Expand description of how gptboot and gptzfsboot choose a partition for

booting.

Reviewed by:	ae
MFC after:	1 week

1 files changed, 28 insertions, 11 deletions

diff --git a/sbin/geom/class/part/gpart.8 b/sbin/geom/class/part/gpart.8
index 61f8275f8436..8843e5324954 100644
--- a/sbin/geom/class/part/gpart.8
+++ b/sbin/geom/class/part/gpart.8
@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd January 3, 2013
+.Dd January 25, 2013
 .Dt GPART 8
 .Os
 .Sh NAME
@@ -807,34 +807,51 @@ Both types of bootstrap code are used to boot from the GUID Partition Table.
 First, a protective MBR is embedded into the first disk sector from the
 .Pa /boot/pmbr
 image.
-It searches the GPT
+It searches through the GPT for a
 .Cm freebsd-boot
 partition (see the
 .Sx "PARTITION TYPES"
-section) in the GPT and runs the next bootstrap stage from it.
+section) and runs the next bootstrap stage from it.
 The
 .Cm freebsd-boot
 partition should be smaller than 545 KB.
+It can be located either before or after other
+.Fx
+partitions on the disk.
 There are two variants of bootstrap code to write to this partition:
 .Pa /boot/gptboot
 and
 .Pa /boot/gptzfsboot .
 .Pa /boot/gptboot
 is used to boot from UFS.
-It searches
+It searches through
+.Cm freebsd-ufs
+partitions in the GPT and boots from the first one with the
+.Cm bootonce
+attribute set.
+If that attribute is not found,
+.Pa /boot/gptboot
+boots from the first
 .Cm freebsd-ufs
-GPT partitions and starts
+partition with the
+.Cm bootme
+attribute set.
+If neither attribute is found,
+.Pa /boot/gptboot
+boots from the first
+.Cm freebsd-ufs
+partition.
 .Pa /boot/loader
 .Pq the third bootstrap stage
-if found.
-The
+is loaded from the first partition that matches these conditions.
 .Pa /boot/gptzfsboot
 is used to boot from ZFS.
-It searches
+It searches through the GPT for
 .Cm freebsd-zfs
-GPT partitions and starts
+partitions, trying to detect ZFS pools.
+After all pools are detected,
 .Pa /boot/zfsloader
-if found.
+is started from the first one found.
 .Pp
 The VTOC8 scheme does not support embedding bootstrap code.
 Instead, the 8 KBytes bootstrap code image
@@ -945,7 +962,7 @@ The GEOM PART class can detect the same partition table visible through
 different GEOM providers, and some of them will be marked as corrupt.
 Be careful when choosing a provider for recovery.
 If you choose incorrectly you can destroy the metadata of another GEOM class,
-e.g.\& GEOM MIRROR or GEOM LABEL.
+e.g.,\& GEOM MIRROR or GEOM LABEL.
 .Sh SYSCTL VARIABLES
 The following
 .Xr sysctl 8