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.
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.
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.
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.
Table 1, “vinum Plex
Organizations” summarizes the
advantages and disadvantages of each plex organization.
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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