diff --git a/docs/images.md b/docs/images.md
index 74ec19c1..af91de11 100644
--- a/docs/images.md
+++ b/docs/images.md
@@ -84,6 +84,25 @@ When using multiple writers, each writer should modify a distinct byte
 range. If two writers modify the same byte range concurrently they will
 overwrite each other data.
 
+### transfer_format
+
+The transfer data format. Always available when using the `nbd` and
+`memory` backends, not available when using the `file` backend. When
+using the `http` backend `transfer_format` is available only if the
+remote server reports it.
+
+Since 2.4.6.
+
+### virtual_size
+
+The underlying image virtual size. Available only when the backend is
+using `raw` transfer format. Always available when using the `nbd` and
+`memory` backends, not available when using the `file` backend.  When
+using the `http` backend the `virtual_size` is available only if the
+remote server reports it.
+
+Since 2.4.6.
+
 ### Errors
 
 Specific errors for OPTIONS request:
@@ -112,9 +131,9 @@ Response:
     Content-Length: 122
 
     {"unix_socket": "\u0000/org/ovirt/imageio", "features": ["extents", "zero", "flush"],
-     "max_readers": 8, "max_writers": 8}
+     "max_readers": 8, "max_writers": 8, "transfer_format": "raw", "virtual_size": 53687091200}
 
-Get options for ticket-id with read-write access using nbd backend:
+Get options for ticket-id with read-write access using `nbd` backend:
 
     $ curl -k -X OPTIONS https://server:54322/images/{ticket-id} | jq
     {
@@ -125,11 +144,13 @@ Get options for ticket-id with read-write access using nbd backend:
         "flush"
       ],
       "max_readers": 8,
-      "max_writers": 8
+      "max_writers": 8,
+      "transfer_format": "raw",
+      "virtual_size": 53687091200
     }
 
-The nbd backend is used when specifying the "raw" transfer format when
-creating an image transfer in oVirt API.
+The nbd backend is used when creating an image transfer with
+format="raw" in oVirt API.
 
 Get options for ticket-id with read-only access using file backend:
 
@@ -143,6 +164,9 @@ Get options for ticket-id with read-only access using file backend:
       "max_writers": 1
     }
 
+Note that `virtual_size` and `transfer_format` are not reported, and this
+backend does not support multiple writers.
+
 Get all available options for the special `*` ticket:
 
     $ curl -sk -X OPTIONS 'https://server:54322/images/*' | jq
@@ -268,7 +292,7 @@ Since 2.0
 
 Request zero extents:
 
-    GET /images/{ticket-id}/extent
+    GET /images/{ticket-id}/extents
 
 Response:
 
@@ -280,7 +304,7 @@ Response:
 
 Request dirty extents:
 
-    GET /images/{ticket-id}/extent?context=dirty
+    GET /images/{ticket-id}/extents?context=dirty
 
 Response:
 
diff --git a/ovirt_imageio/_internal/backends/file.py b/ovirt_imageio/_internal/backends/file.py
index 03626c6e..66edaf24 100644
--- a/ovirt_imageio/_internal/backends/file.py
+++ b/ovirt_imageio/_internal/backends/file.py
@@ -74,6 +74,13 @@ def __init__(self, fio, sparse=False, max_connections=8):
     def max_readers(self):
         return self._max_connections
 
+    @property
+    def format(self):
+        """
+        Currently we don't have access to the underlying disk format.
+        """
+        return None
+
     # io.FileIO interface
 
     def readinto(self, buf):
diff --git a/ovirt_imageio/_internal/backends/http.py b/ovirt_imageio/_internal/backends/http.py
index 93335a0d..3e6d109d 100644
--- a/ovirt_imageio/_internal/backends/http.py
+++ b/ovirt_imageio/_internal/backends/http.py
@@ -63,7 +63,6 @@ def __init__(self, url, cafile=None, secure=True, connect_timeout=10,
         self._connect_timeout = connect_timeout
         self._read_timeout = read_timeout
         self._position = 0
-        self._size = None
         self._extents = {}
 
         # Initlized during connection.
@@ -74,6 +73,8 @@ def __init__(self, url, cafile=None, secure=True, connect_timeout=10,
         self._can_flush = False
         self._max_readers = 1
         self._max_writers = 1
+        self._format = None
+        self._size = None
 
         if connect:
             self._connect()
@@ -104,8 +105,10 @@ def clone(self):
             backend._can_flush = self._can_flush
             backend._max_readers = self._max_readers
             backend._max_writers = self._max_writers
+            backend._format = self._format
 
-            # Copy size and extents to save expensive EXTENTS calls.
+            # Copy extents and size to save expensive EXTENTS calls with old
+            # imageio servers.
             backend._size = self._size
             for ctx in list(self._extents):
                 backend._extents[ctx] = self._extents[ctx].copy()
@@ -133,6 +136,10 @@ def _connect(self):
             # max_writers does not support multiple writers.
             self._max_writers = options.get("max_writers", 1)
 
+            # Newer server reports also transfer_format and virtual_size.
+            self._format = options.get("transfer_format")
+            self._size = options.get("virtual_size")
+
             self._optimize_connection(options.get("unix_socket"))
         except Exception:
             self._con.close()
@@ -155,6 +162,14 @@ def max_readers(self):
     def max_writers(self):
         return self._max_writers
 
+    @property
+    def format(self):
+        """
+        Will be None for old server (imageio < 2.4.6) or when server is using
+        file backend.
+        """
+        return self._format
+
     # Preferred interface.
 
     def read_from(self, reader, length, buf):
@@ -325,10 +340,17 @@ def seek(self, n, how=os.SEEK_SET):
         return self._position
 
     def size(self):
-        # We have 2 bad options:
-        # - Get last extent, may be slow, and may not be neded otherwise.
-        # - Emulate HEAD request, logging tracebacks in the remote server.
-        # Getting extents is more polite, so lets use it if we can.
+        """
+        Return backend size in bytes.
+
+        With newer server (imageio >= 2.4.6) reporting the virtual size the
+        size is intialized in connect().
+
+        Otherwise we have 2 bad options:
+        - Get the last extent, may be slow, and may not be neded otherwise.
+        - Emulate HEAD request, logging tracebacks in the remote server.
+          Getting extents is more polite, so lets use it if we can.
+        """
         if self._size is None:
             if self._can_extents:
                 last = list(self.extents())[-1]
@@ -454,7 +476,7 @@ def _put_header(self, length):
         self._con.putheader("content-length", length)
         self._con.putheader("content-type", "application/octet-stream")
         self._con.putheader("content-range", "bytes {}-{}/*".format(
-                self._position, self._position + length - 1))
+            self._position, self._position + length - 1))
 
         self._con.endheaders()
 
diff --git a/ovirt_imageio/_internal/backends/memory.py b/ovirt_imageio/_internal/backends/memory.py
index 9f54259d..e15e52ea 100644
--- a/ovirt_imageio/_internal/backends/memory.py
+++ b/ovirt_imageio/_internal/backends/memory.py
@@ -72,6 +72,10 @@ def max_writers(self):
         # support more than one writer concurrently.
         return 1
 
+    @property
+    def format(self):
+        return "raw"
+
     # io.BaseIO interface
 
     def readinto(self, buf):
diff --git a/ovirt_imageio/_internal/backends/nbd.py b/ovirt_imageio/_internal/backends/nbd.py
index 3ee683af..5fc156ca 100644
--- a/ovirt_imageio/_internal/backends/nbd.py
+++ b/ovirt_imageio/_internal/backends/nbd.py
@@ -95,6 +95,10 @@ def max_readers(self):
     def max_writers(self):
         return self._max_connections
 
+    @property
+    def format(self):
+        return "raw"
+
     # Backend interface
 
     def readinto(self, buf):
diff --git a/ovirt_imageio/_internal/handlers/images.py b/ovirt_imageio/_internal/handlers/images.py
index cade4e5e..2f4f68ef 100644
--- a/ovirt_imageio/_internal/handlers/images.py
+++ b/ovirt_imageio/_internal/handlers/images.py
@@ -268,5 +268,11 @@ def options(self, req, resp, ticket_id):
             options["max_readers"] = ctx.backend.max_readers
             options["max_writers"] = ctx.backend.max_writers
 
+            # Optional backend options.
+            if ctx.backend.format is not None:
+                options["transfer_format"] = ctx.backend.format
+            if ctx.backend.format == "raw":
+                options["virtual_size"] = ctx.backend.size()
+
         resp.headers["allow"] = ",".join(allow)
         resp.send_json(options)
diff --git a/test/handlers/images_test.py b/test/handlers/images_test.py
index f00875a5..d38325bc 100644
--- a/test/handlers/images_test.py
+++ b/test/handlers/images_test.py
@@ -14,7 +14,9 @@
 import pytest
 
 from ovirt_imageio._internal import config
+from ovirt_imageio._internal import qemu_img
 from ovirt_imageio._internal import server
+from ovirt_imageio._internal.units import KiB, MiB, GiB
 
 from .. import testutil
 from .. import http
@@ -236,7 +238,7 @@ def test_upload_invalid_range(tmpdir, srv, client, content_range):
 
 
 def test_upload_close_connection(tmpdir, srv, client):
-    image_size = 4096
+    image_size = 4 * KiB
     image = testutil.create_tempfile(tmpdir, "image", b"a" * image_size)
     ticket = testutil.create_ticket(
         url="file://" + str(image), size=image_size)
@@ -296,24 +298,24 @@ def test_download(tmpdir, srv, client, rng, start, end):
 
 def test_download_image_size_gt_ticket_size(tmpdir, srv, client):
     image = testutil.create_tempfile(tmpdir, "image", size=8192)
-    ticket = testutil.create_ticket(url="file://" + str(image), size=4096)
+    ticket = testutil.create_ticket(url="file://" + str(image), size=4 * KiB)
     srv.auth.add(ticket)
     res = client.get("/images/" + ticket["uuid"])
     assert res.status == 200
-    assert len(res.read()) == 4096
+    assert len(res.read()) == 4 * KiB
 
 
 def test_download_ticket_size_gt_image_size(tmpdir, srv, client):
-    image = testutil.create_tempfile(tmpdir, "image", size=4096)
+    image = testutil.create_tempfile(tmpdir, "image", size=4 * KiB)
     ticket = testutil.create_ticket(url="file://" + str(image), size=8192)
     srv.auth.add(ticket)
     res = client.get("/images/" + ticket["uuid"])
     assert res.status == 200
-    assert len(res.read()) == 4096
+    assert len(res.read()) == 4 * KiB
 
 
 def test_download_range_forbidden(tmpdir, srv, client):
-    image = testutil.create_tempfile(tmpdir, "image", size=4096)
+    image = testutil.create_tempfile(tmpdir, "image", size=4 * KiB)
     ticket = testutil.create_ticket(url="file://" + str(image), size=8192)
     srv.auth.add(ticket)
     res = client.get("/images/" + ticket["uuid"],
@@ -324,7 +326,7 @@ def test_download_range_forbidden(tmpdir, srv, client):
 
 def test_download_range_unavailable(tmpdir, srv, client):
     image = testutil.create_tempfile(tmpdir, "image", size=8192)
-    ticket = testutil.create_ticket(url="file://" + str(image), size=4096)
+    ticket = testutil.create_ticket(url="file://" + str(image), size=4 * KiB)
     srv.auth.add(ticket)
     res = client.get("/images/" + ticket["uuid"],
                      headers={"Range": "bytes=0-4096"})
@@ -333,7 +335,7 @@ def test_download_range_unavailable(tmpdir, srv, client):
 
 
 def test_download_no_range(tmpdir, srv, client):
-    size = 1024
+    size = KiB
     image = testutil.create_tempfile(tmpdir, "image", size=size)
     ticket = testutil.create_ticket(url="file://" + str(image), size=size)
     srv.auth.add(ticket)
@@ -344,7 +346,7 @@ def test_download_no_range(tmpdir, srv, client):
 
 
 def test_download_extends_ticket(tmpdir, srv, client, fake_time):
-    size = 1024
+    size = KiB
     image = testutil.create_tempfile(tmpdir, "image", size=size)
     ticket = testutil.create_ticket(url="file://" + str(image), size=size)
     srv.auth.add(ticket)
@@ -381,7 +383,7 @@ def test_download_partial_not_satistieble(tmpdir, srv, client):
     # actual image size reported by vdsm - one byte difference is enough to
     # cause a failure.
     # See https://bugzilla.redhat.com/1512315.
-    size = 1024
+    size = KiB
     image = testutil.create_tempfile(tmpdir, "image", size=size)
     ticket = testutil.create_ticket(url="file://" + str(image), size=size + 1)
     srv.auth.add(ticket)
@@ -397,7 +399,7 @@ def test_download_partial_no_range(tmpdir, srv, client):
     # is only an upper limit. Or maybe we should treat the ticket size as the
     # expected size?
     # This is another variant of https://bugzilla.redhat.com/1512315.
-    size = 1024
+    size = KiB
     image = testutil.create_tempfile(tmpdir, "image", size=size)
     ticket = testutil.create_ticket(url="file://" + str(image), size=size + 1)
     srv.auth.add(ticket)
@@ -405,7 +407,7 @@ def test_download_partial_no_range(tmpdir, srv, client):
     assert res.status == http_client.OK
     # Should return the available image data, not the ticket size. Reading
     # this response will fail with IncompleteRead.
-    assert res.length == 1024
+    assert res.length == KiB
 
 
 def test_download_partial_no_range_empty(tmpdir, srv, client):
@@ -413,7 +415,7 @@ def test_download_partial_no_range_empty(tmpdir, srv, client):
     # http response that fail on the client side with BadStatusLine: ''.
     # See https://bugzilla.redhat.com/1512312
     image = testutil.create_tempfile(tmpdir, "image")  # Empty image
-    ticket = testutil.create_ticket(url="file://" + str(image), size=1024)
+    ticket = testutil.create_ticket(url="file://" + str(image), size=KiB)
     srv.auth.add(ticket)
     res = client.get("/images/" + ticket["uuid"])
     assert res.status == http_client.OK
@@ -421,7 +423,7 @@ def test_download_partial_no_range_empty(tmpdir, srv, client):
 
 
 def test_download_no_range_end(tmpdir, srv, client):
-    size = 1024
+    size = KiB
     image = testutil.create_tempfile(tmpdir, "image", size=size)
     ticket = testutil.create_ticket(url="file://" + str(image), size=size)
     srv.auth.add(ticket)
@@ -433,7 +435,7 @@ def test_download_no_range_end(tmpdir, srv, client):
 
 
 def test_download_holes(tmpdir, srv, client):
-    size = 1024
+    size = KiB
     image = testutil.create_tempfile(tmpdir, "image", size=size)
     ticket = testutil.create_ticket(url="file://" + str(image), size=size)
     srv.auth.add(ticket)
@@ -445,7 +447,7 @@ def test_download_holes(tmpdir, srv, client):
 
 
 def test_download_filename_in_ticket(tmpdir, srv, client):
-    size = 1024
+    size = KiB
     filename = "\u05d0.raw"  # hebrew aleph
     image = testutil.create_tempfile(tmpdir, "image", size=size)
     ticket = testutil.create_ticket(url="file://" + str(image), size=size,
@@ -472,7 +474,7 @@ def test_download_out_of_range(tmpdir, srv, client, rng, end):
 
 def test_download_progress(tmpdir, srv, client, monkeypatch):
     # We need to read at least one buffer to update the transfered value.
-    monkeypatch.setattr(srv.config.backend_file, "buffer_size", 1024**2)
+    monkeypatch.setattr(srv.config.backend_file, "buffer_size", MiB)
 
     # And we need to request enough data so the server does not complete before
     # the client read all the data.
@@ -511,7 +513,7 @@ def test_download_progress(tmpdir, srv, client, monkeypatch):
 
 
 def test_download_close_connection(tmpdir, srv, client):
-    image_size = 4096
+    image_size = 4 * KiB
     image = testutil.create_tempfile(tmpdir, "image", b"a" * image_size)
     ticket = testutil.create_ticket(
         url="file://" + str(image), size=image_size)
@@ -704,8 +706,8 @@ def test_options_all(srv, client):
     assert "max_writers" not in options
 
 
-def test_options_read_write(srv, client, tmpdir):
-    size = 128 * 1024
+def test_options_file_read_write(srv, client, tmpdir):
+    size = 128 * KiB
     image = testutil.create_tempfile(tmpdir, "image", size=size)
     ticket = testutil.create_ticket(
         url="file://" + str(image), size=size, ops=["read", "write"])
@@ -716,12 +718,16 @@ def test_options_read_write(srv, client, tmpdir):
     assert set(res.getheader("allow").split(',')) == allows
     options = json.loads(res.read())
     assert set(options["features"]) == ALL_FEATURES
+
+    # File backend specific options.
     assert options["max_readers"] == srv.config.daemon.max_connections
-    assert options["max_writers"] == 1  # Using file backend.
+    assert options["max_writers"] == 1
+    assert "transfer_format" not in options
+    assert "virtual_size" not in options
 
 
-def test_options_read(srv, client, tmpdir):
-    size = 128 * 1024
+def test_options_file_read(srv, client, tmpdir):
+    size = 128 * KiB
     image = testutil.create_tempfile(tmpdir, "image", size=size)
     ticket = testutil.create_ticket(
         url="file://" + str(image), size=size, ops=["read"])
@@ -732,12 +738,16 @@ def test_options_read(srv, client, tmpdir):
     assert set(res.getheader("allow").split(',')) == allows
     options = json.loads(res.read())
     assert set(options["features"]) == BASE_FEATURES
+
+    # File backend specific options.
     assert options["max_readers"] == srv.config.daemon.max_connections
-    assert options["max_writers"] == 1  # Using file backend.
+    assert options["max_writers"] == 1
+    assert "transfer_format" not in options
+    assert "virtual_size" not in options
 
 
-def test_options_write(srv, client, tmpdir):
-    size = 128 * 1024
+def test_options_file_write(srv, client, tmpdir):
+    size = 128 * KiB
     image = testutil.create_tempfile(tmpdir, "image", size=size)
     ticket = testutil.create_ticket(
         url="file://" + str(image), size=size, ops=["write"])
@@ -749,12 +759,81 @@ def test_options_write(srv, client, tmpdir):
     assert set(res.getheader("allow").split(',')) == allows
     options = json.loads(res.read())
     assert set(options["features"]) == ALL_FEATURES
+
+    # File backend specific options.
+    assert options["max_readers"] == srv.config.daemon.max_connections
+    assert options["max_writers"] == 1
+    assert "transfer_format" not in options
+    assert "virtual_size" not in options
+
+
+@pytest.mark.parametrize("fmt", ["raw", "qcow2"])
+def test_options_nbd_read(srv, client, tmpdir, nbd_server, fmt):
+    # Create disk.
+    size = GiB
+    disk = str(tmpdir.join(f"disk.{fmt}"))
+    qemu_img.create(disk, fmt, size=size)
+
+    # Start nbd server exporting the disk.
+    nbd_server.image = disk
+    nbd_server.fmt = fmt
+    nbd_server.read_only = True
+    nbd_server.start()
+
+    # Add ticket using nbd server url.
+    ticket = testutil.create_ticket(
+        url=nbd_server.sock.url(), size=size, ops=["read"])
+    srv.auth.add(ticket)
+
+    # Get OPTIONS.
+    res = client.options("/images/" + ticket["uuid"])
+    allows = {"OPTIONS", "GET"}
+    assert res.status == 200
+    assert set(res.getheader("allow").split(',')) == allows
+    options = json.loads(res.read())
+    assert set(options["features"]) == BASE_FEATURES
+
+    # NBD backend specific options.
+    assert options["max_readers"] == srv.config.daemon.max_connections
+    assert options["max_writers"] == srv.config.daemon.max_connections
+    assert options["transfer_format"] == "raw"
+    assert options["virtual_size"] == size
+
+
+@pytest.mark.parametrize("fmt", ["raw", "qcow2"])
+def test_options_nbd_write(srv, client, tmpdir, nbd_server, fmt):
+    # Create disk.
+    size = GiB
+    disk = str(tmpdir.join(f"disk.{fmt}"))
+    qemu_img.create(disk, fmt, size=size)
+
+    # Start nbd server exporting the disk.
+    nbd_server.image = disk
+    nbd_server.fmt = fmt
+    nbd_server.start()
+
+    # Add ticket using nbd server url.
+    ticket = testutil.create_ticket(
+        url=nbd_server.sock.url(), size=size, ops=["write"])
+    srv.auth.add(ticket)
+
+    # Get OPTIONS.
+    res = client.options("/images/" + ticket["uuid"])
+    allows = {"OPTIONS", "GET", "PUT", "PATCH"}
+    assert res.status == 200
+    assert set(res.getheader("allow").split(',')) == allows
+    options = json.loads(res.read())
+    assert set(options["features"]) == ALL_FEATURES
+
+    # NBD backend specific options.
     assert options["max_readers"] == srv.config.daemon.max_connections
-    assert options["max_writers"] == 1  # Using file backend.
+    assert options["max_writers"] == srv.config.daemon.max_connections
+    assert options["transfer_format"] == "raw"
+    assert options["virtual_size"] == size
 
 
 def test_options_extends_ticket(srv, client, tmpdir, fake_time):
-    size = 128 * 1024
+    size = 128 * KiB
     image = testutil.create_tempfile(tmpdir, "image", size=size)
     ticket = testutil.create_ticket(url="file://" + str(image), size=size)
     srv.auth.add(ticket)
@@ -786,7 +865,7 @@ def test_options_for_nonexistent_ticket(srv, client):
 
 
 def test_options_ticket_expired(srv, client, tmpdir, fake_time):
-    size = 128 * 1024
+    size = 128 * KiB
     image = testutil.create_tempfile(tmpdir, "image", size=size)
     ticket = testutil.create_ticket(
         url="file://" + str(image), size=size, timeout=300)
@@ -828,9 +907,9 @@ def test_response_version_error(tmpdir, srv, client):
 ])
 def test_keep_alive_connection_on_success(tmpdir, srv, client, method, body):
     # After successful request the connection should remain open.
-    image = testutil.create_tempfile(tmpdir, "image", size=1024)
+    image = testutil.create_tempfile(tmpdir, "image", size=KiB)
     ticket = testutil.create_ticket(url="file://" + str(image),
-                                    size=1024)
+                                    size=KiB)
     srv.auth.add(ticket)
     uri = "/images/%(uuid)s" % ticket
     # Disabling auto_open so we can test if a connection was closed.
@@ -915,7 +994,7 @@ def test_cors_options_all(srv, client):
 
 
 def test_cors_get_ok(tmpdir, srv, client):
-    size = 8192
+    size = 8 * KiB
     image = testutil.create_tempfile(tmpdir, "image", size=size)
     ticket = testutil.create_ticket(url="file://" + str(image), size=size)
     srv.auth.add(ticket)
@@ -944,7 +1023,7 @@ def test_cors_get_error(srv, client):
 
 
 def test_cors_put_ok(tmpdir, srv, client):
-    size = 8192
+    size = 8 * KiB
     image = testutil.create_tempfile(tmpdir, "image", size=size)
     ticket = testutil.create_ticket(url="file://" + str(image), size=size)
     srv.auth.add(ticket)