lib/PublicInbox/V2Writable.pm | 22 ++++++++++++++++++++--
 t/v2writable.t                | 11 ++++++++++-

diff --git a/lib/PublicInbox/V2Writable.pm b/lib/PublicInbox/V2Writable.pm
index 3bcea37defde1f588f96066259e7dc87cb6b9e73..7728b91a542a79727eaed2f627639fdad080cbb0 100644
--- a/lib/PublicInbox/V2Writable.pm
+++ b/lib/PublicInbox/V2Writable.pm
@@ -19,7 +19,9 @@ # an estimate of the post-packed size to the raw uncompressed size
 my $PACKING_FACTOR = 0.4;
 
 # assume 2 cores if GNU nproc(1) is not available
-my $NPROC = int($ENV{NPROC} || `nproc 2>/dev/null` || 2);
+sub nproc () {
+	int($ENV{NPROC} || `nproc 2>/dev/null` || 2);
+}
 
 sub new {
 	my ($class, $v2ibx, $creat) = @_;
@@ -32,12 +34,28 @@ 		} else {
 			die "$dir does not exist\n";
 		}
 	}
+
+	my $nparts = 0;
+	my $xpfx = "$dir/xap" . PublicInbox::Search::SCHEMA_VERSION;
+
+	# always load existing partitions in case core count changes:
+	if (-d $xpfx) {
+		foreach my $part (<$xpfx/*>) {
+			-d $part && $part =~ m!/\d+\z! or next;
+			eval {
+				Search::Xapian::Database->new($part)->close;
+				$nparts++;
+			};
+		}
+	}
+	$nparts = nproc() if ($nparts == 0);
+
 	my $self = {
 		-inbox => $v2ibx,
 		im => undef, #  PublicInbox::Import
 		xap_rw => undef, # PublicInbox::V2SearchIdx
 		xap_ro => undef,
-		partitions => $NPROC,
+		partitions => $nparts,
 		transact_bytes => 0,
 		# limit each repo to 1GB or so
 		rotate_bytes => int((1024 * 1024 * 1024) / $PACKING_FACTOR),
diff --git a/t/v2writable.t b/t/v2writable.t
index 2d35aca387d524b145e2fe58844e4a8391778d3d..404c86518fd0382769afa3833c57dbdb90fa67da 100644
--- a/t/v2writable.t
+++ b/t/v2writable.t
@@ -30,7 +30,11 @@ 	],
 	body => "hello world\n",
 );
 
-my $im = PublicInbox::V2Writable->new($ibx, 1);
+my $im = eval {
+	local $ENV{NPROC} = '1';
+	PublicInbox::V2Writable->new($ibx, 1);
+};
+is($im->{partitions}, 1, 'one partition when forced');
 ok($im->add($mime), 'ordinary message added');
 
 if ('ensure git configs are correct') {
@@ -182,5 +186,10 @@ 		is($nn{$mid}++, 0, "MID is unique in NEWNEWS");
 	}
 	is_deeply([sort keys %nn], [sort keys %uniq]);
 };
+{
+	local $ENV{NPROC} = 2;
+	$im = PublicInbox::V2Writable->new($ibx, 1);
+	is($im->{partitions}, 1, 'detected single partition from previous');
+}
 
 done_testing();