GPTBOOT(8)              FreeBSD System Manager's Manual             GPTBOOT(8)

NAME
     gptboot - GPT bootcode for UFS on BIOS-based computers

DESCRIPTION
     gptboot is used on BIOS-based computers to boot from a UFS partition on a
     GPT-partitioned disk.  gptboot is installed in a freebsd-boot partition
     with gpart(8).

     When it starts, gptboot first reads the GPT and determines which drive
     and partition to boot from, as described under BOOTING, below.  If it
     does not find an eligible partition, or if the user hits a key within
     three seconds, gptboot switches from auto-boot to interactive mode.
     Interactive mode allows manual selection of the disk, partition,
     filename, and boot option flags, as described in boot(8).

IMPLEMENTATION NOTES
     The GPT standard allows a variable number of partitions, but gptboot only
     boots from tables with 128 partitions or less.

PARTITION ATTRIBUTES
     gptboot checks and manages several attributes of GPT UFS partitions.

     bootme          Attempt to boot from this partition.  If more than one
                     partition has the bootme attribute set, gptboot will
                     attempt to boot each one until successful.

     bootonce        Attempt to boot from this partition only one time.
                     Setting this attribute with gpart(8) automatically also
                     sets the bootme attribute.  Multiple partitions may have
                     the bootonce and bootme attributes set.

     bootfailed      The bootfailed attribute marks partitions that had the
                     bootonce attribute set, but failed to boot.  This
                     attribute is managed by the system.  See BOOTING and
                     POST-BOOT ACTIONS below for details.

USAGE
     For normal usage, the user does not have to set or manage any of the
     partition attributes.  gptboot will boot from the first UFS partition
     found.

     The bootonce attribute can be used for testing an upgraded operating
     system on an already-working computer.  The existing system partition is
     left untouched, and the new version of the operating system to be tested
     is installed on another partition.  The bootonce attribute is set on that
     new test partition.  The next boot is attempted from the test partition.
     Success or failure will be shown in the system log files.  After a
     successful boot of the test partition, a user script can check the logs
     and change the bootme attributes so the test partition becomes the new
     system partition.  Because the bootonce attribute is cleared after an
     attempted boot, a failed boot will not leave the system attempting to
     boot from a partition that will never succeed.  Instead, the system will
     boot from the older, known-working operating system that has not been
     modified.  If the bootme attribute is set on any partitions, booting will
     be attempted from them first.  If no partitions with bootme attributes
     are found, booting will be attempted from the first UFS partition found.

BOOTING
     gptboot first reads the partition table.  All freebsd-ufs partitions with
     only the bootonce attribute set, indicating a failed boot, are set to
     bootfailed.  gptboot then scans through all of the freebsd-ufs
     partitions.  Boot behavior depends on the combination of bootme and
     bootonce attributes set on those partitions.

     bootonce + bootme          Highest priority: booting is attempted from
                                each of the freebsd-ufs partitions with both
                                of these attributes.  On each partition, the
                                bootme attribute is removed and the boot
                                attempted.

     bootme                     Middle priority: booting is attempted from
                                each of the freebsd-ufs partitions with the
                                bootme attribute.

     If neither bootonce nor bootme attributes are found on any partitions,
     booting is attempted from the first freebsd-ufs partition on the disk.

POST-BOOT ACTIONS
     The startup script /etc/rc.d/gptboot checks the attributes of freebsd-ufs
     partitions on all GPT disks.  Partitions with the bootfailed attribute
     generate a "boot from X failed" system log message.  Partitions with only
     the bootonce attribute, indicating a partition that successfully booted,
     generate a "boot from X succeeded" system log message.  The bootfailed
     attributes are cleared from all the partitions.  The bootonce attribute
     is cleared from the partition that successfully booted.  There is
     normally only one of these.

FILES
     /boot/gptboot  bootcode binary
     /boot.config   parameters for the boot blocks (optional)

EXAMPLES
     gptboot is installed in a freebsd-boot partition, usually the first
     partition on the disk.  A "protective MBR" (see gpart(8)) is typically
     installed in combination with gptboot.

     Install gptboot on the ada0 drive:

           gpart bootcode -b /boot/pmbr -p /boot/gptboot -i 1 ada0

     gptboot can also be installed without the PMBR:

           gpart bootcode -p /boot/gptboot -i 1 ada0

     Set the bootme attribute for partition 2:

           gpart set -a bootme -i 2 ada0

     Set the bootonce attribute for partition 2, automatically also setting
     the bootme attribute:

           gpart set -a bootonce -i 2 ada0

SEE ALSO
     boot.config(5), rc.conf(5), boot(8), gpart(8)

HISTORY
     gptboot appeared in FreeBSD 7.1.

AUTHORS
     This manual page written by Warren Block <wblock@FreeBSD.org>.

FreeBSD 13.1-RELEASE-p6         April 30, 2019         FreeBSD 13.1-RELEASE-p6