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MDCONFIG(8) FreeBSD 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 malloc(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 system is under memory pressure, otherwise they stay in the operating memory. Using swap backing is generally preferred 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 de- tails. -n When printing md(4) device names, print only the unit number without 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 implied 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 performance 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 ini- tially. 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 af- ter each access. The release has the same semantics as the POSIX_FADV_DONTNEED feature of posix_fadvise(2). The result is that with normal (non-zfs) caching, buffers are released from the buffer cache soon after they are con- structed, 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]cluster Enable clustering on this disk. [no]compress Enable/disable compression features to reduce memory us- age. [no]force Disable/enable extra sanity checks to prevent the user from doing something that might adversely affect the sys- tem. This can be used with the -d flag to forcibly de- stroy an md(4) disk that is still in use. [no]readonly Enable/disable readonly mode. [no]verify For vnode backed devices: enable/disable requesting veri- fication of the file used for backing store. The type of verification depends on which security features are available. One example of verification is testing file integrity with checksums or cryptographic signatures. -u unit Request a specific unit number or device name for the md(4) de- vice instead of automatic allocation. If a device name is speci- fied, 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 al- located 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(7) 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 in- formation, positioning md1.nop to the start of the filesystem in the im- age. mdconfig -u md1 -f diskimage.img gnop create -o 512K md1 mount /dev/md1.nop /mnt SEE ALSO open(2), md(4), ffs(7), gpart(8), mdmfs(8), malloc(9) HISTORY The mdconfig utility first appeared in FreeBSD 5.0 as a cleaner replace- ment for the vn kernel module and the vnconfig utility combo. AUTHORS The mdconfig utility was written by Poul-Henning Kamp <phk@FreeBSD.org>. FreeBSD 13.0 November 6, 2020 FreeBSD 13.0
NAME | SYNOPSIS | DESCRIPTION | EXAMPLES | SEE ALSO | HISTORY | AUTHORS
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