To understand the design of this resource manager, it's necessary to understand the basics of a .tar file, and the two common formats.

Way back in the dim dark early days of UNIX (I think it must have been in the roaring twenties or something), huge 30cm (1 foot) diameter 9-track magnetic tapes were all the rage. In order to transport data from one machine to another, or to make backups, many files would be archived onto these tapes. The name “TAR” stood for Tape ARchive, and the format of the .tar file was an attempt to accommodate tape media. The tapes worked best in block mode; rather than write individual bytes of data, the system would read or write 512-byte (or other sized) blocks of data. When you consider the technology of the day, it made a lot of sense—here was this moving magnetic tape that passed over a head at a certain speed. While the tape was moving, it was good to pump data to (or from) the tape. When the tape stopped, it would be awkward to position it exactly into the middle of a block of data, so most tape operations were block-oriented (with the notable exception of incremental tape drives, but we won't go into that here). A typical magnetic tape could hold tens to hundreds of megabytes of data, depending on the density (number of bits per inch).

The .tar format, therefore, was block-oriented, and completely sequential (as opposed to random access). For every file in the archive, there was a block-aligned header describing the file (permissions, name, owner, date and time, etc.), followed by one or more blocks consisting of the file's data.