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If the filesystem is Btrfs and the disk is not an abstracted device such as
LVM, RAID, or encryption, the reserved space in the Btrfs header can be used as
the environment block (see The GRUB environment block). This provides a fixed raw
block that GRUB can reliably write to. The env_block records this
location in GRUB blocklist syntax (see How to specify block lists) so that
grub-editenv and grub.cfg know how to access and use the
external raw block.
This variable is initialized when grubenv is first created by
grub-editenv and is treated as read-only to avoid being overwritten
with an unpredictable value.