The following are LSM terms and definitions.
A plex that uses subdisks on one or more disks to create a virtual contiguous region of storage space that is accessed linearly. If LSM reaches the end of a subdisk while writing data, it continues to write data to the next subdisk, which can physically exist on the same disk or a different disk. This layout allows you to use space on several regions of the same disk, or regions of several disks, to create a single big pool of storage.
A set of files that are saved using the
volsave(8)
command and
can be used to restore an LSM configuration.
By default, an LSM description
set is saved in a time-stamped directory under the
/usr/var/lsm/db
directory.
Disks exist as two entities:
A physical disk on which all data is ultimately stored and which exhibits all the behaviors of the underlying technology.
An LSM representation of disks which, while mapping one-to-one with the physical disks, is just a presentation of units from which allocations of storage are made.
The difference is that a physical disk presents the image of a device with a definable geometry with a definable number of cylinders, heads, and so on while an LSM disk is simply a unit of allocation with a name and a size.
A configuration record that defines the path to a disk. Disk access records most often include a unit number. LSM uses the disk access records stored in a system to find all disks attached to the system. Disk access records do not identify particular physical disks.
Through the use of disk IDs, LSM allows you to move disks between controllers or to different locations on a controller. When you move a disk, a different disk access record is used to access the disk, although the disk media record continues to track the actual physical disk.
On some systems, LSM builds a list of disk access records automatically,
based on the list of devices attached to the system.
On these systems, it
is not necessary to define disk access records explicitly.
On other systems,
you must define disk access records with the
/sbin/voldisk define
command.
Specialty disks, such as RAM disks or floppy disks, are
likely to require explicit
/sbin/voldisk define
commands.
Disk access records are identified by their disk access names (also known as DA names).
See also disk ID, disk media record, volboot file
A group of disks that share a common configuration database. A configuration database consists of a set of records describing objects including disks, volumes, plexes, and subdisks that are associated with one particular disk group. Each disk group has an administrator-assigned name that you use to reference that disk group. Each disk group has an internally defined unique disk group ID, which differentiates two disk groups with the same administrator-assigned name.
Disk groups provide a method to partition the configuration database, so that the database size is not too large and so that database modifications do not affect too many drives. They also allow LSM to operate with groups of physical disk media that can be moved between systems.
Disks and disk groups have a circular relationship: disk groups are formed from disks, and disk group configuration databases are stored on disks. All disks in a disk group are stamped with a disk group ID, which is a unique identifier for naming disk groups. Some or all disks in a disk group also store copies of the configuration database of the disk group.
See also disk group ID, root disk group
A small database that contains all volume, plex, subdisk, and disk media records. Also known as the configuration database. These databases are replicated onto some or all disks in the disk group, often with two copies on each disk. Because these databases pertain to disk groups, record associations cannot span disk groups. Thus, you cannot define a subdisk on a disk in one disk group and associate it with a volume in another disk group.
A 64-byte universally unique identifier that is assigned
to a disk group when the disk group is created with the
/sbin/voldg
init
command.
This identifier is in addition to the disk group name,
which you assigned.
The disk group ID differentiates disk groups that have
the same administrator-assigned name.
A block stored in a private region of a disk that defines several properties of the disk, such as the:
Size of the private region
Location and size of the public region
Unique disk ID for the disk
Disk group ID and disk group name (if the disk is currently associated with a disk group)
Host ID for a host that has exclusive use of the disk
A 64-byte universally unique identifier that is assigned
to a physical disk when its private region is initialized with the
/sbin/voldisk init
command.
The disk ID is stored in the disk media
record so that the physical disk can be related to the disk media record at
system startup.
See also disk media record
A reference to a physical disk, or possibly a disk partition. This record can be thought of as a physical disk identifier for the disk or partition. Disk media records are configuration records that provide a name (known as the disk media name or DM name) that you use to reference a particular disk independent of its location on the system's various disk controllers. Disk media records reference particular physical disks through a disk ID, which is a unique identifier that is assigned to a disk when it is initialized for use with the LSM software.
Operations are provided to set or remove the disk ID stored in a disk media record. Such operations have the effect of removing or replacing disks, with any associated subdisks being removed or replaced along with the disk.
See also disk access record
A name, usually assigned by you, that identifies a particular
host.
Host IDs are used to assign ownership to particular physical disks.
When a disk is part of a disk group that is in active use by a particular
host, the disk is stamped with that host's host ID.
If another host attempts
to access the disk, it detects that the disk has a nonmatching host ID and
disallows access until the host with ownership discontinues use of the disk.
Use the
/sbin/voldisk clearimport
command to clear the
host ID stored on a disk.
If a disk is a member of a disk group and has a host ID that matches a particular host, then that host will import the disk group as part of system startup.
A log kept in the private region on the disk that is written
by LSM kernel.
The log contains records describing the state of volumes in
the disk group.
This log provides a mechanism for the kernel to persistently
register state changes, so that the
vold
daemon can detect
the state changes even in the event of a system failure.
A copy of a volume's logical data address space; also known as a mirror. A volume can have up to 32 plexes associated with it. Each plex is, at least conceptually, a copy of the volume that is maintained consistently in the presence of volume I/O and reconfigurations. Plexes represent the primary means of configuring storage for a volume. Plexes can have a concatenated, striped, or RAID 5 organization (layout).
See also concatenated plex, RAID 5 plex, striped plex
If the plexes of a volume contain different data, then the plexes are said to be inconsistent. This is a problem only if LSM is unaware of the inconsistencies, as the volume can return differing results for consecutive reads.
Plex inconsistency is a serious compromise of data integrity. This inconsistency is caused by write operations that start around the time of a system failure, if parts of the write complete on one plex but not the other. If the plexes are not first synchronized to contain the same data, plexes are inconsistent after creation of a mirrored volume. An important role of LSM is to ensure that consistent data is returned to any application that reads a volume. This might require that plex consistency of a volume be "recovered" by copying data between plexes, so that they have the same contents. Alternatively, you can put a volume into a state such that reads from one plex are automatically written back to the other plexes, making the data consistent for that volume offset.
Disks used by LSM contain two special regions: a private region and a public region. Usually, each region is formed from a complete partition of the disk; however, the private and public regions can be allocated from the same partition.
The private region of a disk contains on-disk structures that are used by LSM for internal purposes. Each private region begins with a disk header that identifies the disk and its disk group. Private regions can also contain copies of a disk group's configuration database and copies of the disk group's kernel log.
See also disk header, kernel log, public region
The public region of a disk is the space reserved for allocating subdisks. Subdisks are defined with offsets that are relative to the beginning of the public region of a disk. Only one contiguous region of a disk can form the public region for a disk.
See also private region
A plex that places data and parity evenly across each of its associated subdisks. A plex has a characteristic number of stripe columns (represented by the number of associated subdisks) and a characteristic stripe width. The stripe width defines how much data with a particular address is allocated to one of the associated subdisks. The parity data is the result of an XOR operation on the data in each stripe unit. The parity data is written to a different column (presumed to be a different disk) for each stripe, left-shifted by one column, so that no one column contains all the parity for the volume. Therefore, if a disk in a RAID 5 plex fails, the volume is still recoverable by recreating the missing data or parity for each stripe.
See also concatenated plex, striped plex
A configurable policy for switching between plexes for volume reads. When a volume has more than one enabled associated plex, LSM distributes reads between the plexes to distribute the I/O load and thus increase total possible bandwidth of reads through the volume. You set the read policy. Read policy choices include:
round-robin read policy
For every other read operation, switches to a different plex from the one used for the previous read operation. Given three plexes, switches between each of the three plexes, in order.
preferred plex read policy
Specifies a particular plex that is used to satisfy read requests. In the event that a read request cannot be satisfied by the preferred plex, this policy changes to the round-robin read policy.
select read policy
The default policy. Adjusts to use an appropriate read policy based on the set of plexes associated with the volume. If only one enabled read-write striped plex is associated with the volume, then that plex is chosen automatically as the preferred plex; otherwise, the round-robin policy is used. If a volume has one striped plex and one nonstriped plex, preferring the striped plex often yields better throughput.
Each system requires one special disk group called rootdg. This group is generally the default for most utilities. In addition to defining the regular disk group information, the configuration database for the root disk group contains local information that is specific to a disk group.
A plex that places data evenly across each of its associated subdisks. A plex has a characteristic number of stripe columns (represented by the number of associated subdisks) and a characteristic stripe width. The stripe width defines how much data with a particular address is allocated to one of the associated subdisks. Given a stripe width of 128 blocks and two stripe columns, the first group of 128 blocks is allocated to the first subdisk, the second group of 128 blocks is allocated to the second subdisk, the third group to the first subdisk, and so on.
See also concatenated plex, RAID 5 plex
A region of storage allocated on a disk for use by a volume. Subdisks are associated with volumes through plexes. You organize one or more subdisks to form plexes based on a plex layout: concatenated, striped or RAID 5. Subdisks are defined relative to disk media records.
The
volboot
file is a special file (usually
stored in
/etc/vol/volboot) that is used to bootstrap the
root disk group and to define a system's host ID.
In addition to a host ID,
the
volboot
file contains a list of disk access records.
On system startup, this list of disks is scanned to find a disk that is a
member of the rootdg disk group and that is stamped with this system's host
ID.
When such a disk is found, its configuration database is read and is used
to get a more complete list of disk access records that are used as a second-stage
bootstrap of the root disk group and to locate all other disk groups.
A virtual disk device that looks to applications and file systems like a physical disk partition device. Volumes present block and raw device interfaces that are compatible in their use. A volume can use mirrors, span several disk drives, and be moved to different disks. You can change the configuration of a volume without causing disruption to applications or file systems that are using the volume.