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(itstool) path: row/entry
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Since <filename>vinum</filename> exists within the <trademark class="registered">UNIX</trademark> disk storage framework, it would be possible to use <trademark class="registered">UNIX</trademark> partitions as the building block for multi-disk plexes. In fact, this turns out to be too inflexible as <trademark class="registered">UNIX</trademark> disks can have only a limited number of partitions. Instead, <filename>vinum</filename> subdivides a single <trademark class="registered">UNIX</trademark> partition, the <emphasis>drive</emphasis>, into contiguous areas called <emphasis>subdisks</emphasis>, which are used as building blocks for plexes.
Subdisks reside on <filename>vinum</filename> <emphasis>drives</emphasis>, currently <trademark class="registered">UNIX</trademark> partitions. <filename>vinum</filename> 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 <filename>vinum</filename>.
Volume Size Considerations
Plexes can include multiple subdisks spread over all drives in the <filename>vinum</filename> configuration. As a result, the size of an individual drive does not limit the size of a plex or a volume.
Redundant Data Storage
<filename>vinum</filename> 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.
Which Plex Organization?
<filename>vinum</filename> implements both concatenation and striping at the plex level:
A <emphasis>concatenated plex</emphasis> 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 <acronym>CPU</acronym> time than striped plexes, though the difference in <acronym>CPU</acronym> 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 <emphasis>striped plex</emphasis> 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 <filename>vinum</filename> does not implement it.
<xref linkend="vinum-comparison"/> summarizes the advantages and disadvantages of each plex organization.
<filename>vinum</filename> 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
High performance in combination with highly concurrent access
Some Examples
<filename>vinum</filename> maintains a <emphasis>configuration database</emphasis> which describes the objects known to an individual system. Initially, the user creates the configuration database from one or more configuration files using <citerefentry><refentrytitle>gvinum</refentrytitle><manvolnum>8</manvolnum></citerefentry>. <filename>vinum</filename> stores a copy of its configuration database on each disk <emphasis>device</emphasis> under its control. This database is updated on each state change, so that a restart accurately restores the state of each <filename>vinum</filename> object.
The Configuration File
The configuration file describes individual <filename>vinum</filename> objects. The definition of a simple volume might be:

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Source information

Source string comment
(itstool) path: row/entry
Flags
read-only
Source string location
article.translate.xml:406
String age
a year ago
Source string age
a year ago
Translation file
articles/vinum.pot, string 61