The following are LSM terms and definitions.
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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 lets you use space on several regions of the same disk, or regions of several disks, to create a single big pool of storage.
See also RAID 5 plex, striped plex
A disk group created by the
volclonedg
command that is an exact duplicate of another LSM disk group on the system
or cluster and whose disks were cloned in hardware before its creation.
A small database that contains all volume, plex, subdisk, and disk media records for a disk group. This database is stored in the private regions of some or all disks in the disk group, possibly with two copies on each disk. Because the database pertains to a disk group, 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.
The configuration database in the
rootdg
disk group
contains records for all other disk groups.
See also description set, private region, root disk group (rootdg)
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A set of files saved by using the
volsave
command
that 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.
The log for a mirrored volume. Typically, the DRL for a mirrored volume is a separate plex containing one subdisk, but the DRL can also be a subdisk associated with a data plex. A DRL maintains a bitmapped representation of the regions of the volume, and marks as dirty all regions being written to. A DRL reduces the time required to restore synchronization of data for all the plexes in the volume when the system restarts after a failure. A volume can have more than one DRL for redundancy. A DRL does not provide any benefit in the event of a disk failure affecting the volume.
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 that, 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, whereas an LSM disk is simply a unit of allocation with a name and a size.
Disks used by LSM usually contain two special regions: a
private region
and a
public region.
Typically, each region is formed from a complete partition of the disk, resulting
in a
sliced
disk; however, the private and public regions
can be allocated from the same partition, resulting in a
simple
disk.
A disk used by LSM can also be a
nopriv
disk, which has only a public region and no private
region.
LSM
nopriv
disks are created as the result of encapsulating
a disk or disk partition.
See also disk group, nopriv disk, simple disk, sliced disk, subdisk, volume
A configuration record identified by a disk access (DA) name 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 lets you move disks between controllers or to different locations on a controller. When you move a disk, a different disk record name 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, you
do not need 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.
See also disk ID, disk media record, volboot file
A group of disks that share a common configuration database. 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 database copies are stored on some or all disks in a disk group. All disks in a disk group are stamped with a disk group ID, which is a unique identifier for naming disk groups.
See also clone disk group, configuration database, disk group ID, root disk group (rootdg)
A 64-byte, universally unique identifier 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:
Location and 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
See also disk ID, disk group ID, host ID, private region
A 64-byte, universally unique identifier 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, identified by a disk media (DM) name. This record can be thought of as a physical disk identifier for a physical disk or partition. Disk media records are configuration records that you can use to reference a particular disk — independent of its location on the system's various disk controllers.
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; any associated subdisks are removed or replaced along with the disk.
See also configuration database, disk access record, disk ID
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A feature used with mirrored volumes that reduces the time required to resynchronize plexes. Used specifically when detaching one plex of a mirrored volume to create a temporary volume for backups, with the intention of later reattaching the plex to the original (primary) volume.
See also primary volume, secondary volume, migrant plex
See FPA log
The log plex for a mirrored volume using the Fast Plex Attach feature, or the log subdisk for a migrant plex detached from a primary volume to create a secondary volume for backups. The FPA log plex for the primary volume tracks only those regions of the volume that change while a migrant plex is detached (as part of a secondary volume). The FPA log subdisk for the secondary volume tracks changes to the secondary volume. When the migrant plex returns to the primary volume, the FPA logs are merged and only the regions marked in the merged log are written to the returning plex, reducing the time required to resynchronize the plex.
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A name, usually assigned by the administrator, that identifies
a particular system or cluster member.
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 ID
and only that host can access 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 hot-spare disk is an LSM disk that you designate to automatically
replace a disk that fails while in use by a mirrored or RAID 5 volume.
You
enable and disable the hot-sparing feature with the
volwatch
command.
You can designate a hot-spare disk when you initialize it for LSM
use or later, as long as the disk is not being used by other LSM objects (volumes
or subdisks).
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A log kept in the
private region
on the disk that is written by the 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.
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A plex that keeps track of write activity for a mirrored or RAID 5 volume or as part of the Fast Plex Attach feature.
See also Dirty Region Log (DRL), FPA log, RAID 5 log
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A plex that, with the support of the Fast Plex Attach feature, is detached from a primary volume and used to create a secondary volume for backups.
See also Fast Plex Attach, primary volume, secondary volume
See plex
A volume that has two or more concatenated or striped data plexes and, typically, at least one log plex. Contrast with simple volume.
See also RAID 5 volume, redundant volume
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A volume that has only one concatenated or striped data plex. If one disk in the volume fails, the entire volume is lost. Contrast with mirrored volume and RAID 5 volume.
See also simple volume
A disk that is configured for use by LSM and has only a public
region and no private region.
A
nopriv
disk is typically
created as a result of encapsulating existing data in a disk or disk partition.
See also private region, public region, simple disk, sliced disk
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A copy of a volume's logical data address space; also known as a mirror. Each plex is, at least conceptually, a copy of the volume that is maintained consistently in the presence of volume I/O and changes to the LSM configuration. Plexes represent the primary means of configuring storage for a volume. Plexes that store data can have a concatenated, striped, or RAID 5 organization (layout).
A log that keeps track of write activity for a mirrored or RAID 5 volume or as part of the Fast Plex Attach feature. A log plex consists of only one associated log subdisk and its layout is always concatenated.
See also concatenated plex, log plex, migrant plex, plex consistency, RAID 5 plex, striped plex
A state of equivalence of the contents of all data plexes in an LSM mirrored volume, so that a read request from one plex returns the same data as a read request from the corresponding region in any other plex. If the plexes of a volume contain different data, then the plexes are said to be inconsistent.
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 and are written to one plex but not the other when the system crashes. 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 so that reads from one plex are automatically written back to the other plexes, making the data consistent for that volume offset.
The volume from which a plex is detached (the migrant plex) with Fast Plex Attach support to create a temporary secondary volume for backups.
See also Fast Plex Attach, secondary volume, migrant plex
The private region of a disk contains on-disk structures that are used by LSM for internal purposes. Each private region is typically 4096 blocks and 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, subdisk
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The log plex for a RAID 5 volume. A RAID 5 log plex stores a copy of the data and parity information for a predefined number of writes. In the event of a system failure, a RAID 5 log reduces the time required to resynchronize data and parity in the volume when the system restarts. When the system restarts after a failure, any write operations that did not complete before the failure are restarted.
See also RAID 5 plex, RAID 5 volume
A plex that distributes data and parity in equal-size units 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, RAID 5 log, RAID 5 volume, striped plex
A volume that uses a RAID 5 plex and, typically, at least one RAID 5 log plex. A RAID 5 volume has only one RAID 5 plex. Also described as a redundant volume, because a RAID 5 volume provides data redundancy in the form of parity.
See also mirrored volume, RAID 5 log, RAID 5 plex, redundant volume, simple volume
A configurable policy for switching between plexes for volume reads (applies only to volumes with striped or concatenated plexes). 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 to be one of the following:
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, sequentially.
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 concatenated plex, preferring the striped plex often yields better throughput.
An LSM volume that provides data redundancy, either through mirroring (as in a mirrored volume, which can use two or more concatenated or striped data plexes, each of which contains a complete set of the volume data), or through striping with parity (as in a RAID 5 volume), and has at least one log plex. Contrast with nonredundant volume.
See also log plex, mirrored volume, RAID 5 volume
LSM creates and requires one special disk group named
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.
See also configuration database
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A temporary volume created for performing backups of another volume, using the Fast Plex Attach feature. The secondary volume is created from a plex (the migrant plex) that was detached from a mirrored volume (the primary volume) with Fast Plex Attach support.
See also Fast Plex Attach, migrant plex, primary volume, snapshot volume
A disk configured for use by LSM that has a public region
and a private region that occupy the same disk partition.
A
simple
disk is created by initializing a disk partition (including the
c
partition), instead of the entire disk (without a partition specifier),
for LSM use.
See also nopriv disk, sliced disk, public region, private region
A volume that uses only one concatenated plex. This type of volume provides no data redundancy and no protection from system failure but does permit you to create a volume using space on multiple disks, move data to other LSM disks, and perform online volume management. Without an LSM license, you can create only simple volumes.
See also concatenated plex, mirrored volume, RAID 5 volume
A disk configured for use by LSM that has a separate public
region (typically the
g
partition of the disk) and a private
region (typically the
h
partition).
See also nopriv disk, simple disk, public region, private region
A volume created from a complete, enabled plex of another
volume (with the
volassist snapshot
command), used to perform
data backups or other operations while leaving the original volume mounted
and usable.
An LSM snapshot volume is a mounted, fully readable and writable
entity that contains a full copy of the data in the original volume, and that
is completely separate from the original volume that supplied the data plex
for its creation.
See also secondary volume
A value that represents the amount of data that LSM writes to the subdisks in each column of a striped or RAID5 data plex. LSM divides the full I/O request into units of the size defined by the stripe width and writes each unit to a different subdisk. The default stripe width for a striped plex is 64K bytes and for a RAID 5 plex is 16K bytes. Stripe widths do not apply to concatenated plexes by definition.
See also striped plex, RAID 5 plex
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 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, stripe width
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.
See also concatenated plex, disk media record, RAID 5 plex, striped plex
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The
volboot
file is a special file (usually
stored in
/etc/vol/volboot) that is used to load the
rootdg
disk group at system startup 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 disk access
records 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 in a second stage
of loading the root disk group and to locate all other disk groups.
See also disk access record
A virtual disk device that applications and file systems can use like a physical disk or disk partition. Volumes present block and raw device interfaces that are compatible in their use with disk partition devices. A volume can span several disk drives, consist of multiple mirrors (plexes), 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.
See also plex