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Manual Pages  — GPTZFSBOOT


gptzfsboot – GPT bootcode for ZFS on BIOS-based computers



gptzfsboot is used on BIOS-based computers to boot from a filesystem in a ZFS pool. gptzfsboot is installed in a freebsd-boot partition of a GPT-partitioned disk with gpart(8).


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


gptzfsboot tries to find all ZFS pools that are composed of BIOS-visible hard disks or partitions on them. gptzfsboot looks for ZFS device labels on all visible disks and in discovered supported partitions for all supported partition scheme types. The search starts with the disk from which gptzfsboot itself was loaded. Other disks are probed in BIOS defined order. After a disk is probed and gptzfsboot determines that the whole disk is not a ZFS pool member, the individual partitions are probed in their partition table order. Currently GPT and MBR partition schemes are supported. With the GPT scheme, only partitions of type freebsd-zfs are probed. The first pool seen during probing is used as a default boot pool.

The filesystem specified by the bootfs property of the pool is used as a default boot filesystem. If the bootfs property is not set, then the root filesystem of the pool is used as the default. zfsloader(8) is loaded from the boot filesystem. If /boot.config or /boot/config is present in the boot filesystem, boot options are read from it in the same way as boot(8).

The ZFS GUIDs of the first successfully probed device and the first detected pool are made available to zfsloader(8) in the vfs.zfs.boot.primary_vdev and vfs.zfs.boot.primary_pool variables.


Normally gptzfsboot will boot in fully automatic mode. However, like boot(8), it is possible to interrupt the automatic boot process and interact with gptzfsboot through a prompt. gptzfsboot accepts all the options that boot(8) supports.

The filesystem specification and the path to zfsloader(8) are different from boot(8). The format is

[zfs:pool/filesystem: ][/path/to/loader ]

Both the filesystem and the path can be specified. If only a path is specified, then the default filesystem is used. If only a pool and filesystem are specified, then /boot/zfsloader is used as a path.

Additionally, the status command can be used to query information about discovered pools. The output format is similar to that of zpool status (seezpool(8)).

The configured or automatically determined ZFS boot filesystem is stored in the zfsloader(8) loaddev variable, and also set as the initial value of the currdev variable.


  boot code binary
/boot.config parameters for the boot block (optional)
/boot/config alternative parameters for the boot block (optional)


gptzfsboot is typically installed in combination with a "protective MBR" ( see gpart(8) ). To install gptzfsboot on the ada0 drive:
gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 ada0

gptzfsboot can also be installed without the PMBR:

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


boot.config(5), boot(8), gpart(8), loader(8), zfsloader(8), zpool(8)


gptzfsboot appeared in FreeBSD 7.3.


This manual page was written by Andriy Gapon <avg@FreeBSD.org>.


gptzfsboot looks for ZFS meta-data only in MBR partitions (known on FreeBSD as slices). It does not look into BSD disklabel(8) partitions that are traditionally called partitions. If a disklabel partition happens to be placed so that ZFS meta-data can be found at the fixed offsets relative to a slice, then gptzfsboot will recognize the partition as a part of a ZFS pool, but this is not guaranteed to happen.

GPTZFSBOOT (8) September 15, 2014

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Please direct any comments about this manual page service to Ben Bullock.