Overview of Solaris Volume Manager Components
The custom JumpStart installation method and Solaris Live Upgrade enable you to create
the following components that are required to replicate data.
This section briefly describes each of these components. For complete information about these
components, see Solaris Volume Manager Administration Guide.
State Database and State Database Replicas
The state database is a database that stores information on a physical disk.
The state database records and tracks changes that are made to your
configuration. Solaris Volume Manager automatically updates the state database when a configuration
or state change occurs. Creating a new volume is an example of
a configuration change. A submirror failure is an example of a state
change.
The state database is actually a collection of multiple, replicated database copies.
Each copy, referred to as a state database replica, ensures that the data in the
database is always valid. Having copies of the state database protects against
data loss from single points of failure. The state database tracks the
location and status of all known state database replicas.
Solaris Volume Manager cannot operate until you have created the state database
and its state database replicas. A Solaris Volume Manager configuration must
have an operating state database.
The state database replicas ensure that the data in the state database
is always valid. When the state database is updated, each state database
replica is also updated. The updates occur one at a time
to protect against corruption of all updates if the system crashes.
If your system loses a state database replica, Solaris Volume Manager must identify
which state database replicas still contain valid data. Solaris Volume Manager determines
this information by using a majority consensus algorithm. This algorithm requires that a majority (half
+ 1) of the state database replicas be available and in agreement
before any of them are considered valid. Because of this majority consensus
algorithm, you must create at least three state database replicas when you
set up your disk configuration. A consensus can be reached if at
least two of the three state database replicas are available.
Each state database replica occupies 4 Mbytes (8192 disk sectors) of disk
storage by default. Replicas can be stored on the following devices:
Replicas cannot be stored on the root (/), swap, or /usr
slices, or on slices that contain existing file systems or data. After
the replicas have been stored, volumes or file systems can be placed
on the same slice.
You can keep more than one copy of a state database on
one slice. However, you might make the system more vulnerable to a
single point of failure by placing state database replicas on a single slice.
RAID-1 Volumes (Mirrors)
A RAID-1 volume, or mirror, is a volume that maintains identical copies
of the data in RAID-0 volumes (single-slice concatenations). After you configure a RAID-1
volume, the volume can be used just as if it were a physical
slice. You can duplicate any file system, including existing file systems. You
can also use a RAID-1 volume for any application, such as a
database.
Using RAID-1 volumes to mirror file systems has advantages and disadvantages:
With RAID-1 volumes, data can be read from both RAID-0 volumes simultaneously (either volume can service any request), providing improved performance. If one physical disk fails, you can continue to use the mirror with no loss in performance or loss of data.
Using RAID-1 volumes requires an investment in disks. You need at least twice as much disk space as the amount of data.
Because Solaris Volume Manager software must write to all RAID-0 volumes, duplicating the data can also increase the time that is required for write requests to be written to disk.
RAID-0 Volumes (Concatenations)
A RAID-0 volume is a single-slice concatenation. The concatenation is a volume whose
data is organized serially and adjacently across components, forming one logical storage
unit. The custom JumpStart installation method and Solaris Live Upgrade do not
enable you to create stripes or other complex Solaris Volume Manager volumes.
During the installation or upgrade, you can create RAID-1 volumes (mirrors) and attach
RAID-0 volumes to these mirrors. The RAID-0 volumes that are mirrored are called
submirrors. A mirror is made of one or more RAID-0 volumes.
After the installation, you can manage the data on separate RAID-0 submirror volumes
by administering the RAID-1 mirror volume through the Solaris Volume Manager software.
The custom JumpStart installation method enables you to create a mirror that
consists of up to two submirrors. Solaris Live Upgrade enables you to create
a mirror that consists of up to three submirrors. Practically, a two-way mirror
is usually sufficient. A third submirror enables you to make online backups
without losing data redundancy while one submirror is offline for the backup.
