19.5. Vinum Objects

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

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

19.5.1. 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, and thus of a volume.

19.5.2. 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.

19.5.3. Performance Issues

Vinum implements both concatenation and striping at the plex level:

  • A concatenated plex uses the address space of each subdisk in turn.

  • A striped plex stripes the data across each subdisk. The subdisks must all have the same size, and there must be at least two subdisks in order to distinguish it from a concatenated plex.

19.5.4. Which Plex Organization?

The version of Vinum supplied with FreeBSD 11.2 implements two kinds of plex:

  • Concatenated plexes are the most flexible: 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.

  • The greatest advantage of striped (RAID-0) plexes is that they reduce hot spots: by choosing an optimum sized stripe (about 256 kB), you can even out the load on the component drives. The disadvantages of this approach are (fractionally) more complex code and restrictions on subdisks: they must be all the same size, and extending a plex by adding new subdisks is so complicated that Vinum currently does not implement it. Vinum imposes an additional, trivial restriction: a striped plex must have at least two subdisks, since otherwise it is indistinguishable from a concatenated plex.

Tabela 19.1, "Vinum Plex Organizations" summarizes the advantages and disadvantages of each plex organization.

Tabela 19.1. Vinum Plex Organizations
Plex typeMinimum subdisksCan add subdisksMust be equal sizeApplication
concatenated1yesnoLarge data storage with maximum placement flexibility and moderate performance
striped2noyesHigh performance in combination with highly concurrent access

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