Logical volumes are composed of a hierarchy of logical storage objects: volumes are composed of subvolumes, subvolumes are composed of plexes, and plexes are composed of volume elements. Volume elements are composed of disk partitions. This hierarchy of storage units is shown in Figure 4-1, an example of a relatively complex logical volume.

Figure 4-1. Logical Volume Example

Figure 4-1 illustrates the relationships between volumes, subvolumes, plexes, and volume elements. In this example, six physical disk drives contain eight disk partitions. The logical volume has a log subvolume, a data subvolume, and a real-time subvolume. The log subvolume has two plexes (copies of the data) for higher reliability and the data and real-time subvolumes are not plexed (meaning that they each consist of a single plex). The log plexes each consist of a volume element which is a disk partition on disk 1. The plex of the data subvolume consists of two volume elements, a partition that is the remainder of disk 1 and a partition that is all of disk 2. The plex used for the real-time subvolume is striped for increased performance. The striped volume element is constructed from four disk partitions, each of which is an entire disk.

The subsections below describe these logical storage objects in more detail.

Volumes

Volumes are composed of subvolumes. For EFS filesystems, a volume consists of just one subvolume. For XFS filesystems, a volume consists of a data subvolume, an optional log subvolume, and an optional real-time subvolume. The breakdown of a volume into subvolumes is shown in Figure 4-2.

Figure 4-2. Volume Composition

Each volume can be used as a single filesystem or as a raw partition. Volume information used by the system during system startup is stored in disk labels on each disk used by the volume. At system startup, volumes won't come up if any of their subvolumes cannot be brought online. You can create volumes, delete them, and move them to another system.

Subvolumes

As explained in the section“Volumes,” each logical volume is composed of one to three subvolumes, as shown in Figure 4-3. A subvolume is made up of one to four plexes.

Figure 4-3. Subvolume Composition

Each subvolume is a distinct address space and a distinct type. The types of subvolumes are:

Data subvolume		The data subvolume is required in all logical volumes. It is the only subvolume present in EFS filesystems.
Log subvolume		The log subvolume contains XFS journaling information. It is a log of filesystem transactions and is used to expedite system recovery after a crash. Log information is sometimes put in the data subvolume rather than in a log subvolume (see the section “Choosing the Log Type and Size” in Chapter 2 and the mkfs_xfs(1M) reference page and its discussion of the –l option for more information).
Real-time subvolume		Real-time subvolumes are generally used for data applications such as video, where guaranteed response time is more important than data integrity. The section “Using the Real-Time Subvolume” in this chapter and Chapter 5, “Guaranteed-Rate I/O,” explain how applications access data on real-time subvolumes.

Subvolumes enforce separation among data types. For example, user data cannot overwrite filesystem log data. Subvolumes also enable filesystem data and user data to be configured to meet goals for performance and reliability. For example, performance can be improved by putting subvolumes on different disk drives.

Each subvolume can be organized independently. For example, the log subvolume can be plexed for fault tolerance and the real-time subvolume can be striped across a large number of disks to give maximum throughput for video playback.

Volume elements that are part of a real-time subvolume should not be on the same disk as volume elements used for data or log subvolumes. This is a recommendation for all files on real-time subvolumes and required for files used for guaranteed-rate I/O with hard guarantees. (See “Hardware Configuration Requirements for GRIO” in Chapter 5 for more information.)

You can create subvolumes, but you cannot detach them from their volumes or delete them. A subvolume is automatically deleted when the volume is deleted.

Plexes

A subvolume can contain from one to four plexes (sometimes called mirrors). Each plex contains a portion or all of the subvolume's data. By creating a volume with multiple plexes, system reliability is increased.

If there is just one plex in a subvolume, that plex spans the entire address space of the subvolume. However, when there are multiple plexes, individual plexes can have holes in their address spaces as long as the union of all plexes spans the entire address space.

Normally, data is written to all plexes. However, when necessary (for example when an additional plex is added to a subvolume), a full plex copy, called a plex revive, is done automatically by the system. See the xlv_assemble(1M) and xlv_plexd(1M) reference pages for more information.

A plex is composed of one or more volume elements, as shown in Figure 4-4, up to a maximum of 128 volume elements. Each volume element represents a range of addresses within the subvolume.

Figure 4-4. Plex Composition

When a plex is composed of two or more volume elements, it is said to have concatenated volume elements. With concatenation, data written sequentially to the plex is also written sequentially to the volume elements; the first volume element is filled, then the second, and so on. Concatenation is useful for creating a filesystem that is larger than the size of a single disk.

You can add plexes to subvolumes, detach them from subvolumes that have multiple plexes (and possibly attach them elsewhere), and delete them from subvolumes that have multiple plexes.

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Note: To have multiple plexes, you must purchase the Disk Plexing Option software option and obtain and install a NetLS license.

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Volume Elements

The simplest type of volume element is a single disk partition. The two other types of volume elements, striped volume elements and multipartition volume elements, are composed of several disk partitions. Figure 4-5 shows a single partition volume element.

Figure 4-5. Single Partition Volume Element Composition

Figure 4-6 shows a striped volume element. Striped volume elements consist of two or more disk partitions, organized so that an amount of data called the stripe unit is written to each disk partition before writing the next stripe unit-worth of data to the next partition.

Figure 4-6. Striped Volume Element Composition

Striping can be used to alternate sections of data among multiple disks. This provides a performance advantage by allowing parallel I/O activity.

Figure 4-7 shows a multipartition volume element in which the volume element is composed of more than one disk partition. In this configuration, the disk partitions are addressed sequentially.

Figure 4-7. Multipartition Volume Element Composition

Any mixture of the three types of volume elements (single partition, striped, and multipartition) can be concatenated in a plex.

Volumes appear as block and character devices in the /dev directory. The device names for logical volumes are /dev/dsk/xlv/<volume_name> and /dev/rdsk/xlv/<volume_name>, where <volume_name> is a volume name specified when the volume is created using xlv_make(1M).

When a volume is created on one system and moved (by moving the disks) to another system, the new volume name is the same as the original volume name with the hostname of the original system prepended. For example, if a volume called xlv0 is moved from a system called engrlab1 to a system called engrlab2, the device name of the volume on the new system is /dev/dsk/xlv/engrlab1.xlv0 (the old system name engrlab1 has been prepended to the volume name xlv0).

The XLV daemons are:

XLV does not require an explicit configuration file, nor is it turned on and off with chkconfig(1M). XLV is able to assemble logical volumes based solely upon information written in the disk labels. During initialization, the system performs a hardware inventory, reads all the disk labels, and automatically assembles the available disks into volumes.

If some disks are missing, XLV checks to see if there are enough volume elements among the available plexes to map the entire address space. If the whole address space is available, XLV brings the volume online even if some of the plexes are incomplete.

For read failures on log and data subvolumes, XLV rereads from a different plex (when available) and attempts to fix the failed plex by rewriting the results. XLV does not retry on failures for real-time data.

For log and data subvolumes, XLV assumes that the write errors it receives are hard errors (the disk driver and controllers handle soft errors). If the volume element with a hard error is plexed, XLV marks the volume element bad and ignores it. If the volume element is not plexed, the volume element remains associated with the volume and an error is returned.

XLV doesn't handle write errors on real-time subvolumes. Incorrect data is returned without error messages.

There are some situations where logical volumes cannot be used or are not recommended:

The basic guidelines for choosing which subvolumes to use with EFS filesystems are:

The basic guidelines for choosing which subvolumes to use with XFS filesystems are:

The basic guidelines for choosing subvolume sizes are:

The basic guidelines for plexing are:

The basic guidelines for striping are:

The basic guidelines for the concatenation of disk partitions are:

This example shows a simple logical volume composed of a data subvolume created from two entire option disks. The disks are on controller 0, units 2 and 3 (use the hinv(1M) command, for example hinv –c disk, to obtain this information). Partition 7 of each disk is normally the entire disk (use the prtvtoc(1M) command, for example prtvtoc /dev/rdsk/dks0d2vh and prtvtoc /dev/rdsk/dks0d3vh, to obtain this information).

This example shows the noninteractive creation of a logical volume from four equal-sized option disks (controller 0, units 2 through 5). Two plexes will be created with the data striped across the two disks in each plex. The stripe unit will be 128 KB.

To get a list of the highest-level volume objects on a system, use selection 41 of the xlv_admin menu, for example:

xlv_admin> 41

==================== Listing Objects =============
Volume Element:  'spare_ve'
Volume:          'xlv0'

In this example, there are two high-level volume objects, a volume element named spare_ve and a logical volume named xlv0. The volume element is a high-level volume object because it is not part of any plex or subvolume.

To display the complete hierarchy of a high-level volume object, use selection 42 of the xlv_admin menu, for example:

xlv_admin> 42
Please enter name of object to be operated on.
xlv_admin> xlv0

============= Displaying Requested Object ==========
vol xlv0
ve xlv0.data.0.0 [active]
        start=0, end=226799, (cat)grp_size=1
        /dev/dsk/dks0d2s7 (226800 blks)
ve xlv0.data.0.1 [active]
        start=226800, end=453599, (cat)grp_size=1
        /dev/dsk/dks0d3s7 (226800 blks)
ve xlv0.data.0.2 [active]
        start=453600, end=680399, (cat)grp_size=1
        /dev/dsk/dks0d4s7 (226800 blks)

This output shows that xlv0 contains only a data subvolume. The data subvolume has one plex that has three volume elements.

Growing a logical volume (increasing its size) can be done in two ways by adding one or more volume elements to the end of one or more of its plexes.

If you have purchased the Disk Plexing Option software option and have installed a NetLS license for it, you can add a plex to an existing subvolume for improved reliability in case of disk failures. The procedure to add a plex to a subvolume is described below. To add more than one plex to a subvolume or to add a plex to each of the subvolumes in a volume, repeat the procedure as necessary.

The plex revive completes and the new plex switches to [active] state automatically, but if you want to check its progress and verify that the plex has become active, follow this procedure:

Detaching a plex from a volume, perhaps because you want to swap disk drives, can be done while the volume is active. However, the entire address range of the subvolume must still covered by active volume elements in the remaining plex or plexes. xlv_admin does not allow you to remove the only active plex in a volume if the other plexes are not yet active. The procedure to detach a plex is:

You can delete a volume or any other XLV object by using selection 31 of the xlv_admin command menu. The procedure is:

Real-time files have some special characteristics that cause standard IRIX utilities to operate in ways that you might not expect. In particular:

To create a real-time file, use the F_FSSETXATTR fcntl(2) system call with the XFS_XFLAG_REALTIME bit set in the fsx_xflags field of the fsxattr structure. This must be done after the file has first been created/opened for writing, but before any data has been written to the file. Once data has been written to a file, it cannot be changed from a standard data file to a real-time file, nor can files created as real-time files be changed to standard data files.

Real-time files can only be read or written using direct I/O. Therefore, read(2) and write(2) operations to a real-time file must meet the requirements specified by the F_DIOINFO fcntl(2) call. See the open(2) reference page for a discussion of the O_DIRECT option to the open() system call.

The real-time subvolume is used by applications for files that require fixed I/O rates. This feature, called guaranteed-rate I/O, is described in Chapter 5, “Guaranteed-Rate I/O.”