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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) device 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.
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 constructing 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 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 normal (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]cluster Enable clustering on this disk.
[no]compress Enable/disable compression features to reduce memory usage.
For vnode backed devices: detect whether hole-punching is supported by the underlying file system. If the file system supports hole-punching, then to handle a BIO_DELETE request, 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-punching, 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 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 of the vnode 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 verification 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) 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 abbreviation of mdconfig -a -t vnode -f file.
EXAMPLES Create a disk with /tmp/boot.flp as backing storage. The name of the allocated 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: 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 information, 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), md(4), ffs(7), 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 14.0-RELEASE-p6 August 27, 2021 FreeBSD 14.0-RELEASE-p6