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# 22.4 `vinum` Objects

In order to address these problems, `vinum` implements a four-level hierarchy of objects:

- The most visible object is the virtual disk, called a volume. Volumes have essentially the same properties as a UNIX® disk drive, though there are some minor differences. For one, they have no size limitations.

- Volumes are composed of plexes, each of which represent the total address space of a volume. This level in the hierarchy provides redundancy. Think of plexes as individual disks in a mirrored array, each containing the same data.

- Since `vinum` exists within the UNIX disk storage framework, it would be possible to use UNIX partitions as the building block for multi-disk plexes. In fact, this turns out to be too inflexible as UNIX disks can have only a limited number of partitions. Instead, `vinum` subdivides a single UNIX partition, the drive, into contiguous areas called subdisks, which are used as building blocks for plexes.

- Subdisks reside on `vinum` drives, currently UNIX partitions. `vinum` drives can contain any number of subdisks. With the exception of a small area at the beginning of the drive, which is used for storing configuration and state information, the entire drive is available for data storage.

The following sections describe the way these objects provide the functionality required of `vinum`.

## 22.4.1 Volume Size Considerations

Plexes can include multiple subdisks spread over all drives in the `vinum` configuration. As a result, the size of an individual drive does not limit the size of a plex or a volume.

## 22.4.2 Redundant Data Storage

`vinum` implements mirroring by attaching multiple plexes to a volume. Each plex is a representation of the data in a volume. A volume may contain between one and eight plexes.

Although a plex represents the complete data of a volume, it is possible for parts of the representation to be physically missing, either by design (by not defining a subdisk for parts of the plex) or by accident (as a result of the failure of a drive). As long as at least one plex can provide the data for the complete address range of the volume, the volume is fully functional.

## 22.4.3 Which Plex Organization?

`vinum` implements both concatenation and striping at the plex level:

- A concatenated plex uses the address space of each subdisk in turn. Concatenated plexes are the most flexible as they can contain any number of subdisks, and the subdisks may be of different length. The plex may be extended by adding additional subdisks. They require less CPU time than striped plexes, though the difference in CPU overhead is not measurable. On the other hand, they are most susceptible to hot spots, where one disk is very active and others are idle.

- A striped plex stripes the data across each subdisk. The subdisks must all be the same size and there must be at least two subdisks in order to distinguish it from a concatenated plex. The greatest advantage of striped plexes is that they reduce hot spots. By choosing an optimum sized stripe, about 256 kB, the load can be evened out on the component drives. Extending a plex by adding new subdisks is so complicated that `vinum` does not implement it.

summarizes the advantages and disadvantages of each plex organization.

Table 22-1. `vinum` Plex Organizations

Plex type | Minimum subdisks | Can add subdisks | Must be equal size | Application
--- | --- | --- | --- | ---
concatenated | 1 | yes | no | Large data storage with maximum placement flexibility and moderate performance
striped | 2 | no | yes | High performance in combination with highly concurrent access

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