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man: document the new grow-file-system flag

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Lennart Poettering 2021-04-21 18:03:07 +02:00
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docs/DISCOVERABLE_PARTITIONS.md
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@ -94,24 +94,48 @@ localized.
## Partition Flags
For the root, `/usr/`, server data, home, variable data, temporary data and swap
partitions, the partition flag bit 63 ("*no-auto*") may be used to turn off
auto-discovery for the specific partition. If set, the partition will not be
automatically mounted or enabled.
This specification defines three GPT partition flags that may be set for the
partition types defined above:
For the root, `/usr/`, server data, home, variable data and temporary data
partitions, the partition flag bit 60 ("*read-only*") may be used to mark a
partition for read-only mounts only. If set, the partition will be mounted
read-only instead of read-write. Note that the variable data partition and the
temporary data partition will generally not be able to serve their purpose if
marked read-only, since by their very definition they are supposed to be
mutable. (The home and server data partitions are generally assumed to be
mutable as well, but the requirement for them is not equally strong.) Because
of that, while the read-only flag is defined and supported, it's almost never a
good idea to actually use it for these partitions.
1. For the root, `/usr/`, Verity, home, server data, variable data, temporary data,
swap and extended boot loader partitions, the partition flag bit 63
("*no-auto*") may be used to turn off auto-discovery for the specific
partition. If set, the partition will not be automatically mounted or
enabled.
Note that these two flag definitions happen to map nicely to the ones used by
Microsoft Basic Data Partitions.
2. For the root, `/usr/`, Verity, home, server data, variable data, temporary
data and extended boot loader partitions, the partition flag bit 60
("*read-only*") may be used to mark a partition for read-only mounts only.
If set, the partition will be mounted read-only instead of read-write. Note
that the variable data partition and the temporary data partition will
generally not be able to serve their purpose if marked read-only, since by
their very definition they are supposed to be mutable. (The home and server
data partitions are generally assumed to be mutable as well, but the
requirement for them is not equally strong.) Because of that, while the
read-only flag is defined and supported, it's almost never a good idea to
actually use it for these partitions. Also note that Verity partitions are
by their semantics always read-only. The flag is hence of little effect for
them, and it is recommended to set it unconditionally for the Verity
partition types.
3. For the root, `/usr/`, home, server data, variable data, temporary data and
extended boot loader partitions, the partition flag bit 59
("*grow-file-system*") may be used to mark a partition for automatic growing
of the contained file system to the size of the partition when
mounted. Tools that automatically mount disk image with a GPT partition
table are suggested to implicitly grow the contained file system to the
partition size they are contained in. This flag is without effect on
partitions marked read-only.
Note that the first two flag definitions happen to map nicely to the ones used
by Microsoft Basic Data Partitions.
All three of these flags generally affect only auto-discovery and automatic
mounting of disk images. If partitions marked with these flags are mounted
using low-level commands like
[mount(8)](https://man7.org/linux/man-pages/man2/mount.8.html) or directly with
[mount(2)](https://man7.org/linux/man-pages/man2/mount.2.html), they typically
have no effect.
## Suggested Mode of Operation