diff --git a/btrbk b/btrbk
index 20fe8f6..34908d9 100755
--- a/btrbk
+++ b/btrbk
@@ -2358,9 +2358,10 @@ sub filefrag_extentmap($)
 sub aio_extentmap($)
 {
   my $vol = shift || die;
+  my $starttime = time;
 
   INFO("Fetching extent information (IO::AIO) for: $vol->{PRINT}");
-  my $starttime = time;
+
   # NOTE: this returns exitstatus=0 if file is not found, or no files found
   my $ret = run_cmd( cmd => [ 'find', { unsafe => $vol->{PATH} }, '-xdev', '-type', 'f' ],
                      large_output => 1 );
@@ -2369,44 +2370,42 @@ sub aio_extentmap($)
     return undef;
   }
 
-  DEBUG("Reading ioctl FIEMAP on all files");
+  DEBUG("Reading ioctl FIEMAP of " . scalar(@$ret) . " files");
 
-  IO::AIO::max_outstanding(512);  # < 1024 (max file descriptors)
+  IO::AIO::max_outstanding(128);  # < 1024 (max file descriptors)
+  IO::AIO::max_poll_reqs(32);
 
-  # not sure how IO::AIO does its threading, it says we should not care about it (?).
-  # anyway, it works without "my @range :shared" (from threads::shared).
   # Note: aio_fiemap returns byte range (not blocks)
   my @range;
-  my $relax = 0;
   my $count = 0;
+  my $inline = 0;
   foreach my $file (@$ret) {
     IO::AIO::aio_open($file, IO::AIO::O_RDONLY(), 0, sub {
-      my $fh = shift or die "failed to open $file: $!";
-      #aioreq_pri 4;
-      # aio_fiemap $fh, $start, $length, $flags, $count, $cb->(\@extents)
-      IO::AIO::aio_fiemap($fh, 0, undef, 0, undef, sub {
-        my $aref = shift; # [$logical, $physical, $length, $flags]
-        # ignore inline data: for some reason, filefrag reports different size (x 4096 ?)
-        push @range, map { ($_->[3] & IO::AIO::FIEMAP_EXTENT_DATA_INLINE()) ? () :
-                           [ $_->[1], $_->[1] + $_->[2] - 1 ] } @$aref;
+      return unless($_[0]); # graceful abort on file open errors (check $count below)
+      IO::AIO::aio_fiemap($_[0], 0, undef, 0, undef, sub {
         $count++;
-        close $fh;
+        foreach(@{$_[0]}) { # [$logical, $physical, $length, $flags]
+          if($_->[3] & IO::AIO::FIEMAP_EXTENT_DATA_INLINE() ) {
+            # ignore inline data: for some reason, filefrag reports different size (x 4096 ?)
+            $inline++;
+            next;
+          }
+          push @range, [ $_->[1], $_->[1] + $_->[2] - 1 ];
+        }
       });
     });
 
-    # the above eats up all our filedescriptors, relax every now and then
-    $relax++;
-    if($relax > 256) {
-      # poll_cb is slow, no need to call it every time
-      IO::AIO::poll_cb(); # max_outstanding is only unsed if poll_cb is called
-      TRACE "aio_fiemap: processed $count files, " . scalar(@range) . " regions" if($loglevel >= 4);
-      $relax = 0;
-    }
+    # poll, or the above eats up all our filedescriptors
+    IO::AIO::poll_cb(); # takes "max_outstanding" and "max_poll_reqs" settings
+    #TRACE "aio_fiemap: processed $ret : $count files (inline=$inline), " . scalar(@range) . " regions" if($loglevel >= 4);
   }
 
   IO::AIO::flush();
 
-  DEBUG("parsed FIEMAP of $count files: " . scalar(@range) . " regions in " . (time - $starttime) . "s for: $vol->{PRINT}");
+  WARN "Failed to open $count / " . scalar(@$ret) . " files" if($count != scalar(@$ret));
+
+  DEBUG("Parsed " . scalar(@range) . " regions for $count files in " . (time - $starttime) . "s for: $vol->{PRINT}");
+  DEBUG("Ignored $inline \"inline\" extents for: $vol->{PRINT}") if($inline);
   return extentmap_merge(\@range);
 }