcow-backup --rm \
+          -v ${RESTORE_LOCATION}:/backup:z \
+          -v $(docker volume ls -qf name=^${CMPS_PRJ}_vmail-vol-1$):/vmail:z \
+          ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="${DECOMPRESS_PROG}" -Pxvf /backup/${ARCHIVE_FILE}
+      fi
       docker start $(docker ps -aqf name=dovecot-mailcow)
       echo
       echo "In most cases it is not required to run a full resync, you can run the command printed below at any time after testing wether the restore process broke a mailbox:"
@@ -218,31 +238,50 @@ function restore() {
       ;;
     redis)
       docker stop $(docker ps -qf name=redis-mailcow)
-      docker run -i --name mailcow-backup --rm \
-        -v ${RESTORE_LOCATION}:/backup:z \
-        -v $(docker volume ls -qf name=^${CMPS_PRJ}_redis-vol-1$):/redis:z \
-        ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="pigz -d -p ${THREADS}" -Pxvf /backup/backup_redis.tar.gz
+      ARCHIVE_INFO=$(get_archive_info "backup_redis" "${RESTORE_LOCATION}")
+      if [[ -z "${ARCHIVE_INFO}" ]]; then
+        echo -e "\e[31mError: No backup file found for redis (searched for .tar.zst and .tar.gz)\e[0m"
+      else
+        ARCHIVE_FILE=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f1)
+        DECOMPRESS_PROG=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f2)
+        docker run -i --name mailcow-backup --rm \
+          -v ${RESTORE_LOCATION}:/backup:z \
+          -v $(docker volume ls -qf name=^${CMPS_PRJ}_redis-vol-1$):/redis:z \
+          ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="${DECOMPRESS_PROG}" -Pxvf /backup/${ARCHIVE_FILE}
+      fi
       docker start $(docker ps -aqf name=redis-mailcow)
       ;;
     crypt)
       docker stop $(docker ps -qf name=dovecot-mailcow)
-      docker run -i --name mailcow-backup --rm \
-        -v ${RESTORE_LOCATION}:/backup:z \
-        -v $(docker volume ls -qf name=^${CMPS_PRJ}_crypt-vol-1$):/crypt:z \
-        ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="pigz -d -p ${THREADS}" -Pxvf /backup/backup_crypt.tar.gz
+      ARCHIVE_INFO=$(get_archive_info "backup_crypt" "${RESTORE_LOCATION}")
+      if [[ -z "${ARCHIVE_INFO}" ]]; then
+        echo -e "\e[31mError: No backup file found for crypt (searched for .tar.zst and .tar.gz)\e[0m"
+      else
+        ARCHIVE_FILE=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f1)
+        DECOMPRESS_PROG=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f2)
+        docker run -i --name mailcow-backup --rm \
+          -v ${RESTORE_LOCATION}:/backup:z \
+          -v $(docker volume ls -qf name=^${CMPS_PRJ}_crypt-vol-1$):/crypt:z \
+          ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="${DECOMPRESS_PROG}" -Pxvf /backup/${ARCHIVE_FILE}
+      fi
       docker start $(docker ps -aqf name=dovecot-mailcow)
       ;;
     rspamd)
-      if [[ $(find "${RESTORE_LOCATION}" \( -name '*x86*' -o -name '*aarch*' \) -exec basename {} \; | sed 's/^\.//' | sed 's/^\.//') == "" ]]; then
+      ARCHIVE_INFO=$(get_archive_info "backup_rspamd" "${RESTORE_LOCATION}")
+      if [[ -z "${ARCHIVE_INFO}" ]]; then
+        echo -e "\e[31mError: No backup file found for rspamd (searched for .tar.zst and .tar.gz)\e[0m"
+      elif [[ $(find "${RESTORE_LOCATION}" \( -name '*x86*' -o -name '*aarch*' \) -exec basename {} \; | sed 's/^\.//' | sed 's/^\.//') == "" ]]; then
         echo -e "\e[33mCould not find a architecture signature of the loaded backup... Maybe the backup was done before the multiarch update?"
         sleep 2
         echo -e "Continuing anyhow. If rspamd is crashing upon boot try remove the rspamd volume with docker volume rm ${CMPS_PRJ}_rspamd-vol-1 after you've stopped the stack.\e[0m"
         sleep 2
         docker stop $(docker ps -qf name=rspamd-mailcow)
+        ARCHIVE_FILE=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f1)
+        DECOMPRESS_PROG=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f2)
         docker run -i --name mailcow-backup --rm \
           -v ${RESTORE_LOCATION}:/backup:z \
           -v $(docker volume ls -qf name=^${CMPS_PRJ}_rspamd-vol-1$):/rspamd:z \
-          ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="pigz -d -p ${THREADS}" -Pxvf /backup/backup_rspamd.tar.gz
+          ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="${DECOMPRESS_PROG}" -Pxvf /backup/${ARCHIVE_FILE}
         docker start $(docker ps -aqf name=rspamd-mailcow)
       elif [[ $ARCH != $(find "${RESTORE_LOCATION}" \( -name '*x86*' -o -name '*aarch*' \) -exec basename {} \; | sed 's/^\.//' | sed 's/^\.//') ]]; then
         echo -e "\e[31mThe Architecture of the backed up mailcow OS is different then your restoring mailcow OS..."
@@ -250,19 +289,28 @@ function restore() {
         echo -e "Skipping rspamd due to compatibility issues!\e[0m"
       else
         docker stop $(docker ps -qf name=rspamd-mailcow)
+        ARCHIVE_FILE=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f1)
+        DECOMPRESS_PROG=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f2)
         docker run -i --name mailcow-backup --rm \
           -v ${RESTORE_LOCATION}:/backup:z \
           -v $(docker volume ls -qf name=^${CMPS_PRJ}_rspamd-vol-1$):/rspamd:z \
-          ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="pigz -d -p ${THREADS}" -Pxvf /backup/backup_rspamd.tar.gz
+          ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="${DECOMPRESS_PROG}" -Pxvf /backup/${ARCHIVE_FILE}
         docker start $(docker ps -aqf name=rspamd-mailcow)
       fi
       ;;
     postfix)
       docker stop $(docker ps -qf name=postfix-mailcow)
-      docker run -i --name mailcow-backup --rm \
-        -v ${RESTORE_LOCATION}:/backup:z \
-        -v $(docker volume ls -qf name=^${CMPS_PRJ}_postfix-vol-1$):/postfix:z \
-        ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="pigz -d -p ${THREADS}" -Pxvf /backup/backup_postfix.tar.gz
+      ARCHIVE_INFO=$(get_archive_info "backup_postfix" "${RESTORE_LOCATION}")
+      if [[ -z "${ARCHIVE_INFO}" ]]; then
+        echo -e "\e[31mError: No backup file found for postfix (searched for .tar.zst and .tar.gz)\e[0m"
+      else
+        ARCHIVE_FILE=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f1)
+        DECOMPRESS_PROG=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f2)
+        docker run -i --name mailcow-backup --rm \
+          -v ${RESTORE_LOCATION}:/backup:z \
+          -v $(docker volume ls -qf name=^${CMPS_PRJ}_postfix-vol-1$):/postfix:z \
+          ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="${DECOMPRESS_PROG}" -Pxvf /backup/${ARCHIVE_FILE}
+      fi
       docker start $(docker ps -aqf name=postfix-mailcow)
       ;;
     mysql|mariadb)
@@ -305,14 +353,19 @@ function restore() {
           echo Restoring... && \
           gunzip < backup/backup_mysql.gz | mysql -uroot && \
           mysql -uroot -e SHUTDOWN;"
-        elif [[ -f "${RESTORE_LOCATION}/backup_mariadb.tar.gz" ]]; then
-        docker run --name mailcow-backup --rm \
-          -v $(docker volume ls -qf name=^${CMPS_PRJ}_mysql-vol-1$):/backup_mariadb/:rw,z \
-          --entrypoint= \
-          -v ${RESTORE_LOCATION}:/backup:z \
-          ${SQLIMAGE} /bin/bash -c "shopt -s dotglob ; \
-            /bin/rm -rf /backup_mariadb/* ; \
-            /bin/tar -Pxvzf /backup/backup_mariadb.tar.gz"
+        else
+          ARCHIVE_INFO=$(get_archive_info "backup_mariadb" "${RESTORE_LOCATION}")
+          if [[ -n "${ARCHIVE_INFO}" ]]; then
+            ARCHIVE_FILE=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f1)
+            DECOMPRESS_PROG=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f2)
+            docker run --name mailcow-backup --rm \
+              -v $(docker volume ls -qf name=^${CMPS_PRJ}_mysql-vol-1$):/backup_mariadb/:rw,z \
+              --entrypoint= \
+              -v ${RESTORE_LOCATION}:/backup:z \
+              ${SQLIMAGE} /bin/bash -c "shopt -s dotglob ; \
+                /bin/rm -rf /backup_mariadb/* ; \
+                /bin/tar --use-compress-program='${DECOMPRESS_PROG}' -Pxvf /backup/${ARCHIVE_FILE}"
+          fi
         fi
         echo "Modifying mailcow.conf..."
         source ${RESTORE_LOCATION}/mailcow.conf
@@ -363,8 +416,8 @@ elif [[ ${1} == "restore" ]]; then
   fi
 
   echo "[ 0 ] - all"
-  # find all files in folder with *.gz extension, print their base names, remove backup_, remove .tar (if present), remove .gz
-  FILE_SELECTION[0]=$(find "${FOLDER_SELECTION[${input_sel}]}" -maxdepth 1 \( -type d -o -type f \) \( -name '*.gz' -o -name 'mysql' \) -printf '%f\n' | sed 's/backup_*//' | sed 's/\.[^.]*$//' | sed 's/\.[^.]*$//')
+  # find all files in folder with *.zst or *.gz extension, print their base names, remove backup_, remove .tar (if present), remove .zst/.gz
+  FILE_SELECTION[0]=$(find "${FOLDER_SELECTION[${input_sel}]}" -maxdepth 1 \( -type d -o -type f \) \( -name '*.zst' -o -name '*.gz' -o -name 'mysql' \) -printf '%f\n' | sed 's/backup_*//' | sed 's/\.[^.]*$//' | sed 's/\.[^.]*$//' | sort -u)
   for file in $(ls -f "${FOLDER_SELECTION[${input_sel}]}"); do
     if [[ ${file} =~ vmail ]]; then
       echo "[ ${i} ] - Mail directory (/var/vmail)"

helper-scripts/dev_tests/test_backup_and_restore.sh

@@ -0,0 +1,301 @@
+#!/usr/bin/env bash
+
+# Test script for backup_and_restore.sh
+# Tests backward compatibility with .tar.gz and new .tar.zst format
+
+set -e
+
+SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+BACKUP_IMAGE="${BACKUP_IMAGE:-ghcr.io/mailcow/backup:latest}"
+TEST_DIR="/tmp/mailcow_backup_test_$$"
+THREADS=2
+
+echo "=== Mailcow Backup & Restore Test Suite ==="
+echo "Test directory: ${TEST_DIR}"
+echo "Backup image: ${BACKUP_IMAGE}"
+echo ""
+
+# Cleanup function
+cleanup() {
+  echo "Cleaning up test files..."
+  rm -rf "${TEST_DIR}"
+  docker rmi mailcow-backup-test 2>/dev/null || true
+}
+trap cleanup EXIT
+
+# Create test directory structure
+mkdir -p "${TEST_DIR}"/{test_data,backup_zst,backup_gz,restore_zst,restore_gz,backup_large_zst,backup_large_gz}
+echo "Test data for mailcow backup compatibility test" > "${TEST_DIR}/test_data/test.txt"
+echo "Additional file to verify complete restore" > "${TEST_DIR}/test_data/test2.txt"
+
+# Build test backup image with zstd support
+echo "=== Building backup image with zstd support ==="
+docker build -t mailcow-backup-test "${SCRIPT_DIR}/../data/Dockerfiles/backup/" || {
+  echo "ERROR: Failed to build backup image"
+  exit 1
+}
+
+# Test 1: Create .tar.zst backup
+echo ""
+echo "=== Test 1: Creating .tar.zst backup ==="
+docker run --rm \
+  -w /data \
+  -v "${TEST_DIR}/test_data:/data:ro" \
+  -v "${TEST_DIR}/backup_zst:/backup" \
+  mailcow-backup-test \
+  /bin/tar --use-compress-program="zstd --rsyncable -T${THREADS}" \
+  -cvpf /backup/backup_test.tar.zst . \
+  > /dev/null
+echo "✓ .tar.zst backup created: $(ls -lh ${TEST_DIR}/backup_zst/backup_test.tar.zst | awk '{print $5}')"
+
+# Test 2: Create .tar.gz backup
+echo ""
+echo "=== Test 2: Creating .tar.gz backup (legacy) ==="
+docker run --rm \
+  -w /data \
+  -v "${TEST_DIR}/test_data:/data:ro" \
+  -v "${TEST_DIR}/backup_gz:/backup" \
+  mailcow-backup-test \
+  /bin/tar --use-compress-program="pigz --rsyncable -p ${THREADS}" \
+  -cvpf /backup/backup_test.tar.gz . \
+  > /dev/null
+echo "✓ .tar.gz backup created: $(ls -lh ${TEST_DIR}/backup_gz/backup_test.tar.gz | awk '{print $5}')"
+
+# Test 3: Test get_archive_info function
+echo ""
+echo "=== Test 3: Testing get_archive_info function ==="
+
+# Extract and test the function directly
+get_archive_info() {
+  local backup_name="$1"
+  local location="$2"
+
+  if [[ -f "${location}/${backup_name}.tar.zst" ]]; then
+    echo "${backup_name}.tar.zst|zstd -d -T${THREADS}"
+  elif [[ -f "${location}/${backup_name}.tar.gz" ]]; then
+    echo "${backup_name}.tar.gz|pigz -d -p ${THREADS}"
+  else
+    echo ""
+  fi
+}
+
+# Test with .tar.zst
+result=$(get_archive_info "backup_test" "${TEST_DIR}/backup_zst")
+if [[ "${result}" =~ "zstd" ]]; then
+  echo "✓ Correctly detects .tar.zst and returns zstd decompressor"
+else
+  echo "✗ Failed to detect .tar.zst"
+  exit 1
+fi
+
+# Test with .tar.gz
+result=$(get_archive_info "backup_test" "${TEST_DIR}/backup_gz")
+if [[ "${result}" =~ "pigz" ]]; then
+  echo "✓ Correctly detects .tar.gz and returns pigz decompressor"
+else
+  echo "✗ Failed to detect .tar.gz"
+  exit 1
+fi
+
+# Test with no file
+result=$(get_archive_info "backup_test" "${TEST_DIR}")
+if [[ -z "${result}" ]]; then
+  echo "✓ Correctly returns empty when no backup file found"
+else
+  echo "✗ Should return empty but got: ${result}"
+  exit 1
+fi
+
+# Test 4: Restore from .tar.zst
+echo ""
+echo "=== Test 4: Restoring from .tar.zst ==="
+docker run --rm \
+  -w /restore \
+  -v "${TEST_DIR}/backup_zst:/backup:ro" \
+  -v "${TEST_DIR}/restore_zst:/restore" \
+  mailcow-backup-test \
+  /bin/tar --use-compress-program="zstd -d -T${THREADS}" -xvpf /backup/backup_test.tar.zst \
+  > /dev/null 2>&1
+
+if [[ -f "${TEST_DIR}/restore_zst/test.txt" ]] && \
+   [[ -f "${TEST_DIR}/restore_zst/test2.txt" ]]; then
+  echo "✓ Successfully restored from .tar.zst"
+else
+  echo "✗ Failed to restore from .tar.zst"
+  ls -la "${TEST_DIR}/restore_zst/" || true
+  exit 1
+fi
+
+# Test 5: Restore from .tar.gz
+echo ""
+echo "=== Test 5: Restoring from .tar.gz (backward compatibility) ==="
+docker run --rm \
+  -w /restore \
+  -v "${TEST_DIR}/backup_gz:/backup:ro" \
+  -v "${TEST_DIR}/restore_gz:/restore" \
+  mailcow-backup-test \
+  /bin/tar --use-compress-program="pigz -d -p ${THREADS}" -xvpf /backup/backup_test.tar.gz \
+  > /dev/null 2>&1
+
+if [[ -f "${TEST_DIR}/restore_gz/test.txt" ]] && \
+   [[ -f "${TEST_DIR}/restore_gz/test2.txt" ]]; then
+  echo "✓ Successfully restored from .tar.gz (backward compatible)"
+else
+  echo "✗ Failed to restore from .tar.gz"
+  ls -la "${TEST_DIR}/restore_gz/" || true
+  exit 1
+fi
+
+# Test 6: Verify content integrity
+echo ""
+echo "=== Test 6: Verifying content integrity ==="
+original_content=$(cat "${TEST_DIR}/test_data/test.txt")
+zst_content=$(cat "${TEST_DIR}/restore_zst/test.txt")
+gz_content=$(cat "${TEST_DIR}/restore_gz/test.txt")
+
+if [[ "${original_content}" == "${zst_content}" ]] && \
+   [[ "${original_content}" == "${gz_content}" ]]; then
+  echo "✓ Content integrity verified for both formats"
+else
+  echo "✗ Content mismatch detected"
+  exit 1
+fi
+
+# Test 7: Compare compression ratios
+echo ""
+echo "=== Test 7: Compression comparison ==="
+zst_size=$(stat -f%z "${TEST_DIR}/backup_zst/backup_test.tar.zst" 2>/dev/null || stat -c%s "${TEST_DIR}/backup_zst/backup_test.tar.zst")
+gz_size=$(stat -f%z "${TEST_DIR}/backup_gz/backup_test.tar.gz" 2>/dev/null || stat -c%s "${TEST_DIR}/backup_gz/backup_test.tar.gz")
+improvement=$(echo "scale=2; (${gz_size} - ${zst_size}) * 100 / ${gz_size}" | bc)
+
+echo "  Small files - .tar.gz size: ${gz_size} bytes"
+echo "  Small files - .tar.zst size: ${zst_size} bytes"
+echo "  Small files - Improvement: ${improvement}% smaller with zstd"
+
+# Test 8: Error handling - missing backup file
+echo ""
+echo "=== Test 8: Error handling - Missing backup file ==="
+result=$(get_archive_info "nonexistent_backup" "${TEST_DIR}/backup_zst")
+if [[ -z "${result}" ]]; then
+  echo "✓ Correctly handles missing backup files"
+else
+  echo "✗ Should return empty for missing files"
+  exit 1
+fi
+
+# Test 9: Error handling - Empty directory
+echo ""
+echo "=== Test 9: Error handling - Empty directory ==="
+mkdir -p "${TEST_DIR}/empty_dir"
+result=$(get_archive_info "backup_test" "${TEST_DIR}/empty_dir")
+if [[ -z "${result}" ]]; then
+  echo "✓ Correctly handles empty directories"
+else
+  echo "✗ Should return empty for empty directories"
+  exit 1
+fi
+
+# Test 10: Priority test - .tar.zst preferred over .tar.gz
+echo ""
+echo "=== Test 10: Format priority - .tar.zst preferred ==="
+mkdir -p "${TEST_DIR}/both_formats"
+touch "${TEST_DIR}/both_formats/backup_test.tar.gz"
+touch "${TEST_DIR}/both_formats/backup_test.tar.zst"
+result=$(get_archive_info "backup_test" "${TEST_DIR}/both_formats")
+if [[ "${result}" =~ "zstd" ]]; then
+  echo "✓ Correctly prefers .tar.zst when both formats exist"
+else
+  echo "✗ Should prefer .tar.zst over .tar.gz"
+  exit 1
+fi
+
+# Test 11: Large file compression test
+echo ""
+echo "=== Test 11: Large file compression test ==="
+mkdir -p "${TEST_DIR}/large_data"
+# Create ~10MB of compressible data (log-like content)
+for i in {1..50000}; do
+  echo "[$(date '+%Y-%m-%d %H:%M:%S')] INFO: Processing email message $i from user@example.com to recipient@domain.com" >> "${TEST_DIR}/large_data/maillog.txt"
+  echo "[$(date '+%Y-%m-%d %H:%M:%S')] DEBUG: SMTP connection established from 192.168.1.$((i % 255))" >> "${TEST_DIR}/large_data/maillog.txt"
+done 2>/dev/null
+
+# Get size (portable: works on Linux and macOS)
+if du --version 2>/dev/null | grep -q GNU; then
+  original_size=$(du -sb "${TEST_DIR}/large_data" | cut -f1)
+else
+  # macOS
+  original_size=$(find "${TEST_DIR}/large_data" -type f -exec stat -f%z {} \; | awk '{sum+=$1} END {print sum}')
+fi
+echo "  Original data size: $(echo "scale=2; ${original_size} / 1024 / 1024" | bc) MB"
+
+# Backup with zstd
+docker run --rm \
+  -w /data \
+  -v "${TEST_DIR}/large_data:/data:ro" \
+  -v "${TEST_DIR}/backup_large_zst:/backup" \
+  mailcow-backup-test \
+  /bin/tar --use-compress-program="zstd --rsyncable -T${THREADS}" \
+  -cvpf /backup/backup_large.tar.zst . \
+  > /dev/null 2>&1
+
+# Backup with pigz
+docker run --rm \
+  -w /data \
+  -v "${TEST_DIR}/large_data:/data:ro" \
+  -v "${TEST_DIR}/backup_large_gz:/backup" \
+  mailcow-backup-test \
+  /bin/tar --use-compress-program="pigz --rsyncable -p ${THREADS}" \
+  -cvpf /backup/backup_large.tar.gz . \
+  > /dev/null 2>&1
+
+zst_large_size=$(stat -f%z "${TEST_DIR}/backup_large_zst/backup_large.tar.zst" 2>/dev/null || stat -c%s "${TEST_DIR}/backup_large_zst/backup_large.tar.zst" 2>/dev/null || echo "0")
+gz_large_size=$(stat -f%z "${TEST_DIR}/backup_large_gz/backup_large.tar.gz" 2>/dev/null || stat -c%s "${TEST_DIR}/backup_large_gz/backup_large.tar.gz" 2>/dev/null || echo "0")
+
+if [[ ${zst_large_size} -gt 0 ]] && [[ ${gz_large_size} -gt 0 ]]; then
+  large_improvement=$(echo "scale=2; (${gz_large_size} - ${zst_large_size}) * 100 / ${gz_large_size}" | bc)
+
+  echo "  .tar.gz compressed: $(echo "scale=2; ${gz_large_size} / 1024 / 1024" | bc) MB"
+  echo "  .tar.zst compressed: $(echo "scale=2; ${zst_large_size} / 1024 / 1024" | bc) MB"
+  echo "  Improvement: ${large_improvement}% smaller with zstd"
+else
+  echo "  ✗ Failed to get file sizes"
+  exit 1
+fi
+
+if [[ $(echo "${large_improvement} > 0" | bc) -eq 1 ]]; then
+  echo "✓ zstd provides better compression on realistic data"
+else
+  echo "⚠ zstd compression similar or worse than gzip (unusual but not critical)"
+fi
+
+# Test 12: Thread scaling test
+echo ""
+echo "=== Test 12: Multi-threading verification ==="
+# This test verifies that different thread counts work (not measuring speed difference)
+for thread_count in 1 4; do
+  THREADS=${thread_count}
+  result=$(get_archive_info "backup_test" "${TEST_DIR}/backup_zst")
+  if [[ "${result}" =~ "-T${thread_count}" ]]; then
+    echo "✓ Thread count ${thread_count} correctly configured"
+  else
+    echo "✗ Thread count not properly applied"
+    exit 1
+  fi
+done
+
+echo ""
+echo "=== All tests passed! ==="
+echo ""
+echo "Summary:"
+echo "  ✓ zstd compression working"
+echo "  ✓ pigz compression working (legacy)"
+echo "  ✓ zstd decompression working"
+echo "  ✓ pigz decompression working (backward compatible)"
+echo "  ✓ Archive detection working"
+echo "  ✓ Content integrity verified"
+echo "  ✓ Format priority correct (.tar.zst preferred)"
+echo "  ✓ Error handling for missing files"
+echo "  ✓ Error handling for empty directories"
+echo "  ✓ Multi-threading configuration verified"
+echo "  ✓ Large file compression: ${large_improvement}% improvement"
+echo "  ✓ Small file compression: ${improvement}% improvement"