diff --git a/mb/mb/conf.py b/mb/mb/conf.py
index d5b10e4a..ff819d1e 100644
--- a/mb/mb/conf.py
+++ b/mb/mb/conf.py
@@ -8,6 +8,7 @@
 boolean_opts = ['zsync_hashes', 'chunked_hashes']
 
 DEFAULTS = {'zsync_hashes': False,
+            'zsync_block_size_for_1G': None,
             'chunked_hashes': True,
             'chunk_size': 262144,
             'apache_documentroot': None}
@@ -74,6 +75,11 @@ def __init__(self, conffile='/etc/mirrorbrain.conf', instance=None):
                         'cannot parse setting in [%s] section: %r' % (i, b + str(e)), conffile)
                 except configparser.NoOptionError as e:
                     pass
+            try:
+                self.general[i]['zsync_block_size_for_1G'] = adjust_zsync_block_size_for_1G(cp.getint(i, 'zsync_block_size_for_1G'))
+            except configparser.NoOptionError as e:
+                pass
+
             # set default values where the config didn't define anything
             for d in DEFAULTS:
                 try:
@@ -95,3 +101,19 @@ def __init__(self, conffile='/etc/mirrorbrain.conf', instance=None):
         # take care of the [mirrorprobe] section
         #
         self.mirrorprobe = dict(cp.items('mirrorprobe'))
+
+def adjust_zsync_block_size_for_1G(n):
+    if n < 1024:
+        print("zsync_block_size_for_1G is too small, ignoring", file=sys.stderr);
+        return DEFAULTS['zsync_block_size_for_1G'] 
+    if (n & (n-1) == 0) and n != 0:
+        return n
+
+    exponent = 0
+    while n >= 2:
+        n /= 2
+        exponent += 1
+    n = 2 ** exponent
+
+    print("zsync_block_size_for_1G must be power of 2 (512, 1024, 2048, ...), adjusting down to: " + repr(n)) # , file=sys.stderr);
+    return n
diff --git a/mb/mb/hashes.py b/mb/mb/hashes.py
index 2df4a925..405b78be 100644
--- a/mb/mb/hashes.py
+++ b/mb/mb/hashes.py
@@ -29,7 +29,7 @@ class Hasheable:
 
     def __init__(self, basename, src_dir=None,
                  base_dir=None, do_zsync_hashes=False,
-                 do_chunked_hashes=True, chunk_size=DEFAULT_PIECESIZE, do_chunked_with_zsync=False, skip_metadata=False):
+                 do_chunked_hashes=True, chunk_size=DEFAULT_PIECESIZE, do_chunked_with_zsync=False, skip_metadata=False, zsync_block_size_for_1G=None):
         self.basename = basename
         if src_dir:
             self.src_dir = src_dir
@@ -55,6 +55,7 @@ def __init__(self, basename, src_dir=None,
            self.hb.do_chunked_hashes = do_chunked_hashes
            self.hb.do_chunked_with_zsync = do_chunked_with_zsync
            self.hb.chunk_size = chunk_size
+           self.hb.zsync_block_size_for_1G = zsync_block_size_for_1G
 
     def islink(self):
         return stat.S_ISLNK(self.mode)
@@ -114,7 +115,7 @@ def check_db(self, conn, verbose=False, dry_run=False, force=False):
 
             zsums = ''
             for i in self.hb.zsums:
-                zsums = zsums + i.hexdigest()
+                zsums = zsums + i.hex()
 
             c.execute("""UPDATE files SET 
                             mtime  = to_timestamp(%s), 
@@ -162,7 +163,7 @@ def check_db(self, conn, verbose=False, dry_run=False, force=False):
 
             zsums = ''
             for i in self.hb.zsums:
-                zsums = zsums + i.hexdigest()
+                zsums = zsums + i.hex()
 
             c.execute("""UPDATE files set mtime = to_timestamp(%s), size = %s,
                                          md5 = decode(%s, 'hex'),
@@ -333,7 +334,9 @@ def zs_guess_zsync_params(self):
         import math
 
         size = self.h.size
-        if size < 100000000:
+        if size > 1024*1024*1024 and self.zsync_block_size_for_1G is not None:
+            blocksize = self.zsync_block_size_for_1G
+        elif size < 100000000:
             blocksize = 2048
         else:
             blocksize = 4096
@@ -389,6 +392,7 @@ def zs_get_block_sums(self, buf):
             c = md4.digest()
 
             if self.do_zsync_hashes:
+                import zsync
                 r = zsync.rsum06(block)
 
                 # save only some trailing bytes
diff --git a/mb/scripts/mb b/mb/scripts/mb
index 93b5422e..02f39a2f 100755
--- a/mb/scripts/mb
+++ b/mb/scripts/mb
@@ -1158,7 +1158,9 @@ class MirrorDoctor(cmdln.Cmdln):
                                                         'chunked_hashes'),
                                                     chunk_size=chunk_size,
                                                     do_chunked_with_zsync=do_chunked_with_zsync,
-                                                    skip_metadata=skip_metadata)
+                                                    skip_metadata=skip_metadata,
+                                                    zsync_block_size_for_1G=self.config.dbconfig.get(
+                                                        'zsync_block_size_for_1G'))
                 except OSError as e:
                     if e.errno == errno.ENOENT:
                         sys.stderr.write('File vanished: %r\n' % src)
diff --git a/mb/tests/conf_tests.py b/mb/tests/conf_tests.py
new file mode 100644
index 00000000..618b7774
--- /dev/null
+++ b/mb/tests/conf_tests.py
@@ -0,0 +1,22 @@
+import unittest
+
+from mb.conf import adjust_zsync_block_size_for_1G
+
+
+class TestConfig(unittest.TestCase):
+
+    def test_adjust_zsync_block_size_for_1G(self):
+        cases = {
+            0: None,
+            1023: None,
+            1025: 1024,
+            3*1024: 2*1024,
+            4*1024: 4*1024,
+            4*1024+1: 4*1024,
+            1024*1024*1024+1: 1024*1024*1024
+        }
+        for n in cases:
+            self.assertEqual(cases[n], adjust_zsync_block_size_for_1G(n), "for input " + repr(n))
+
+if __name__ == '__main__':
+    unittest.main()