.\" ----------------------------------------------------------------------------
.\" "THE BEER-WARE LICENSE" (Revision 42):
.\" <phk@FreeBSD.org> wrote this file.  As long as you retain this notice you
.\" can do whatever you want with this stuff. If we meet some day, and you think
.\" this stuff is worth it, you can buy me a beer in return.   Poul-Henning Kamp
.\" ----------------------------------------------------------------------------
.\"
.\" $FreeBSD$
.\"
.Dd December 26, 2017
.Dt MD 4
.Os
.Sh NAME
.Nm md
.Nd memory disk
.Sh SYNOPSIS
To compile this driver into the kernel,
place the following lines in your
kernel configuration file:
.Bd -ragged -offset indent
.Cd "device md"
.Ed
.Pp
Alternatively, to load the driver as a
module at boot time, place the following line in
.Xr loader.conf 5 :
.Bd -literal -offset indent
geom_md_load="YES"
.Ed
.Sh DESCRIPTION
The
.Nm
driver provides support for four kinds of memory backed virtual disks:
.Bl -tag -width preload
.It Cm malloc
Backing store is allocated using
.Xr malloc 9 .
Only one malloc-bucket is used, which means that all
.Nm
devices with
.Cm malloc
backing must share the malloc-per-bucket-quota.
The exact size of this quota varies, in particular with the amount
of RAM in the
system.
The exact value can be determined with
.Xr vmstat 8 .
.It Cm preload
A file loaded by
.Xr loader 8
with type
.Sq md_image
is used for backing store.
For backwards compatibility the type
.Sq mfs_root
is also recognized.
If the kernel is created with option
.Dv MD_ROOT
the first preloaded image found will become the root file system.
.It Cm vnode
A regular file is used as backing store.
This allows for mounting ISO images without the tedious
detour over actual physical media.
.It Cm swap
Backing store is allocated from buffer memory.
Pages get pushed out to the swap when the system is under memory
pressure, otherwise they stay in the operating memory.
Using
.Cm swap
backing is generally preferable over
.Cm malloc
backing.
.El
.Pp
For more information, please see
.Xr mdconfig 8 .
.Sh EXAMPLES
To create a kernel with a ramdisk or MD file system, your kernel config
needs the following options:
.Bd -literal -offset indent
options 	MD_ROOT			# MD is a potential root device
options 	MD_ROOT_READONLY	# disallow mounting root writeable
options 	MD_ROOT_SIZE=8192	# 8MB ram disk
makeoptions	MFS_IMAGE=/h/foo/ARM-MD
options 	ROOTDEVNAME=\\"ufs:md0\\"
.Ed
.Pp
The image in
.Pa /h/foo/ARM-MD
will be loaded as the initial image each boot.
To create the image to use, please follow the steps to create a file-backed
disk found in the
.Xr mdconfig 8
man page.
Other tools will also create these images, such as NanoBSD.
.Sh ARM KERNEL OPTIONS
On armv6 and armv7 architectures, an MD_ROOT image larger than
approximately 55 MiB may require building a custom kernel using
several tuning options related to kernel memory usage.
.Bl -tag -width indent
.It Cd options LOCORE_MAP_MB=<num>
This configures how much memory is mapped for the kernel during
the early initialization stages.
The value must be at least as large as the kernel plus all preloaded
modules, including the root image.
There is no downside to setting this value too large, as long
as it does not exceed the amount of physical memory.
The default is 64 MiB.
.It Cd options NKPT2PG=<num>
This configures the number of kernel L2 page table pages to
preallocate during kernel initialization.
Each L2 page can map 4 MiB of kernel space.
The value must be large enough to map the kernel plus all preloaded
modules, including the root image.
The default value is 32, which is sufficient to map 128 MiB.
.It Cd options VM_KMEM_SIZE_SCALE=<num>
This configures the amount of kernel virtual address (KVA) space to
dedicate to the kmem_arena map.
The scale value is the ratio of physical to virtual pages.
The default value of 3 allocates a page of KVA for each 3 pages
of physical ram in the system.

The kernel and modules, including the root image, also consume KVA.
The combination of a large root image and the default scaling
may preallocate so much KVA that there is not enough
remaining address space to allocate kernel stacks, IO buffers,
and other resources that are not part of kmem_arena.
Overallocating kmem_arena space is likely to manifest as failure to
launch userland processes with "cannot allocate kernel stack" messages.

Setting the scale value too high may result in kernel failure to allocate
memory because kmem_arena is too small, and the failure may require
significant runtime to manifest.
Empirically, a value of 5 works well for a 200 MiB root image on
a system with 2 GiB of physical ram.
.El
.Sh SEE ALSO
.Xr gpart 8 ,
.Xr loader 8 ,
.Xr mdconfig 8 ,
.Xr mdmfs 8 ,
.Xr newfs 8 ,
.Xr vmstat 8
.Sh HISTORY
The
.Nm
driver first appeared in
.Fx 4.0
as a cleaner replacement
for the MFS functionality previously used in
.Tn PicoBSD
and in the
.Fx
installation process.
.Pp
The
.Nm
driver did a hostile takeover of the
.Xr vn 4
driver in
.Fx 5.0 .
.Sh AUTHORS
The
.Nm
driver was written by
.An Poul-Henning Kamp Aq Mt phk@FreeBSD.org .