From 74d4ffbba09aab2f73538a37b05ee3abcc921374 Mon Sep 17 00:00:00 2001
From: Eric Wong <e@80x24.org>
Date: Thu, 7 Aug 2025 00:50:05 +0000
Subject: [PATCH] nodatacow: warn about CoW being disabled on btrfs

While Xapian and SQLite performance is untenable with CoW on
btrfs, disabling CoW is not without caveats on btrfs.  So warn
users about the possibility of data corruption.
---
 lib/PublicInbox/Syscall.pm | 14 ++++++++++++--
 t/nodatacow.t              | 15 ++++++++++++++-
 2 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/lib/PublicInbox/Syscall.pm b/lib/PublicInbox/Syscall.pm
index a7fa37363..b1cb4e688 100644
--- a/lib/PublicInbox/Syscall.pm
+++ b/lib/PublicInbox/Syscall.pm
@@ -435,6 +435,7 @@ sub rename_noreplace ($$) {
 	}
 }
 
+# returns "0 but true" on success, undef or
 sub nodatacow_fh ($) {
 	my ($fh) = @_;
 	my $buf = "\0" x 120;
@@ -453,8 +454,17 @@ sub nodatacow_fh ($) {
 		warn("FS_IOC_SETFLAGS: $!\n");
 }
 
-sub nodatacow_dir {
-	if (open my $fh, '<', $_[0]) { nodatacow_fh($fh) }
+sub nodatacow_dir ($) {
+	my ($f) = @_;
+	if (open my $fh, '<', $f) {
+		my $rc = nodatacow_fh($fh); # returns "0 but true" on success
+		$rc && $rc == 0 and warn <<EOM;
+W: Disabling copy-on-write (CoW) on `$f'
+W: to avoid pathological slowdowns.  Data corruption may occur on unclean
+W: shutdowns, especially if using any form of BTRFS RAID.  Periodic defrag
+W: is recommended for *.sqlite3 and *.glass files to maintain performance.
+EOM
+	}
 }
 
 use constant \%CONST;
diff --git a/t/nodatacow.t b/t/nodatacow.t
index 0940d908e..b482a0ec6 100644
--- a/t/nodatacow.t
+++ b/t/nodatacow.t
@@ -34,9 +34,22 @@ SKIP: {
 
 	$name = "$dn/pp.d";
 	mkdir($name) or BAIL_OUT "mkdir($name) $!";
-	PublicInbox::Syscall::nodatacow_dir($name);
+
+	{
+		my @w;
+		local $SIG{__WARN__} = sub { push @w, @_ };
+		PublicInbox::Syscall::nodatacow_dir($name);
+		like "@w", qr/^W: Disabling copy-on-write/sm,
+			'warned about disabling CoW';
+	}
 	$res = xqx([$lsattr, '-d', $name]);
 	like($res, qr/C.*\Q$name\E/, "`C' attribute set on dir with pure Perl");
+	{
+		my @w;
+		local $SIG{__WARN__} = sub { push @w, @_ };
+		PublicInbox::Syscall::nodatacow_dir($name);
+		is_deeply \@w, [], 'no warnings if CoW already disabled';
+	}
 };
 
 done_testing;
-- 
2.47.3