diff --git a/cdssync/AGENTS.md b/cdssync/AGENTS.md
index b5a1731..3e01c73 100644
--- a/cdssync/AGENTS.md
+++ b/cdssync/AGENTS.md
@@ -15,6 +15,10 @@ For migration test datasets in this workspace, follow this process by default:
 
 - Use `/home/aw/code/cds/cdssync/generate_migration_test_dataset.sh` to create the dataset unless the user explicitly asks for a different method.
 - Prefer `/home/aw/code/cds/cdssync/migration-test-dataset` as the local canonical dataset location unless the user specifies another target.
+- The generator script accepts an optional `UPDATE_INTERVAL_SECONDS` argument:
+  - omit it to create the dataset once and exit
+  - use `0` for continuous random content updates
+  - use any integer greater than `0` to rewrite mutable files every `N` seconds
 - If ACL/xattr coverage matters, ensure the generation host has:
   - `acl` installed for `setfacl` and `getfacl`
   - `attr` installed for `setfattr` and `getfattr`
diff --git a/cdssync/generate_migration_test_dataset.sh b/cdssync/generate_migration_test_dataset.sh
index 61ce13c..0b3f946 100755
--- a/cdssync/generate_migration_test_dataset.sh
+++ b/cdssync/generate_migration_test_dataset.sh
@@ -5,7 +5,7 @@ set -euo pipefail
 usage() {
   cat <<'EOF'
 Usage:
-  generate_migration_test_dataset.sh TARGET_DIR
+  generate_migration_test_dataset.sh TARGET_DIR [UPDATE_INTERVAL_SECONDS]
 
 Creates a compact filesystem migration test dataset under TARGET_DIR.
 The dataset matches the manifest in migration-test-manifest.md.
@@ -14,16 +14,28 @@ Notes:
   - Existing TARGET_DIR contents are left in place unless they collide.
   - ACL and xattr cases are created only if the local tools are available.
   - Sparse files are created with logical size but low physical allocation.
+  - If UPDATE_INTERVAL_SECONDS is provided, the script keeps rewriting
+    mutable files with random content after the initial dataset creation.
+  - An interval of 0 means continuous updates with no sleep between passes.
+  - Update mode rewrites content-bearing regular files only.
+  - Update mode does not rewrite script files, sparse files, symlinks,
+    hard links, or empty files.
 EOF
 }
 
-if [[ $# -ne 1 ]]; then
+if [[ $# -lt 1 || $# -gt 2 ]]; then
   usage
   exit 1
 fi
 
 TARGET_DIR=$1
 ROOT=$(realpath -m "$TARGET_DIR")
+UPDATE_INTERVAL=${2:-}
+
+if [[ -n "$UPDATE_INTERVAL" && ! "$UPDATE_INTERVAL" =~ ^[0-9]+$ ]]; then
+  echo "UPDATE_INTERVAL_SECONDS must be a non-negative integer" >&2
+  exit 1
+fi
 
 mkdir -p "$ROOT"
 
@@ -42,6 +54,10 @@ create_dir() {
   mkdir -p "$ROOT/$1"
 }
 
+log() {
+  printf '[%s] %s\n' "$(date '+%Y-%m-%d %H:%M:%S')" "$*"
+}
+
 set_times() {
   local rel=$1
   local stamp=$2
@@ -114,6 +130,96 @@ apply_mode() {
   chmod "$2" "$ROOT/$1"
 }
 
+set_acl_and_xattr_metadata() {
+  if (( have_setfattr )); then
+    setfattr -n user.migration_case -v "xattr-text" "$ROOT/metadata/xattr_text_1mb_644.txt"
+    setfattr -n user.migration_case -v "xattr-random" "$ROOT/metadata/xattr_random_3mb_600.bin"
+  else
+    echo "Skipping xattr assignment: setfattr not available"
+  fi
+
+  if (( have_setfacl )); then
+    setfacl -m u:nobody:r-- "$ROOT/metadata/acl_text_1mb_644.txt"
+    setfacl -m u:nobody:r-x "$ROOT/metadata/acl_script_1mb_755.sh"
+  else
+    echo "Skipping ACL assignment: setfacl not available"
+  fi
+}
+
+rewrite_file_with_random_data() {
+  local rel=$1
+  local path="$ROOT/$rel"
+  local size
+  local mode
+
+  size=$(stat -c '%s' "$path")
+  mode=$(stat -c '%a' "$path")
+
+  chmod u+w "$path"
+  if (( size > 0 )); then
+    head -c "$size" /dev/urandom >"$path"
+  else
+    : >"$path"
+  fi
+  chmod "$mode" "$path"
+}
+
+update_mutable_files_pass() {
+  local rel
+  local mutable_files=(
+    "regular/text_1mb_644.txt"
+    "regular/text_3mb_600.txt"
+    "regular/text_5mb_755.txt"
+    "regular/random_1mb_600.bin"
+    "regular/random_3mb_644.bin"
+    "regular/random_5mb_755.bin"
+    "regular/compressible_1mb_644.log"
+    "regular/compressible_3mb_600.log"
+    "regular/compressible_5mb_755.log"
+    "hidden/.hidden_text_1mb_644.txt"
+    "hidden/.hidden_random_3mb_600.bin"
+    "spaces in name/file with spaces text 1mb 644.txt"
+    "spaces in name/file with spaces random 3mb 600.bin"
+    "regular/longname_aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa_text_1mb_644.txt"
+    "regular/longname_bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb_random_3mb_600.bin"
+    "regular/longname_cccccccccccccccccccccccccccccccc_compressible_5mb_755.log"
+    "deep/tree/level1/level2/level3/deep_text_1mb_644.txt"
+    "deep/tree/level1/level2/level3/deep_random_3mb_600.bin"
+    "regular/dup_source_text_3mb_644.txt"
+    "regular/dup_copy_a_text_3mb_600.txt"
+    "deep/tree/level1/level2/dup_copy_b_text_3mb_755.txt"
+    "regular/old_text_1mb_644.txt"
+    "regular/recent_text_1mb_644.txt"
+    "regular/futureish_text_1mb_644.txt"
+    "readonly-dir/locked_text_1mb_444.txt"
+    "readonly-dir/locked_random_3mb_400.bin"
+    "metadata/xattr_text_1mb_644.txt"
+    "metadata/xattr_random_3mb_600.bin"
+    "metadata/acl_text_1mb_644.txt"
+  )
+
+  for rel in "${mutable_files[@]}"; do
+    rewrite_file_with_random_data "$rel"
+  done
+
+  set_acl_and_xattr_metadata
+}
+
+run_update_loop() {
+  local iteration=1
+
+  log "Starting update loop for $ROOT with interval ${UPDATE_INTERVAL}s"
+  while true; do
+    update_mutable_files_pass
+    log "Completed random update pass $iteration"
+    iteration=$((iteration + 1))
+
+    if (( UPDATE_INTERVAL > 0 )); then
+      sleep "$UPDATE_INTERVAL"
+    fi
+  done
+}
+
 make_file() {
   local path=$1
   local type=$2
@@ -232,19 +338,7 @@ create_metadata_cases() {
   make_file "metadata/acl_text_1mb_644.txt" text 1 0644
   make_file "metadata/acl_script_1mb_755.sh" script 1 0755
 
-  if (( have_setfattr )); then
-    setfattr -n user.migration_case -v "xattr-text" "$ROOT/metadata/xattr_text_1mb_644.txt"
-    setfattr -n user.migration_case -v "xattr-random" "$ROOT/metadata/xattr_random_3mb_600.bin"
-  else
-    echo "Skipping xattr assignment: setfattr not available"
-  fi
-
-  if (( have_setfacl )); then
-    setfacl -m u:nobody:r-- "$ROOT/metadata/acl_text_1mb_644.txt"
-    setfacl -m u:nobody:r-x "$ROOT/metadata/acl_script_1mb_755.sh"
-  else
-    echo "Skipping ACL assignment: setfacl not available"
-  fi
+  set_acl_and_xattr_metadata
 }
 
 write_summary() {
@@ -273,3 +367,7 @@ create_metadata_cases
 write_summary
 
 echo "Created migration test dataset at: $ROOT"
+
+if [[ -n "$UPDATE_INTERVAL" ]]; then
+  run_update_loop
+fi
diff --git a/cdssync/migration-test-manifest.md b/cdssync/migration-test-manifest.md
index 4ed6f7a..fda6c69 100644
--- a/cdssync/migration-test-manifest.md
+++ b/cdssync/migration-test-manifest.md
@@ -2,6 +2,18 @@
 
 This manifest defines a compact, high-value filesystem test set for validating file migration behavior. It is intended to cover common file-content, naming, metadata, and directory edge cases without generating an unnecessarily large corpus.
 
+The generator script can also run in continuous update mode after initial creation. In that mode, mutable content files are rewritten with random data on a fixed interval:
+
+- omit the interval argument to create the dataset once and exit
+- use `0` for continuous rewrites with no sleep between passes
+- use any integer greater than `0` to rewrite mutable files every `N` seconds
+
+Important implementation detail for update mode:
+
+- the update loop rewrites content-bearing regular files that are intended to simulate active data churn
+- it does not rewrite script files, sparse files, symlinks, hard links, or empty files
+- this preserves the special-case filesystem structure while still generating ongoing content changes
+
 ## Recommended Root Layout
 
 - `regular/`