diff --git a/models/background.py b/models/background.py
index d3a28463..ee9ab001 100644
--- a/models/background.py
+++ b/models/background.py
@@ -294,8 +294,9 @@ def save( self, filename=None, **kwargs ):
                         f"Variance shape {self.variance.shape} does not match image shape {self.image_shape}"
                     )
 
-                bggrp.create_dataset( 'counts', data=self.counts )
-                bggrp.create_dataset( 'variance', data=self.variance )
+                opts = dict(compression='gzip', compression_opts=1, chunks=(128, 128))
+                bggrp.create_dataset( 'counts', data=self.counts, **opts )
+                bggrp.create_dataset( 'variance', data=self.variance, **opts )
             elif self.format == 'polynomial':
                 raise NotImplementedError('Currently we do not support a polynomial background model. ')
                 bggrp.create_dataset( 'coeffs', data=self.counts )
diff --git a/models/base.py b/models/base.py
index ac20b483..fc894f4b 100644
--- a/models/base.py
+++ b/models/base.py
@@ -1,13 +1,10 @@
 import warnings
-import sys
 import os
 import math
 import types
 import hashlib
 import pathlib
-import logging
 import json
-import shutil
 import datetime
 from uuid import UUID
 
diff --git a/tests/conftest.py b/tests/conftest.py
index a3cd2bff..8a614d21 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -111,7 +111,7 @@ def pytest_sessionfinish(session, exitstatus):
 
         dbsession.commit()
 
-        verify_archive_database_empty = False  # set to False to avoid spurious errors at end of tests (when debugging)
+        verify_archive_database_empty = True  # set to False to avoid spurious errors at end of tests (when debugging)
 
         if any_objects and verify_archive_database_empty:
             raise RuntimeError('There are objects in the database. Some tests are not properly cleaning up!')
diff --git a/tests/fixtures/datastore_factory.py b/tests/fixtures/datastore_factory.py
index fef12f45..60f241ec 100644
--- a/tests/fixtures/datastore_factory.py
+++ b/tests/fixtures/datastore_factory.py
@@ -168,7 +168,7 @@ def make_datastore(
                     if image_cache_path is not None and output_path != image_cache_path:
                         warnings.warn(f'cache path {image_cache_path} does not match output path {output_path}')
                     else:
-                        cache_base_name = output_path[:-10]  # remove the '.image.fits' part
+                        cache_base_name = output_path[:-16]  # remove the '.image.fits.json' part
                         ds.cache_base_name = output_path
                         SCLogger.debug(f'Saving image to cache at: {output_path}')
                         use_cache = True  # the two other conditions are true to even get to this part...
diff --git a/tests/models/test_background.py b/tests/models/test_background.py
index fc58d3b6..dbd441c2 100644
--- a/tests/models/test_background.py
+++ b/tests/models/test_background.py
@@ -1,3 +1,8 @@
+import os
+
+import sep
+import time
+
 import pytest
 import numpy as np
 import h5py
@@ -72,9 +77,15 @@ def test_save_load_backgrounds(decam_raw_image, code_version):
         with pytest.raises(RuntimeError, match='Counts shape .* does not match image shape .*'):
             b2.save()
 
-        b2.counts = np.random.normal(bg_mean, 1, size=image.data.shape)
-        b2.variance = np.random.normal(bg_var, 1, size=image.data.shape)
+        # use actual background measurements so we can get a realistic estimate of the compression
+        back = sep.Background(image.data)
+        b2.counts = back.back()
+        b2.variance = back.rms() ** 2
+
+        t0 = time.perf_counter()
         b2.save()
+        # print(f'Background save time: {time.perf_counter() - t0:.3f} s')
+        # print(f'Background file size: {os.path.getsize(b2.get_fullpath()) / 1024 ** 2:.3f} MB')
 
         # check the filename contains the provenance hash
         assert prov.id[:6] in b2.get_fullpath()
diff --git a/util/cache.py b/util/cache.py
index 4f4611f4..d93de5be 100644
--- a/util/cache.py
+++ b/util/cache.py
@@ -8,6 +8,7 @@
 # ======================================================================
 # Functions for copying FileOnDisk objects to/from cache
 
+
 def copy_to_cache(FoD, cache_dir, filepath=None):
     """Save a copy of the object (and, potentially, associated files) into a cache directory.
 
@@ -79,6 +80,7 @@ def copy_to_cache(FoD, cache_dir, filepath=None):
 
     return json_filepath
 
+
 def copy_list_to_cache(obj_list, cache_dir, filepath=None):
     """Copy a correlated list of objects to the cache directory.
 
@@ -137,6 +139,7 @@ def copy_list_to_cache(obj_list, cache_dir, filepath=None):
 
     return json_filepath
 
+
 def copy_from_cache(cls, cache_dir, filepath):
     """Copy and reconstruct an object from the cache directory.
 
@@ -200,6 +203,7 @@ def copy_from_cache(cls, cache_dir, filepath):
 
     return output
 
+
 def copy_list_from_cache(cls, cache_dir, filepath):
     """Copy and reconstruct a list of objects from the cache directory.