A unit of data supplied or requested by an initiator is called a logical block. Logical blocks are stored according to the specifications of the format for the volume and may be recorded as one or more physical blocks on the medium. When the physical block and the logical block are not recorded in a one-to-one relationship, it is the responsibility of the device to perform all blocking, de-blocking, padding, stripping, splitting or rebuilding of the logical data block(s) sent by an initiator.
Filemarks are special recorded elements containing no user data. The filemark format is defined in some American National Standards. Initiators traditionally use filemarks to separate user data from labels and logical groupings of data from each other. Since some format standards do not define an explicit end-of-data (EOD), host software has often used conventions with filemarks to represent an EOD indication. At least one American National Standard specifically defines filemark use for this purpose. In some implementations, the device's EOD definition may be specified the initiator using the MODE SELECT command.
A setmark is another type of special recorded element containing no user data, providing a segmentation scheme hierarchically superior to filemarks. This level of segmentation is useful for some high capacity storage devices to provide concise addressing and fast positioning to specific sets of data within a partition. In some implementations, the detection and reporting of setmarks may be controlled by the initiator using the MODE SELECT command.
Inter-block gaps, the gaps between blocks, filemarks, and setmarks, are introduced on the medium at the time a block or mark is written without explicit action by the initiator. Minimum and maximum lengths for inter- block gaps are defined in some American National Standards. In some devices, the length of inter-block gaps may be selected by the initiator using the MODE SELECT command while in other devices the gaps are fixed and non-changeable. In addition to blocks, filemarks, and setmarks, erase gaps may be recorded on the medium through use of the ERASE command or device-initiated error recovery actions. Although explicitly recorded on the medium, there is normally no distinction between two contiguous erase gaps. An erase gap may be a length of erased medium or a recorded pattern not distinguishable as a block or mark. Minimum and maximum lengths for erase gaps are defined in some American National Standards while some devices may have no implementation of an erase gap.
After writing data from BOP x, the medium is considered to be a contiguous grouping of blocks, filemarks, setmarks, and gaps. Certain American National Standards define gap lengths which, if exceeded, are to be considered as having reached blank medium. Depending on the format, this blank medium may be treated as an end-of-data indication, an error recovery area, or an unrecoverable medium error causing an interchange error. Unrecorded volumes (new or erased) may exhibit blank medium characteristics if an attempt is made to read or space the volume before data has been written.
A sequential-access device may be capable of supporting fixed or variable length blocks. The concept of fixed or variable mode for writing and reading blocks only indicates the method by which the initiator specifies the size of a logical block for transfer and not the method of recording physical blocks on the medium. However, a device that supports only fixed-length physical blocks may only be capable of supporting logical blocks of the same length. The length of a logical block is always described in bytes. The length of a physical block may or may not be recorded as an exact byte count, depending on the format.