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.

diff --git a/lib/PublicInbox/Syscall.pm b/lib/PublicInbox/Syscall.pm
index a7fa37363ec5e30650f2258de2f3ac1c3b012cac..b1cb4e688fee1558207cb30f94da2f92561ba761 100644 (file)
--- 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 0940d908e935772183fda449be0ff7d6b152e75c..b482a0ec6c5f5aacf754403fd9cb5597666c1763 100644 (file)
--- 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;