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MDCONFIG(8) System Manager's Manual MDCONFIG(8) NAME mdconfig -- create and control memory disks SYNOPSIS mdconfig -a -t type [-n] [-o [no]option] ... [-f file] [-s size] [-S sectorsize] [-u unit] [-x sectors/track] [-y heads/cylinder] [-L label] mdconfig -d -u unit [-o [no]force] mdconfig -r -u unit -s size [-o [no]force] mdconfig -l [-n] [-v] [-f file] [-u unit] mdconfig file DESCRIPTION The mdconfig utility creates and controls md(4) devices. Options indicate an action to be performed: -a Attach a memory disk. This will configure and attach a memory disk with the parameters specified and attach it to the system. If the -u unit option is not provided, the newly created device name will be printed on stdout. -d Detach a memory disk from the system and release all resources. -r Resize a memory disk. -t type Select the type of the memory disk. malloc Storage for this type of memory disk is allocated with mal- loc(9). This limits the size to the malloc bucket limit in the kernel. If the -o reserve option is not set, creating and filling a large malloc-backed memory disk is a very easy way to panic the system. vnode A file specified with -f file becomes the backing store for this memory disk. swap Storage for this type of memory disk is allocated from buffer memory. Pages get pushed out to swap when the sys- tem is under memory pressure, otherwise they stay in the operating memory. Using swap backing is generally pre- ferred instead of using malloc backing. null Bitsink; all writes do nothing, all reads return zeroes. -f file Filename to use for the vnode type memory disk. The -a and -t vnode options are implied if not specified. -l List configured devices. If given with -u, display details about that particular device. If given with -f file, display md(4) de- vice names of which file is used as the backing store. If both of -u and -f options are specified, display devices which match the two conditions. If the -v option is specified, show all details. -n When printing md(4) device names, print only the unit number with- out the md(4) prefix. -s size Size of the memory disk. Size is the number of 512 byte sectors unless suffixed with a b, k, m, g, t, or p which denotes byte, kilobyte, megabyte, gigabyte, terabyte and petabyte respectively. When used without the -r option, the -a and -t swap options are im- plied if not specified. -S sectorsize Sectorsize to use for the memory disk, in bytes. -x sectors/track See the description of the -y option below. -y heads/cylinder For malloc or vnode backed devices, the -x and -y options can be used to specify a synthetic geometry. This is useful for con- structing bootable images for later download to other devices. -L label Associate a label (arbitrary string) with the new memory disk. The label can then be inspected with mdconfig -l -v -o [no]option Set or reset options. [no]async For vnode backed devices: avoid IO_SYNC for increased per- formance but at the risk of deadlocking the entire kernel. [no]cache For vnode backed devices: enable/disable caching of data in system caches. The default is to not cache. Accesses via the device are converted to accesses via the vnode. The caching policy for the vnode is used initially. This is normally to cache. This caching policy is retained if the cache option is used. Otherwise, caching is limited by releasing data from caches soon after each access. The release has the same semantics as the POSIX_FADV_DONTNEED feature of posix_fadvise(2). The result is that with nor- mal (non-zfs) caching, buffers are released from the buffer cache soon after they are constructed, but their data is kept in the page cache at lower priority. The cache option tends to waste memory by giving unwanted double caching, but it saves time if there is memory to spare. [no]reserve Allocate and reserve all needed storage from the start, rather than as needed. [no]compress Enable/disable compression features to reduce memory usage. [no]force Disable/enable extra sanity checks to prevent the user from doing something that might adversely affect the system. This can be used with the -d flag to forcibly destroy an md(4) disk that is still in use. [no]mustdealloc For vnode backed devices: detect whether hole-punching is supported by the underlying file system. If the file sys- tem supports hole-punching, then to handle a BIO_DELETE re- quest, some or all of the request's operation range may be turned into a hole in the file used for backing store. Any parts which are not turned into holes are zero-filled in the file. If the file system does not support hole-punch- ing, BIO_DELETE requests to the device are not handled and will fail with EOPNOTSUPP. When mustdealloc is not specified or [no]mustdealloc is specified, for a BIO_DELETE request, if the file system supports hole-punching, some or all of the request's opera- tion range may be turned into a hole in the file used for backing store. Any parts which are not turned into holes are zero-filled in the file. If the file system of the vn- ode type memory disk does not support hole-punching, the request's operation range is zero-filled in the file. [no]readonly Enable/disable readonly mode. [no]verify For vnode backed devices: enable/disable requesting verifi- cation of the file used for backing store. The type of verification depends on which security features are avail- able. One example of verification is testing file in- tegrity with checksums or cryptographic signatures. -u unit Request a specific unit number or device name for the md(4) device instead of automatic allocation. If a device name is specified, it must start with "md" followed by the unit number. The last form, mdconfig file, is provided for convenience as an abbrevia- tion of mdconfig -a -t vnode -f file. EXAMPLES Create a disk with /tmp/boot.flp as backing storage. The name of the allo- cated unit will be printed on stdout, such as "md0": mdconfig /tmp/boot.flp Create a 1 gigabyte swap backed memory disk named "md3": mdconfig -s 1g -u md3 Detach and free all resources used by /dev/md3: mdconfig -du md3 Show detailed information on current memory disks: mdconfig -lv Resize the "md3" memory disk to 2 gigabytes: mdconfig -rs 2g -u md3 Create a 1 gigabyte swap backed disk, initialize an ffs(4) file system on it, and mount it on /tmp: mdconfig -s 1g -u md10 newfs -U /dev/md10 mount /dev/md10 /tmp chmod 1777 /tmp Create a memory disk out of an ISO 9660 CD image file, using the first available md(4) device, and then mount it: mount -t cd9660 /dev/`mdconfig -f cdimage.iso` /mnt Create a file-backed device from a hard disk image that begins with 512K of raw header information. gnop(8) is used to skip over the header informa- tion, positioning md1.nop to the start of the filesystem in the image. mdconfig -u md1 -f diskimage.img gnop create -o 512K md1 mount /dev/md1.nop /mnt SEE ALSO fpathconf(2), fspacectl(2), open(2), ffs(4), md(4), gpart(8), mdmfs(8), malloc(9), vn_deallocate(9) HISTORY The mdconfig utility first appeared in FreeBSD 5.0 as a cleaner replacement for the vn kernel module and the vnconfig utility combo. AUTHORS The mdconfig utility was written by Poul-Henning Kamp <phk@FreeBSD.org>. FreeBSD ports 15.quarterly June 1, 2024 MDCONFIG(8)
NAME | SYNOPSIS | DESCRIPTION | EXAMPLES | SEE ALSO | HISTORY | AUTHORS
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