Plexes

Volume Manager uses subdisks to build virtual objects called plexes. A plex is a structured or ordered collection of subdisks that represents one copy of the data in a volume. A plex consists of one or more subdisks located on one or more physical disks.

A plex is also called a mirror. The terms plex and mirror can be used interchangeably, even though a plex is only one copy of the data. The terms "mirrored" or "mirroring" imply two or more copies of data.

Volume Manager Storage Layout

A volume's layout refers to the organization of plexes in a volume. Volume layout is the way plexes are configured to remap the volume address space through which I/O is redirected at run-time. Volume layouts are based on the concept of disk spanning, which is the ability to logically combine physical disks in order to store data across multiple disks.

A variety of volume layouts are available, and each layout has different advantages and disadvantages. The layouts that you choose depend on the levels of performance and reliability required by your system.

With Volume Manager, you can change the volume layout without disrupting applications or file systems that are using the volume. A volume layout can be configured, reconfigured, resized, and tuned while the volume remains accessible.

Common volume layouts include:

• Concatenated
• Striped
• Mirrored
• RAID-5
• Layered

Concatenated

In a concatenated volume, subdisks are arranged both sequentially and contiguously within a plex. Concatenation allows a volume to be created from multiple regions of one or more disks if there is not enough space for an entire volume on a single region of a disk.

Striped

In a striped volume, data is spread evenly across multiple disks. Stripes are equally-sized fragments that are allocated alternately and evenly to the subdisks of a single plex. There must be at least two subdisks in a striped plex, each of which must exist on a different disk. Throughput increases with the number of disks across which a plex is striped. Striping helps to balance I/O load in cases where high traffic areas exist on certain subdisks.

Mirrored

A mirrored volume uses multiple plexes to duplicate the information contained in a volume. Although a volume can have a single plex, at least two are required for true mirroring (redundancy of data). Each of these plexes should contain disk space from different disks for the redundancy to be useful.

RAID-5

A RAID-5 volume uses striping to spread data and parity evenly across multiple disks in an array. Each stripe contains a parity stripe unit and data stripe units. Parity can be used to reconstruct data if one of the disks fails. In comparison to the performance of striped volumes, write throughput of RAID-5 volumes decreases since parity information needs to be updated each time data is accessed. However, in comparison to mirroring, the use of parity reduces the amount of space required.

Layered

A layered volume is a virtual Volume Manager object that nests volumes within volumes to create more complex volume structures that mirror data at a more granular level.

How Does Volume Manager Work?

With Volume Manager, you enable virtual data storage by bringing a disk under Volume Manager control. To bring a disk under Volume Manager control means that Volume Manager creates virtual objects and establishes logical connections between those objects and the underlying physical objects, or disks.

When a disk is brought under Volume Manager control:

1. Volume Manager removes all of the partition table entries from the VTOC, except for partition table entry 2 (backup slice). Partition table entry 2 contains the entire disk, including the VTOC, and is used to determine the size of the disk.

Note: The boot disk is a special case and is discussed in a later lesson.

2. Volume Manager then rewrites the VTOC and creates two partitions on the physical disk. One partition contains the private region, and the other contains the public region.
   • Private region: The private region stores information, such as disk headers, configuration copies, and kernel logs, that Volume Manager uses to manage virtual objects. The private region represents a small management overhead.

The minimum size for the private region is 1024 sectors (512K) for disks with active configuration databases, but VxVM uses 2048 sectors (1024K) by default. This default value is rounded up to the next cylinder boundary.

The maximum size for the private region is 524288 blocks (512K sectors).

• Public region: The public region consists of the remainder of the space on the disk. The public region represents the available space that Volume Manager can use to assign to volumes and is where an application stores data. Volume Manager never overwrites this area unless specifically instructed to do so.

By convention, the public region of a Volume Manager-controlled disk is referred to as a Volume Manager disk, or VxVM disk. The true definition of a VxVM disk is a Volume Manager-controlled disk.

3. Partition Tags: VxVM sets the partition tags, the numeric values that describe the file system mounted on a partition, for the public and private regions:
   • Tag 14 is always used for the public region of the disk.
   • Tag 15 is always used for the private region of the disk.

If the disk has no partitions that are being placed under Volume Manager control, then Volume Manager creates the private region first, and the public region second, on the disk.

4. Volume Manager updates the VTOC with information about the removal of the existing partitions and the addition of the new partitions during the initialization process.

Source: VERITAS Volume Manager Fundamentals I

ERITAS Volume Manager Fundamentals I

Physical Disk Naming for SAN

We can  locate and access the data on a physical disk by using a device name that specifies the controller, target ID, and disk number. 

A typical device name uses the format: c#t#d#.

• c# is the controller number. 
• t# is the target ID. 
• d# is the logical unit number (LUN) of the drive attached to the target. 

If a disk is divided into partitions, then we also specify the partition number in the device name:

s# is the partition (slice) number. 

For example, device name c0t0d0s1 is connected to controller number 0 in the system, with a target ID of 0, physical disk number 0, and partition number 1 on the disk.

Problem to mount NFS : mount: wrong fs type, bad option, bad superblock

Sometime, we may get error as  mount: wrong fs type, bad option, bad superblock while trying to mount NFS file in ubuntu. For my case, i faced the problem showing the error as:

The reason behind it that for several filesystems (e.g. nfs, cifs) we might need a /sbin/mount. helper program.
As we are trying to mount NFS. The /sbin/mount.nfs helper program is provided by nfs-common. We can install it with:

sudo apt-get update
sudo apt install nfs-common

On the other hand, if we are trying to mount CIFS, the helper program is provided by cifs-utils. We can install it using below command:

sudo apt install cifs-utils


Then we can mount our target as:
We will have to create a directory where to mount the nfs,

sudo mount :/ /