8    Administering the Logical Storage Manager

The Logical Storage Manager (LSM) software provides disk management capabilities that increase data availability and improve disk I/O performance. System administrators use LSM to perform disk management functions dynamically without disrupting users or applications accessing data on those disks.

LSM replaces the Logical Volume Manager (LVM) on Digital UNIX systems. Refer to the Logical Storage Manager manual for information about how to migrate from LVM to LSM.

8.1    Features and Benefits

Table 8-1 summarizes the LSM features and benefits.

Table 8-1:  LSM Features and Benefits

Feature Benefit
Manages disks Frees you from the task of partitioning disks and allocating space. However, LSM allows you to keep control over disk partitioning and space allocation, if desired.
Allows transparent disk configuration changes Allows you to change the disk configuration without rebooting or otherwise interrupting users. Also allows routine administrative tasks, such as file system backup, with reduced down time.
Stores large file systems Enables multiple physical disks to be combined to form a single, larger logical volume. This capability, called concatenation, removes limitations imposed by the actual physical properties of individual disk sizes. It does this by combining the storage potential of several devices.
  Note that disk concatenation is available on all systems, including those that do not have an LSM software license.
Ease of system management Simplifies the management of disk configurations by providing convenient interfaces and utilities to add, move, replace, and remove disks.
Protects against data loss Protects against data loss due to hardware malfunction by creating a mirror (duplicate) image of important file systems and databases.
Increases disk performance Improves disk I/O performance through the use of striping, which is the interleaving of data within the volume across several physical disks.

This chapter provides an overview of LSM concepts and some commonly used commands. The volintro(8) reference page provides a quick reference of LSM terminology and command usage. Refer to the manual Digital UNIX Logical Storage Manager for more complete information on LSM concepts and commands.

8.2    Understanding the LSM Components

LSM consists of physical disk devices, logical entities, and the mappings that connect the physical and logical objects.

LSM builds virtual disks, called volumes, on top of UNIX system disks. A volume is a special device that contains data managed by a UNIX file system, a database, or other application. LSM transparently places a volume between a physical disk and an application, which then operates on the volume rather than on the physical disk. A file system, for instance, is created on the LSM volume rather than a physical disk. Figure 8-1 shows disk storage management in an LSM configuration.

Figure 8-1:  LSM Disk Storage Management

On a system that does not have LSM installed, I/O activity from the UNIX system kernel is passed through disk device drivers that control the flow of data to and from disks. When LSM is installed, the I/O passes from the kernel to the LSM volume device driver, then to the disk device drivers.

The LSM software maps the logical configuration of the system to the physical disk configuration. This is done transparently to the file systems, databases, and applications above it because LSM supports the standard block device and character device interfaces to store and retrieve data on LSM volumes. Thus, you do not have to change applications to access data on LSM volumes.

The block device special files associated with LSM volumes exist in the /dev/vol directory and the character device special files associated with LSM volumes exist in the /dev/svol directory.

8.2.1    LSM Objects

LSM logically binds together the disk devices into a volume that represents the disks as a single virtual device to applications and users. LSM uses a structure of LSM objects to organize and optimize disk usage and guard against media failures. The structure is built with the objects in the following logical order:

  1. Subdisks

  2. Plexes (mirrors)

  3. Volumes

Each object has a dependent relationship on the next-higher element, with subdisks being the lowest-level objects in the structure and volumes the highest level. LSM maintains a configuration database that describes the objects in the LSM configuration and implements utilities to manage the configuration database. Multiple mirrors, striping, and concatenation are additional techniques you can perform with the LSM objects to further enhance the capabilities of LSM.

Table 8-2 describes the LSM objects used to represent portions of the physical disks.

Table 8-2:  LSM Objects

Object Description
Volume Represents an addressable range of disk blocks used by applications, file systems, or databases. A volume is a virtual disk device that looks to applications and file systems like a regular disk-partition device. In fact, volumes are logical devices that appear as devices in the /dev directory. The volumes are labeled fsgen or gen according to their usage and content type. Each volume can be composed of from one to eight plexes (two or more plexes mirror the data within the volume).
  Due to its virtual nature, a volume is not restricted to a particular disk or a specific area thereof. You can change the configuration of a volume (using LSM utilities) without disrupting applications or file systems using that volume.
Plex A collection of one or more subdisks that represent specific portions of physical disks. When more than one plex is present, each plex is a replica of the volume; the data contained at any given point on each is identical (although the subdisk arrangement may differ). Plexes can have a striped or concatenated organization.
Subdisk A logical representation of a set of contiguous disk blocks on a physical disk. Subdisks are associated with plexes to form volumes. Subdisks are the basic components of LSM volumes that form a bridge between physical disks and virtual volumes.
Disk A collection of nonvolatile, read/write data blocks that are indexed and can be quickly and randomly accessed. LSM supports standard disk devices including SCSI and DSA disks. Each disk LSM uses is given two identifiers: a disk access name and an administrative name.
Disk Group A collection of disks that share the same LSM configuration database. The rootdg disk group is a special disk group that always exists.

LSM objects have the following relationships:

Figure 8-2 shows an LSM configuration that includes two plexes to protect a file system or a database against data loss.

Figure 8-2:  LSM Objects and Their Relationships

8.2.2    LSM Disks

You must add physical disks to the LSM environment as LSM disks before you can use them to create LSM volumes. Refer to Section 8.6.3 and the voldiskadd(8) reference page for information about adding physical disks to LSM.

An LSM disk typically uses the following two regions on each physical disk:

Figure 8-3 illustrates the three types of LSM disks: simple, sliced, and nopriv. You can add all of these types of disks into an LSM disk group.

Figure 8-3:  Types of LSM Disks

In Figure 8-3:

LSM configuration databases are stored on the private region of each LSM disk except the nopriv disk. The public regions of the LSM disks collectively form the storage space for application use. For purposes of availability, each simple and sliced disk contains two copies of the configuration database. A sliced disk takes up the entire physical disk, but simple and nopriv disks can reside on the same physical disk. The disk label tags identify the partitions to LSM as LSM disks.

8.2.3    Naming LSM Disks

When you perform disk operations, you should understand the disk-naming conventions for a disk access name and disk media name. Disk access names and disk media names are treated internally as two types of LSM disk objects. Some operations require that you specify the disk access name, while others require the disk media name.

The following definitions describe these disk-naming conventions:

8.2.4    LSM Disk Groups

You can organize a collection of physical disks that share a common configuration or function into disk groups. LSM volumes are created within a disk group and are restricted to using disks within that disk group.

Use disk groups to simplify management and provide data availability. For example:

All systems with LSM installed have the rootdg disk group. By default, operations are directed to this disk group. Most systems do not need to use more than one disk group.

Note

You do not have to add disks to disk groups when a disk is initialized; disks can be initialized and kept on standby as replacements for failed disks. Use a disk that is initialized but has not been added to a disk group to immediately replace a failing disk in any disk group.

Each disk group maintains an LSM configuration database that contains detailed records and attributes about the existing disks, volumes, plexes, and subdisks in the disk group.

8.2.5    LSM Configuration Databases

An LSM configuration database contains records describing all the objects (volumes, plexes, subdisks, disk media names, and disk access names) being used in a disk group.

Two identical copies of the LSM configuration database are located in the private region of each disk within a disk group. LSM maintains two identical copies of the configuration database in case of full or partial disk failure.

The contents of the rootdg configuration database is slightly different from that of an ordinary database in that the rootdg configuration database contains records for disks outside of the rootdg disk group in addition to the ordinary disk-group configuration information. Specifically, a rootdg configuration includes disk-access records that define the disks and disk groups on the system.

The LSM volume daemon, vold, uses the volboot file during startup to locate copies of the rootdg configuration database. This file may list disks that contain configuration copies in standard locations, and can also contain direct pointers to configuration copy locations. The volboot file is located in /etc/vol.

8.2.6    Moving and Replacing LSM Disks in a Disk Group

When a disk is added to a disk group it is given a disk media name, such as disk02. This name relates directly to the physical disk. LSM uses this naming convention (described in Section 8.2.3) because it makes the disk independent of the manner in which the volume is mapped onto physical disks. If a physical disk is moved to a different target address or to a different controller, the name disk02 continues to refer to it. You can replace disks by first associating a different physical disk with the name of the disk to be replaced, and then recovering any volume data that was stored on the original disk (from mirrors or backup copies).

8.3    LSM System Administration

Once a disk is under the control of LSM, all system administration tasks relating to that disk must be performed using LSM utilities and commands. For instance, if you install a file system on an LSM-controlled disk using physical disk paths rather than the LSM interfaces, LSM will be unaware that the new file system exists and will reallocate its space.

LSM provides three interfaces for managing LSM disks: a command line interface, a menu interface, and a graphical user interface. You can use any of these interfaces (or a combination of the interfaces) to change volume size, add plexes, and perform backups or other administrative tasks. You can use the LSM interfaces interchangeably. LSM objects created by one interface are fully interoperable and compatible with objects created by the other interfaces. Table 8-3 describes these LSM interfaces.

Table 8-3:  LSM Administration Interfaces

Interface Type Description
Visual Administrator (dxlsm) Graphical Uses windows, icons, and menus to manage LSM volumes. The dxlsm graphical interface requires a workstation. The interface interprets the mouse-based icon operations into LSM commands. The Visual Administrator (dxlsm) interface requires the LSM software license.
Support Operations (voldiskadm) Menu Provides a menu of disk operations. Each entry in the main menu leads you through a particular operation by providing you with information and asking you questions. Default answers are provided for many questions. This character-cell interface does not require a workstation.
Command line Command Provides two approaches to LSM administration. With the top-down approach, you use the LSM volassist command to automatically build the underlying LSM objects. With the bottom-up approach, you use individual commands to build individual objects to customize the construction of an LSM volume.

8.4    LSM System Administration Commands

The following sections summarize some useful commands from the command line interface. Examples of how to use some of these commands are included in Section 8.6.

See also the appropriate reference pages and the manual Logical Storage Manager for detailed information and examples.

8.4.1    Top-Down Command

The top-down approach to managing storage means placing disks in one large pool of free storage space. You then use the volassist utility to specify to LSM what you need, and LSM allocates the space from this free pool. You can use volassist to create, mirror, grow, or shrink a volume. With volassist, you can use the defaults that the utility provides, or you can specify volume attributes on the command line.

The volassist command has the following syntax:

volassist [- b ] [- g diskgroup ] [- U usetype ] [- d file ] keyword argument ...

8.4.2    Bottom-Up Commands

The bottom-up approach to storage management allows you to control the placement and definition of subdisks, plexes, and volumes. Bottom-up commands allow a great deal of precision control over how LSM creates and connects objects together. You should have a detailed knowledge of the LSM architecture before using these commands.

Bottom-up commands include volmake to create LSM objects, and volume, volplex, and volsd to manipulate volume, plex, and subdisk objects. The syntax for these commands is as follows:

volmake [-Uusetype] [-ouseopt] [-dfile] [type name[attribute]]...

volume [-Uusetype] [-ouseopt] [-Vq] keyword argument...

volplex [-Uusetype] [-ouseopt] [-V] [-vvolume] keyword argument...

volsd [-Uutype] [-ouopt] [-V] [-vvolume] [-pplex] keyword argument...

8.4.3    Information Command

The volprint command, which has built-in parsing and formatting features, displays most of the LSM configuration and status information. The volprint command has the following syntax:

volprint [-AvpsdGhnlafmtqQ] [-gdiskgroup] [-epattern] [-Ddatabase] [-F[type:]format-spec] [name...]

8.5    Planning an LSM Configuration

Before setting up LSM volumes, plexes, and subdisks, you should consider the needs of your site, the hardware available to you, and the rationale for creating volumes and disk groups.

Table 8-4 presents some configuration options and describes the planning considerations that apply to LSM configurations.

Table 8-4:  LSM Configuration Options

Configuration Description
Concatenated volumes You concatenate multiple LSM disks together to form a big volume. You can use a concatenated volume to store a large file or file systems that span more than one disk. Disk concatenation frees you from being limited by the actual physical sizes of individual disks so that you can combine the storage potential of several devices. Use the default disk group, rootdg, to create a concatenated volume from the public regions available. You can also add more LSM disks and create volumes from the new disks you added.
Mirrored volumes You associate multiple plexes with the same volume to create a mirrored volume. If you are concerned about the availability of your data, then plan to mirror data on your system. You should map plexes that are associated with the same volume to different physical disks. For systems with multiple disk controllers, you should map a volume's plexes to different controllers.
  The volassist command will fail if you specify a device that is already in the volume as the mirrored plex; the bottom-up commands will not fail.
Striped volumes For faster read/write throughput, use a volume with a striped plex. On a physical disk drive, the drive performs only one I/O operation at a time. On an LSM volume with its data striped across multiple physical disks, multiple I/Os (one for each physical disk) can be performed simultaneously.
  The basic components of a striped plex are the size of the plex in multiples of the stripe width used, the actual stripe width, and number of stripes. Stripe blocks of the stripe width size are interleaved among the subdisks, resulting in an even distribution of accesses among the subdisks. The stripe width defaults to 128 sectors, but you can tune the size to specific application needs. The volassist command automatically rounds up the volume length to multiples of stripe width.
Mirrored and striped volumes Use mirrored and striped volumes when speed and availability are important. LSM supports mirroring of striped plexes. This configuration offers the improved I/O performance of striping while also providing data availability.
  The different striped plexes in a mirrored volume do not have to be symmetrical. For instance, a three-way striped plex can be mirrored with a two-way striped plex as long as the plex size is the same. Reads can be serviced by any plex in a mirrored volume. Thus, a mirrored volume provides increased read performance. However, LSM issues writes to all plexes in a mirrored volume. Because the writes are issued in parallel, there is a small amount of additional overhead as the result of a write I/O to a mirrored volume.

8.6    Implementing an LSM Configuration

After installing and licensing the LSM software (as described in the Installation Guide), you can use the information in the following sections to quickly get LSM up and running.

The following sections provide quick reference information to help you reenable LSM after an installation, start up LSM for the first time, and perform several common LSM operations. The examples provided use the command-line interface. See the Logical Storage Manager guide for complete information about using the command line interface, and for information about the LSM graphical user interface and menu interface.

8.6.1    Reenabling LSM

If you are already running LSM and the rootdg disk group is already initialized, you do not need to reenable LSM. For example, if you performed an upgrade installation, skip this section.

If you had LSM initialized on a system before doing a full installation, you can reenable the LSM configuration by performing the following steps:

  1. Copy the /etc/volboot file from a backup: 

    # cp /backup/volboot /etc/volboot

  2. Create the LSM special device file: 

    
# /sbin/volinstall

  3. Start the LSM daemons and volumes: 

    
# /sbin/vol-startup

8.6.2    Setting up LSM

If you are setting up LSM for the first time, you can use the volsetup utility to initialize LSM and create the LSM configuration database for the first time. Then, use the voldiskadd utility to add more disks into LSM. This is the simplest method to set up an LSM configuration.

The volsetup utility automatically modifies disk labels, initializes disks for LSM, creates the default disk group, rootdg, and configures disks into the rootdg disk group. You invoke the volsetup utility only once. To later add more disks, use the voldiskadd utility.

The volsetup utility prompts you to estimate how many disks will be managed by LSM. The utility uses the estimate to define optimal values for the private region size (in sectors), and the number of configuration and log copies per disk.

Follow these steps to use volsetup:

  1. If you are in single-user mode, set the host name for your system before initializing LSM.

  2. Execute the /sbin/volsetup interactive utility by entering the following command: 

    
# /sbin/volsetup rz1

    In this example, the rz1 disk is used to initialize the rootdg disk group. If you do not give the name of a disk, LSM prompts you for one.

    Note

    When you are first setting up LSM, do not include the boot disk in the disks you specify to volsetup. After you initialize LSM, you can encapsulate the root and swap partitions and add them to the rootdg disk group or another disk group.

  3. The volsetup utility modifies the /etc/inittab file. When the system reboots, LSM is started automatically by the initialization process when it reads the LSM entries in the inittab file. (See inittab(4) for more information.)

  4. The LSM /sbin/lsmbstartup script starts the LSM vold daemon and the voliod error demon. After running the volsetup procedure, check that the vold daemon is running.

The volsetup utility creates the /etc/vol/volboot file. This file is used to locate copies of the rootdg disk group configuration when the system starts up.

Note

Do not delete or manually update the /etc/vol/volboot file; it is critical for starting LSM.

8.6.3    Adding a Disk to a Disk Group

Once LSM has been initialized with the /sbin/volsetup utility, you can add more physical disks or disk partitions to the rootdg disk group or add a new disk group by executing the interactive voldiskadd utility. This utility requires that a disklabel already exist on the device. Refer to the disklabel(8) reference page for complete information. For example, you could add a disk partition to the rootdg disk group by executing the following command: 

# voldiskadd rz3

To initialize a disk without adding it to a disk group, use the voldisksetup(8) command. This command allows you to add an LSM simple disk or sliced disk.

To add a physical disk to LSM with a specific private region size, use the voldisksetup(8) command. For example, use the following command to initialize a sliced LSM disk with a private region size of 2048 sectors: 

# voldisksetup -i rz3 privlen=2048

Use the voldg command to add the LSM disk to a disk group.

8.6.4    Creating a Volume in a Disk Group

After you create a disk group and add disks, use the volassist command to create volumes. For example: 


# volassist -g disk_group make volume length attribute=value

To create a volume in a disk group, use the instructions in the following list, or use the dxlsm graphical user interface (GUI).

8.6.5    Mirroring a Volume

Once a volume is created and enabled, use the volassist utility to create and attach new plexes to the volume.

8.6.6    Changing the Size of a Volume

You can use the volassist utility to increase or decrease the size of a volume. To change the size of a volume, use the following examples as guidelines:

Caution

The following restrictions apply to grown LSM volumes: