lib/PublicInbox/Msgmap.pm | 2 ++

diff --git a/lib/PublicInbox/Msgmap.pm b/lib/PublicInbox/Msgmap.pm
index 51b6a68c8547e81a4eaf9168000354fea548085a..c85e7796cbd4512f2c286628a4eff8f04aed9388 100644
--- a/lib/PublicInbox/Msgmap.pm
+++ b/lib/PublicInbox/Msgmap.pm
@@ -14,6 +14,7 @@ use DBI;
 use DBD::SQLite;
 use File::Temp qw(tempfile);
 use PublicInbox::Over;
+use PublicInbox::Spawn;
 
 sub new {
 	my ($class, $git_dir, $writable) = @_;
@@ -50,6 +51,7 @@ # used to keep track of used numeric mappings for v2 reindex
 sub tmp_clone {
 	my ($self, $dir) = @_;
 	my ($fh, $fn) = tempfile('msgmap-XXXXXXXX', EXLOCK => 0, DIR => $dir);
+	PublicInbox::Spawn::set_nodatacow(fileno($fh));
 	my $tmp;
 	if ($self->{dbh}->can('sqlite_backup_to_dbh')) {
 		$tmp = ref($self)->new_file($fn, 2);