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A hybrid volume is any volume that intentionally and opaquely makes use of two separate physical volumes. For instance, a workload may consist of random seeks so an SSD may be used to permanently store frequently used or recently written data, while using higher-capacity rotational magnetic media for long-term storage of rarely needed data.
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.
Allocation is an ongoing process based on flow or volume measurements, and gives the distribution of contributing sources, often with a final calculation per day, which in turn provides the basis for a daily production report in the case of a field that produces hydrocarbons.
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.
The extent allocation tree acts as an allocation map for the file system. Unlike other trees, items in this tree do not have object ids. Unlike other trees, items in this tree do not have object ids. They represent regions of space: their key values hold the starting offsets and lengths of the regions they represent.
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.
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.
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.