1.\" ---------------------------------------------------------------------------- 2.\" "THE BEER-WARE LICENSE" (Revision 42): 3.\" <phk@FreeBSD.org> wrote this file. As long as you retain this notice you 4.\" can do whatever you want with this stuff. If we meet some day, and you think 5.\" this stuff is worth it, you can buy me a beer in return. Poul-Henning Kamp 6.\" ---------------------------------------------------------------------------- 7.\" 8.\" $FreeBSD$ 9.\" 10.Dd December 26, 2017 11.Dt MD 4 12.Os 13.Sh NAME 14.Nm md 15.Nd memory disk 16.Sh SYNOPSIS 17To compile this driver into the kernel, 18place the following lines in your 19kernel configuration file: 20.Bd -ragged -offset indent 21.Cd "device md" 22.Ed 23.Pp 24Alternatively, to load the driver as a 25module at boot time, place the following line in 26.Xr loader.conf 5 : 27.Bd -literal -offset indent 28geom_md_load="YES" 29.Ed 30.Sh DESCRIPTION 31The 32.Nm 33driver provides support for four kinds of memory backed virtual disks: 34.Bl -tag -width preload 35.It Cm malloc 36Backing store is allocated using 37.Xr malloc 9 . 38Only one malloc-bucket is used, which means that all 39.Nm 40devices with 41.Cm malloc 42backing must share the malloc-per-bucket-quota. 43The exact size of this quota varies, in particular with the amount 44of RAM in the 45system. 46The exact value can be determined with 47.Xr vmstat 8 . 48.It Cm preload 49A file loaded by 50.Xr loader 8 51with type 52.Sq md_image 53is used for backing store. 54For backwards compatibility the type 55.Sq mfs_root 56is also recognized. 57If the kernel is created with option 58.Dv MD_ROOT 59the first preloaded image found will become the root file system. 60.It Cm vnode 61A regular file is used as backing store. 62This allows for mounting ISO images without the tedious 63detour over actual physical media. 64.It Cm swap 65Backing store is allocated from buffer memory. 66Pages get pushed out to the swap when the system is under memory 67pressure, otherwise they stay in the operating memory. 68Using 69.Cm swap 70backing is generally preferable over 71.Cm malloc 72backing. 73.El 74.Pp 75For more information, please see 76.Xr mdconfig 8 . 77.Sh EXAMPLES 78To create a kernel with a ramdisk or MD file system, your kernel config 79needs the following options: 80.Bd -literal -offset indent 81options MD_ROOT # MD is a potential root device 82options MD_ROOT_READONLY # disallow mounting root writeable 83options MD_ROOT_SIZE=8192 # 8MB ram disk 84makeoptions MFS_IMAGE=/h/foo/ARM-MD 85options ROOTDEVNAME=\\"ufs:md0\\" 86.Ed 87.Pp 88The image in 89.Pa /h/foo/ARM-MD 90will be loaded as the initial image each boot. 91To create the image to use, please follow the steps to create a file-backed 92disk found in the 93.Xr mdconfig 8 94man page. 95Other tools will also create these images, such as NanoBSD. 96.Sh ARM KERNEL OPTIONS 97On armv6 and armv7 architectures, an MD_ROOT image larger than 98approximately 55 MiB may require building a custom kernel using 99several tuning options related to kernel memory usage. 100.Bl -tag -width indent 101.It Cd options LOCORE_MAP_MB=<num> 102This configures how much memory is mapped for the kernel during 103the early initialization stages. 104The value must be at least as large as the kernel plus all preloaded 105modules, including the root image. 106There is no downside to setting this value too large, as long 107as it does not exceed the amount of physical memory. 108The default is 64 MiB. 109.It Cd options NKPT2PG=<num> 110This configures the number of kernel L2 page table pages to 111preallocate during kernel initialization. 112Each L2 page can map 4 MiB of kernel space. 113The value must be large enough to map the kernel plus all preloaded 114modules, including the root image. 115The default value is 32, which is sufficient to map 128 MiB. 116.It Cd options VM_KMEM_SIZE_SCALE=<num> 117This configures the amount of kernel virtual address (KVA) space to 118dedicate to the kmem_arena map. 119The scale value is the ratio of physical to virtual pages. 120The default value of 3 allocates a page of KVA for each 3 pages 121of physical ram in the system. 122 123The kernel and modules, including the root image, also consume KVA. 124The combination of a large root image and the default scaling 125may preallocate so much KVA that there is not enough 126remaining address space to allocate kernel stacks, IO buffers, 127and other resources that are not part of kmem_arena. 128Overallocating kmem_arena space is likely to manifest as failure to 129launch userland processes with "cannot allocate kernel stack" messages. 130 131Setting the scale value too high may result in kernel failure to allocate 132memory because kmem_arena is too small, and the failure may require 133significant runtime to manifest. 134Empirically, a value of 5 works well for a 200 MiB root image on 135a system with 2 GiB of physical ram. 136.El 137.Sh SEE ALSO 138.Xr gpart 8 , 139.Xr loader 8 , 140.Xr mdconfig 8 , 141.Xr mdmfs 8 , 142.Xr newfs 8 , 143.Xr vmstat 8 144.Sh HISTORY 145The 146.Nm 147driver first appeared in 148.Fx 4.0 149as a cleaner replacement 150for the MFS functionality previously used in 151.Tn PicoBSD 152and in the 153.Fx 154installation process. 155.Pp 156The 157.Nm 158driver did a hostile takeover of the 159.Xr vn 4 160driver in 161.Fx 5.0 . 162.Sh AUTHORS 163The 164.Nm 165driver was written by 166.An Poul-Henning Kamp Aq Mt phk@FreeBSD.org . 167