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Volume management treats each PV as being composed of a sequence of chunks called physical extents (PEs). Some volume managers (such as that in HP-UX and Linux) have PEs of a uniform size; others (such as that in Veritas) have variably-sized PEs that can be split and merged at will. Normally, PEs simply map one-to-one to logical extents (LEs ...
Host-based virtualization requires additional software running on the host, as a privileged task or process. In some cases volume management is built into the operating system, and in other instances it is offered as a separate product. Volumes (LUN's) presented to the host system are handled by a traditional physical device driver.
The allocation map is stored in a form of data runs with compressed encoding. Each data run represents a contiguous group of clusters that store the attribute value. For files on a multi-GB volume, each entry can be encoded as 5 to 7 bytes, which means a 1 KB MFT record can store about 100 such data runs.
Additionally, it contains an entry for every area of contiguous free space on the volume. The third record on the first track of the first cylinder of any DASD (e.g., disk) volume is known as the volume label and must contain a pointer to the location of the VTOC. The location of the VTOC may be specified when the volume is initialized.
Over-allocation or over-subscription is a mechanism that allows a server to view more storage capacity than has been physically reserved on the storage array itself. This allows flexibility in growth of storage volumes, without having to predict accurately how much a volume will grow. Instead, block growth becomes sequential.
In computer data storage, a volume or logical drive is a single accessible storage area with a single file system, typically (though not necessarily) resident on a single partition of a hard disk. Although a volume might be different from a physical disk drive, it can still be accessed with an operating system's logical interface.
The extent allocation tree, as with all other trees in the file system, is copy-on-write. Writes to the file system may thus cause a cascade whereby changed tree nodes and file data result in new extents being allocated, causing the extent tree itself to change.
To bring a volume group online, the "vgchange" tool: Searches for PVs in all available block devices. Parses the metadata header in each PV found. Computes the layouts of all visible volume groups. Loops over each logical volume in the volume group to be brought online and: Checks if the logical volume to be brought online has all its PVs visible.