diff --git a/.coveragerc b/.coveragerc
deleted file mode 100644
index 398ff08..0000000
--- a/.coveragerc
+++ /dev/null
@@ -1,2 +0,0 @@
-[run]
-branch = True
diff --git a/.github/workflows/python-app.yml b/.github/workflows/python-app.yml
index 4a290ff..0389f2a 100644
--- a/.github/workflows/python-app.yml
+++ b/.github/workflows/python-app.yml
@@ -7,7 +7,6 @@ on:
   push:
     branches: [ main ]
   pull_request:
-    branches: [ main ]
 
 jobs:
   check-poetry:
diff --git a/changelog.d/.gitkeep b/changelog.d/.gitkeep
new file mode 100644
index 0000000..e69de29
diff --git a/changelog.d/20250611_120003_jb_PL_133453_rbd_whole_object.rst b/changelog.d/20250611_120003_jb_PL_133453_rbd_whole_object.rst
new file mode 100644
index 0000000..e09f7ac
--- /dev/null
+++ b/changelog.d/20250611_120003_jb_PL_133453_rbd_whole_object.rst
@@ -0,0 +1,3 @@
+.. A new scriv changelog fragment.
+
+- Fix whole object detection (PL-133453)
diff --git a/poetry.lock b/poetry.lock
index 8f970e0..b812bc0 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -771,21 +771,21 @@ pytest = ">=2.2"
 
 [[package]]
 name = "pytest-cov"
-version = "4.1.0"
+version = "6.1.1"
 description = "Pytest plugin for measuring coverage."
 optional = false
-python-versions = ">=3.7"
+python-versions = ">=3.9"
 files = [
-    {file = "pytest-cov-4.1.0.tar.gz", hash = "sha256:3904b13dfbfec47f003b8e77fd5b589cd11904a21ddf1ab38a64f204d6a10ef6"},
-    {file = "pytest_cov-4.1.0-py3-none-any.whl", hash = "sha256:6ba70b9e97e69fcc3fb45bfeab2d0a138fb65c4d0d6a41ef33983ad114be8c3a"},
+    {file = "pytest_cov-6.1.1-py3-none-any.whl", hash = "sha256:bddf29ed2d0ab6f4df17b4c55b0a657287db8684af9c42ea546b21b1041b3dde"},
+    {file = "pytest_cov-6.1.1.tar.gz", hash = "sha256:46935f7aaefba760e716c2ebfbe1c216240b9592966e7da99ea8292d4d3e2a0a"},
 ]
 
 [package.dependencies]
-coverage = {version = ">=5.2.1", extras = ["toml"]}
+coverage = {version = ">=7.5", extras = ["toml"]}
 pytest = ">=4.6"
 
 [package.extras]
-testing = ["fields", "hunter", "process-tests", "pytest-xdist", "six", "virtualenv"]
+testing = ["fields", "hunter", "process-tests", "pytest-xdist", "virtualenv"]
 
 [[package]]
 name = "pytest-flake8"
@@ -1065,4 +1065,4 @@ files = [
 [metadata]
 lock-version = "2.0"
 python-versions = "~3.12"
-content-hash = "666d6cd72e7e3de3d31bd41749548da77fb8d39aa99aca4092f75c919c5d129f"
+content-hash = "069ccfd6291927d495545f080eb094b1105be2701d67a78feed07e0cf657f608"
diff --git a/pyproject.toml b/pyproject.toml
index c26edd7..a689793 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -13,6 +13,18 @@ version = "literal: pyproject.toml: tool.poetry.version"
 entry_title_template = "{% if version %}{{ version }} {% endif %}({{ date.strftime('%Y-%m-%d') }})"
 categories = ""
 
+
+[tool.pytest.ini_options]
+addopts = "--timeout=30 --tb=native --cov=src --cov-report=html --cov-config=pyproject.toml src -r w"
+markers = "slow: This is a non-unit test and thus is not run by default. Use ``-m slow`` to run these, or ``-m 1`` to run all tests."
+log_level = "NOTSET"
+asyncio_mode = "auto"
+filterwarnings = [ "ignore::DeprecationWarning:telnetlib3.*:" ]
+
+[tool.coverage.run]
+branch = true
+omit = [ "*/tests/*" ]
+
 [tool.poetry]
 name = "backy"
 version = "2.5.4.dev0"
@@ -59,7 +71,7 @@ pre-commit = "^3.3.3"
 pytest = "^7.4.0"
 pytest-asyncio = "^0.21.1"
 pytest-cache = "^1.0"
-pytest-cov = "^4.1.0"
+pytest-cov = "^6.1.0"
 pytest-flake8 = "^1.1.1"
 pytest-timeout = "^2.1.0"
 scriv = "^1.3.1"
diff --git a/pytest.ini b/pytest.ini
deleted file mode 100644
index d3c553f..0000000
--- a/pytest.ini
+++ /dev/null
@@ -1,9 +0,0 @@
-[pytest]
-addopts = --timeout=30 --tb=native --cov=src --cov-report=html src -r w
-markers = slow: This is a non-unit test and thus is not run by default. Use ``-m slow`` to run these, or ``-m 1`` to run all tests.
-log_level = NOTSET
-asyncio_mode = auto
-
-
-filterwarnings =
-    ignore::DeprecationWarning:telnetlib3.*:
diff --git a/src/backy/sources/ceph/__init__.py b/src/backy/sources/ceph/__init__.py
index 623db0e..91b1aa4 100644
--- a/src/backy/sources/ceph/__init__.py
+++ b/src/backy/sources/ceph/__init__.py
@@ -1,14 +1 @@
-from subprocess import PIPE, run
-
-
-def detect_whole_object_support():
-    result = run(
-        ["rbd", "help", "export-diff"], stdout=PIPE, stderr=PIPE, check=True
-    )
-    return "--whole-object" in result.stdout.decode("ascii")
-
-
-try:
-    CEPH_RBD_SUPPORTS_WHOLE_OBJECT_DIFF = detect_whole_object_support()
-except Exception:
-    CEPH_RBD_SUPPORTS_WHOLE_OBJECT_DIFF = False
+# Make this a package.
diff --git a/src/backy/sources/ceph/rbd.py b/src/backy/sources/ceph/rbd.py
index 9186371..7bd74a0 100644
--- a/src/backy/sources/ceph/rbd.py
+++ b/src/backy/sources/ceph/rbd.py
@@ -1,12 +1,11 @@
 import contextlib
+import functools
 import json
 import subprocess
 from typing import IO, Iterator, cast
 
 from structlog.stdlib import BoundLogger
 
-import backy.sources.ceph
-
 from ...ext_deps import RBD
 from ...utils import CHUNK_SIZE
 from .diff import RBDDiffV1
@@ -66,6 +65,10 @@ def _rbd_stream(self, cmd: list[str]) -> Iterator[IO[bytes]]:
             if rc:
                 raise subprocess.CalledProcessError(rc, proc.args)
 
+    @functools.cached_property
+    def _supports_whole_object(self):
+        return "--whole-object" in self._rbd(["help", "export-diff"])
+
     def exists(self, snapspec):
         try:
             return self._rbd(["info", snapspec], format="json")
@@ -134,7 +137,7 @@ def snap_rm(self, image):
 
     @contextlib.contextmanager
     def export_diff(self, new: str, old: str) -> Iterator[RBDDiffV1]:
-        if backy.sources.ceph.CEPH_RBD_SUPPORTS_WHOLE_OBJECT_DIFF:
+        if self._supports_whole_object:
             EXPORT_WHOLE_OBJECT = ["--whole-object"]
         else:
             EXPORT_WHOLE_OBJECT = []
diff --git a/src/backy/sources/ceph/tests/conftest.py b/src/backy/sources/ceph/tests/conftest.py
index befa272..f1190de 100644
--- a/src/backy/sources/ceph/tests/conftest.py
+++ b/src/backy/sources/ceph/tests/conftest.py
@@ -4,7 +4,6 @@
 
 import pytest
 
-import backy.sources.ceph
 from backy.sources.ceph.rbd import RBDClient
 
 
@@ -242,11 +241,8 @@ def unmap(self, device):
 
 @pytest.fixture(params=[CephJewelCLI, CephLuminousCLI, CephNautilusCLI])
 def rbdclient(request, tmpdir, monkeypatch, log):
-    monkeypatch.setattr(
-        backy.sources.ceph, "CEPH_RBD_SUPPORTS_WHOLE_OBJECT_DIFF", True
-    )
-
     client = RBDClient(log)
+    client._supports_whole_object = True
     client._ceph_cli = request.param(tmpdir)
 
     return client