.\" Copyright 1998 Andries E. Brouwer (aeb@cwi.nl)
.\"
.\" May be distributed under the GNU General Public License
-.\" Rewritten for 2.1.117, aeb, 981010.
.\"
.TH MKSWAP 8 "March 2009" "util-linux" "System Administration"
.SH NAME
.RE
.fi
-Note that a swap file must not contain any holes (so, using
+Note that a swap file must not contain any holes. Using
.BR cp (1)
-to create the file is not acceptable).
+to create the file is not acceptable. Neither is use of
+.BR fallocate (1)
+on file systems that support preallocated files, such as
+.BR XFS " or " ext4 ,
+or on copy-on-write filesystems like
+.BR btrfs .
+It is recommended to use
+.BR dd (1)
+and /dev/zero in these cases. Please read notes from
+.BR swapon (8)
+before adding a swap file to copy-on-write filesystems.
.SH ENVIRONMENT
.IP LIBBLKID_DEBUG=all
.BR swapon (8)
.SH AVAILABILITY
The mkswap command is part of the util-linux package and is available from
-ftp://ftp.kernel.org/pub/linux/utils/util-linux/.
+https://www.kernel.org/pub/linux/utils/util-linux/.