MANIFEST                     |  2 ++
 lib/PublicInbox/NDC_PP.pm    | 29 +++++++++++++++++++++++++++++
 lib/PublicInbox/Over.pm      |  5 +++++
 lib/PublicInbox/SearchIdx.pm |  5 ++++-
 lib/PublicInbox/Spawn.pm     | 63 ++++++++++++++++++++++++++++++++++++++++++++++++++---
 t/nodatacow.t                | 34 ++++++++++++++++++++++++++++++++++

diff --git a/MANIFEST b/MANIFEST
index f46a0776ddbcd9901365cabf33fa68f4bfc8531e..d312e305cc0149ee1bcbe8bd5a34a13891c582ea 100644
--- a/MANIFEST
+++ b/MANIFEST
@@ -156,6 +156,7 @@ lib/PublicInbox/MboxGz.pm
 lib/PublicInbox/MsgIter.pm
 lib/PublicInbox/MsgTime.pm
 lib/PublicInbox/Msgmap.pm
+lib/PublicInbox/NDC_PP.pm
 lib/PublicInbox/NNTP.pm
 lib/PublicInbox/NNTPD.pm
 lib/PublicInbox/NNTPdeflate.pm
@@ -309,6 +310,7 @@ t/multi-mid.t
 t/nntp.t
 t/nntpd-tls.t
 t/nntpd.t
+t/nodatacow.t
 t/nulsubject.t
 t/over.t
 t/plack-2-txt-bodies.eml
diff --git a/lib/PublicInbox/NDC_PP.pm b/lib/PublicInbox/NDC_PP.pm
new file mode 100644
index 0000000000000000000000000000000000000000..0d20030d512903896dd2266fb73e2c8a4bfd369a
--- /dev/null
+++ b/lib/PublicInbox/NDC_PP.pm
@@ -0,0 +1,29 @@
+# Copyright (C) 2020 all contributors <meta@public-inbox.org>
+# License: AGPL-3.0+ <https://www.gnu.org/licenses/agpl-3.0.txt>
+
+# Pure-perl class for Linux non-Inline::C users to disable COW for btrfs
+package PublicInbox::NDC_PP;
+use strict;
+use v5.10.1;
+
+sub set_nodatacow ($) {
+	my ($fd) = @_;
+	return if $^O ne 'linux';
+	defined(my $path = readlink("/proc/self/fd/$fd")) or return;
+	open my $mh, '<', '/proc/self/mounts' or return;
+	for (grep(/ btrfs /, <$mh>)) {
+		my (undef, $mnt_path, $type) = split(/ /);
+		next if $type ne 'btrfs'; # in case of false-positive from grep
+
+		# weird chars are escaped as octal
+		$mnt_path =~ s/\\(0[0-9]{2})/chr(oct($1))/egs;
+		$mnt_path .= '/' unless $mnt_path =~ m!/\z!;
+		if (index($path, $mnt_path) == 0) {
+			# error goes to stderr, but non-fatal for us
+			system('chattr', '+C', $path);
+			last;
+		}
+	}
+}
+
+1;
diff --git a/lib/PublicInbox/Over.pm b/lib/PublicInbox/Over.pm
index f32743c05a8a461baf0c4f5a52c983d5fecd25af..0146414cf4f2ebcf9575714b090e98992b201797 100644
--- a/lib/PublicInbox/Over.pm
+++ b/lib/PublicInbox/Over.pm
@@ -18,7 +18,12 @@ 	my ($self, $rw) = @_;
 	my $f = delete $self->{filename};
 	if (!-f $f) { # SQLite defaults mode to 0644, we want 0666
 		if ($rw) {
+			require PublicInbox::Spawn;
 			open my $fh, '+>>', $f or die "failed to open $f: $!";
+			PublicInbox::Spawn::set_nodatacow(fileno($fh));
+			my $j = "$f-journal";
+			open $fh, '+>>', $j or die "failed to open $j: $!";
+			PublicInbox::Spawn::set_nodatacow(fileno($fh));
 		} else {
 			$self->{filename} = $f; # die on stat() below:
 		}
diff --git a/lib/PublicInbox/SearchIdx.pm b/lib/PublicInbox/SearchIdx.pm
index 1fc574106c543a81d5fc4eef190746948a44e929..aa8d8ce325cf203d1ea77abc9e89d7bb5b73091b 100644
--- a/lib/PublicInbox/SearchIdx.pm
+++ b/lib/PublicInbox/SearchIdx.pm
@@ -125,8 +125,11 @@ 		$self->lock_acquire;
 
 		# don't create empty Xapian directories if we don't need Xapian
 		my $is_shard = defined($self->{shard});
-		if (!$is_shard || ($is_shard && need_xapian($self))) {
+		if (!-d $dir && (!$is_shard ||
+				($is_shard && need_xapian($self)))) {
 			File::Path::mkpath($dir);
+			opendir my $dh, $dir or die "opendir($dir): $!\n";
+			PublicInbox::Spawn::set_nodatacow(fileno($dh));
 		}
 	}
 	return unless defined $flag;
diff --git a/lib/PublicInbox/Spawn.pm b/lib/PublicInbox/Spawn.pm
index db679b770cfd81d985c4416db9b7186d718587a4..50f3185156a3d9160edc002508ee0e874997a3d0 100644
--- a/lib/PublicInbox/Spawn.pm
+++ b/lib/PublicInbox/Spawn.pm
@@ -10,6 +10,9 @@ # Currently, we only use this for code intended for long running
 # daemons (inside the PSGI code (-httpd) and -nntpd).  The short-lived
 # scripts (-mda, -index, -learn, -init) either use IPC::run or standard
 # Perl routines.
+#
+# There'll probably be more OS-level C stuff here, down the line.
+# We don't want too many DSOs: https://udrepper.livejournal.com/8790.html
 
 package PublicInbox::Spawn;
 use strict;
@@ -25,6 +28,7 @@ #include <sys/time.h>
 #include <sys/resource.h>
 #include <unistd.h>
 #include <stdlib.h>
+#include <errno.h>
 
 /* some platforms need alloca.h, but some don't */
 #if defined(__GNUC__) && !defined(alloca)
@@ -144,12 +148,51 @@ 	return (int)pid;
 }
 VFORK_SPAWN
 
+# btrfs on Linux is copy-on-write (COW) by default.  As of Linux 5.7,
+# this still leads to fragmentation for SQLite and Xapian files where
+# random I/O happens, so we disable COW just for SQLite files and Xapian
+# directories.  Disabling COW disables checksumming, so we only do this
+# for regeneratable files, and not canonical git storage (git doesn't
+# checksum refs, only data under $GIT_DIR/objects).
+my $set_nodatacow = $^O eq 'linux' ? <<'SET_NODATACOW' : '';
+#include <sys/ioctl.h>
+#include <sys/vfs.h>
+#include <linux/magic.h>
+#include <linux/fs.h>
+#include <errno.h>
+#include <stdio.h>
+#include <string.h>
+
+void set_nodatacow(int fd)
+{
+	struct statfs buf;
+	int val = 0;
+
+	if (fstatfs(fd, &buf) < 0) {
+		fprintf(stderr, "fstatfs: %s\\n", strerror(errno));
+		return;
+	}
+
+	/* only btrfs is known to have this problem, so skip for non-btrfs */
+	if (buf.f_type != BTRFS_SUPER_MAGIC)
+		return;
+
+	if (ioctl(fd, FS_IOC_GETFLAGS, &val) < 0) {
+		fprintf(stderr, "FS_IOC_GET_FLAGS: %s\\n", strerror(errno));
+		return;
+	}
+	val |= FS_NOCOW_FL;
+	if (ioctl(fd, FS_IOC_SETFLAGS, &val) < 0)
+		fprintf(stderr, "FS_IOC_SET_FLAGS: %s\\n", strerror(errno));
+}
+SET_NODATACOW
+
 my $inline_dir = $ENV{PERL_INLINE_DIRECTORY} //= (
 		$ENV{XDG_CACHE_HOME} //
 		( ($ENV{HOME} // '/nonexistent').'/.cache' )
 	).'/public-inbox/inline-c';
 
-$vfork_spawn = undef unless -d $inline_dir && -w _;
+$set_nodatacow = $vfork_spawn = undef unless -d $inline_dir && -w _;
 if (defined $vfork_spawn) {
 	# Inline 0.64 or later has locking in multi-process env,
 	# but we support 0.5 on Debian wheezy
@@ -158,14 +201,21 @@ 	eval {
 		my $f = "$inline_dir/.public-inbox.lock";
 		open my $fh, '>', $f or die "failed to open $f: $!\n";
 		flock($fh, LOCK_EX) or die "LOCK_EX failed on $f: $!\n";
-		eval 'use Inline C => $vfork_spawn';
+		eval 'use Inline C => $vfork_spawn . $set_nodatacow';
 		my $err = $@;
+		my $ndc_err;
+		if ($err && $set_nodatacow) { # missing Linux kernel headers
+			$ndc_err = $err;
+			undef $set_nodatacow;
+			eval 'use Inline C => $vfork_spawn';
+		}
 		flock($fh, LOCK_UN) or die "LOCK_UN failed on $f: $!\n";
 		die $err if $err;
+		warn $ndc_err if $ndc_err;
 	};
 	if ($@) {
 		warn "Inline::C failed for vfork: $@\n";
-		$vfork_spawn = undef;
+		$set_nodatacow = $vfork_spawn = undef;
 	}
 }
 
@@ -173,6 +223,13 @@ unless (defined $vfork_spawn) {
 	require PublicInbox::SpawnPP;
 	*pi_fork_exec = \&PublicInbox::SpawnPP::pi_fork_exec
 }
+unless ($set_nodatacow) {
+	require PublicInbox::NDC_PP;
+	no warnings 'once';
+	*set_nodatacow = \&PublicInbox::NDC_PP::set_nodatacow;
+}
+undef $set_nodatacow;
+undef $vfork_spawn;
 
 sub which ($) {
 	my ($file) = @_;
diff --git a/t/nodatacow.t b/t/nodatacow.t
new file mode 100644
index 0000000000000000000000000000000000000000..87b6bdf72142a72a008e3366f301a1e30a0a517f
--- /dev/null
+++ b/t/nodatacow.t
@@ -0,0 +1,34 @@
+#!perl -w
+# Copyright (C) 2020 all contributors <meta@public-inbox.org>
+# License: AGPL-3.0+ <https://www.gnu.org/licenses/agpl-3.0.txt>
+use strict;
+use Test::More;
+use File::Temp qw(tempfile);
+use PublicInbox::TestCommon;
+use PublicInbox::Spawn qw(which);
+use_ok 'PublicInbox::NDC_PP';
+
+SKIP: {
+	my $nr = 2;
+	skip 'test is Linux-only', $nr if $^O ne 'linux';
+	my $dir = $ENV{BTRFS_TESTDIR};
+	skip 'BTRFS_TESTDIR not defined', $nr unless defined $dir;
+	skip 'chattr(1) not installed', $nr unless which('chattr');
+	my $lsattr = which('lsattr') or skip 'lsattr(1) not installed', $nr;
+	my ($fh, $name) = tempfile(DIR => $dir, UNLINK => 1);
+	BAIL_OUT "tempfile: $!" unless $fh && defined($name);
+	my $pp_sub = \&PublicInbox::NDC_PP::set_nodatacow;
+	$pp_sub->(fileno($fh));
+	my $res = xqx([$lsattr, $name]);
+	like($res, qr/C/, "`C' attribute set with pure Perl");
+
+	my $ic_sub = \&PublicInbox::Spawn::set_nodatacow;
+	$pp_sub == $ic_sub and
+		skip 'Inline::C or Linux kernel headers missing', 1;
+	($fh, $name) = tempfile(DIR => $dir, UNLINK => 1);
+	$ic_sub->(fileno($fh));
+	$res = xqx([$lsattr, $name]);
+	like($res, qr/C/, "`C' attribute set with Inline::C");
+};
+
+done_testing;