Backing store is allocated using malloc(9). Only one malloc-bucket is used, which means that all md devices with 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 vmstat(8).
A file loaded by loader(8) with type 'md_image' is used for backing store. For backwards compatibility the type 'mfs_root' is also recognized. If the kernel is created with option MD_ROOT the first preloaded image found will become the root file system.
A regular file is used as backing store. This allows for mounting ISO images without the tedious detour over actual physical media.