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Storage Concepts and Terminology: Dell™ Flexible Array Storage Tool User's Guide

Disks

Freespace

Partitions

Containers

Multilevel Containers

File Systems

Volume Set (JBOD)

Stripe Set (RAID 0)

Mirror Set (RAID 1)

RAID-5 Set

Stripe Set of Mirror Sets (RAID 0/1)

Volume Set of Mirror Sets

Volume Set of Stripe Sets

Volume Set of RAID-5 Sets

• • • • •

This chapter introduces you to RAID controller storage concepts and terminology. Read the entire chapter in the order presented to gain a complete understanding of the controller management subsystem structure.

The RAID controller subsystem is made up of the following layers:

Disks
Freespace
Partitions
Containers
Multilevel containers
File systems
Note: The figures in this chapter are for demonstration purposes only. They are not intended to represent typical configurations.

The term disk refers to the physical disk drives on the SCSI bus. In a controller subsystem, the SCSI disks are addressed by their disk ID, which includes the following:

Bus number. Indicates to which bus on the controller the SCSI disk is attached.
SCSI device ID (also known as target ID). Identifies the disk on the SCSI bus.
Logical unit number (LUN). The number assigned to a subdevice (logical unit) of a SCSI device, which is usually zero for a disk drive.

A disk can be in one of the following states:

Not initialized. The disk, such as a brand new disk, has no recognizable data on it.
Legacy disk (DOS). The disk contains a DOS partition table. The controller manages the disk as a legacy disk container, where there is a one-to-one logical-to-physical mapping of container to disk. (See Understanding Legacy Disks for additional information.)
Initialized. The disk is initialized for use as part of a container, or as a failover drive. Initializing Disks tells you how to use FAST to initialize SCSI disks.

Freespace is the space on an initialized disk that is not in use by other controller components. Containers are created from freespace. When a container is deleted, its space is returned to freespace.

A partition is a contiguous area of a physical disk that makes up some or all of a container. Unlike FAT, FAT32, or NTFS partitions, which are created and managed by the operating system, container partitions are created and managed by the controller as by-products of container creation or failover¹ operations. That is, when the controller creates a container, the controller automatically converts freespace on a disk into one or more container partitions (Figure ).

Figure 1. Partitions are by-products of container creation

Each container partition is identified by the disk on which it resides, its offset², and its size. A container partition cannot be used by more than one container at a time.

Containers are logical disks (also known as arrays) created from freespace and made up of one or more partitions on one or more physical disks. A container that spans multiple physical disks can be larger than any one of the physical disks.

Containers differ from most other RAID arrays in that their underlying partitions can be smaller than a physical disk. Consequently, several containers' partitions can reside on a single physical disk.

Containers are categorized as follows:

A single-level container is made up of one or more partitions.
A multilevel container is made up of one or more containers.

Figure shows a single-level container made up of equal-sized partitions that reside on two separate disks. Figure shows a single-level container made up of unequal-sized partitions that reside on two separate disks.

Figure 2. Single-level container made up of two equal-sized partitions

Figure 3. Single-level container made up of two unequal-sized partitions

On Windows NT and Windows 2000, a container's file system appears in Windows Explorer as a disk drive with its own drive letter. Containers with multiple operating system partitions are discussed in Managing Partitions with Microsoft Windows Utilities and in Properties of Containers with Multiple Operating-System Partitions (Windows NT and Windows 2000 Only). File systems are covered on File Systems.

Table lists the single-level container types supported by the controller management software. Table lists the supported multi-level container types.

Table 1. Single-level container types


Volume Set (JBOD)
Stripe Set (RAID 0)
Mirror Set (RAID 1)
RAID-5 Set

Table 2. Multi-level container types

Stripe Set of Mirror Sets (RAID 0/1)

Volume Set of Mirror Sets

Volume Set of Stripe Sets

Volume Set of RAID-5 Sets

For information on creating containers, see Creating Containers. For information on modifying containers, see Modifying Containers.

A volume set is a single-level container that is a concatenation of one or more partitions on one or more disks. The partitions in a volume set do not have to be the same size. The volume set is the simplest container that can be created with the controller management software.

A volume set does not have the superior performance of a stripe set or the redundancy (the capability of preventing data loss in the event of disk failure) of a mirror set or RAID-5 set.

Figure shows a volume set made up of two partitions, each on a different disk.

Figure 4. Volume set made up of two partitions
on two different disks

A stripe set is a single-level container made up of two or more equal-sized partitions that reside on different disks. (A stripe set can be created from one partition, but it is functionally equivalent to a single-drive volume set.) The stripe set distributes data evenly across its respective disks in equal-sized sections called chunks.

Whereas a volume set combines the data on different-sized partitions by concatenation, a stripe set distributes the data among the partitions in a way that optimizes access speed. A stripe set does not have the redundancy of a mirror set or RAID-5 set.

Figure shows a stripe set made up of three partitions on three separate disks.

Figure 5. Stripe set made up of three partitions,
each on a different disk

A mirror set is a single-level container made up of two equal-sized partitions that reside on two different disks. A mirror set stores and maintains the same, or redundant, data in both partitions.

Figure shows a mirror set.

Figure 6. Mirror set must be made up of two equal-sized
partitions residing on different disks

An existing mirror set can be manipulated using the following commands:

The Split command divides a mirror set into two identical volume sets, and is usually used for archiving data. This command works for both single-level or multilevel mirror set containers. (See Splitting a Mirror Set for more information.)
Note: The Split command is not supported on Windows 2000 and NetWare^TM systems.
The Unmirror command changes a mirror set into a volume set by converting the redundant halves of the mirror set to freespace. This command is used when redundancy is no longer required, and works for both single-level and multilevel mirror set containers. (See Unmirroring a Mirror Set for more information.)

The RAID-5 set is similar to a stripe set, except that its data is redundant. A RAID-5 set must be made up of at least three equal-sized partitions.

In a RAID-5 set, one chunk is used for parity data. The parity chunk is distributed across all disks containing partitions of the RAID-5 set so parity operations are evenly divided among all the partitions in the container. The space required for parity data is not available for user data, and is not included in the overall container size.

Figure shows a RAID-5 set made up of four partitions, each on a different disk.

Figure 7. RAID-5 set made up of four partitions
on four different disks

A multilevel container is a combination of two or more single-level containers. Multilevel containers made up of two container types combine the beneficial characteristics of the two types. For example, a stripe set of mirror sets combines the performance of a stripe set with the redundancy of a mirror set. (See , for more information about the advantages of various container types.)

The controller management software supports the following multilevel containers:

Stripe set of mirror sets (RAID 0/1)
Volume set of mirror sets
Volume set of stripe sets
Volume set of RAID-5 sets
Note: To create a volume set of stripe sets and a volume set of RAID-5 sets, you must use either CLI commands or perform the tasks manually in FAST. These containers are not supported by the Container Creation Wizard. For more information on creating these types of containers, see .

Figure shows a multilevel container created from two single-level containers. The multilevel container is referred to as the top-level container, and single-level containers A and B are referred to as underlying containers.

Figure 8. Multilevel container created from two single-level containers

On Windows NT and Windows 2000, multilevel containers appear in Windows Explorer as disk drives. The single-level containers that make up the multilevel container (the underlying containers) do not appear in Windows Explorer; only the multilevel container (the top-level container) that is using the underlying containers can access them.

The top-level container of a multilevel container can be only a volume set or a stripe set.

A stripe set of mirror sets (also called RAID 0/1 or RAID 10) is a multilevel container made up of two or more equal-sized mirror sets. A stripe set of mirror sets' data is redundant.

Figure shows a stripe set of mirror sets created from three equal-sized mirror sets (A, B, and C) and striped across six disks. Each mirror set is made up of two equal-sized partitions on two separate disks.

The stripe set of mirror sets is the top-level container, and mirror sets A, B, and C are the underlying containers.

Figure 9. Stripe set of mirror sets created from three
equal-sized mirror sets and striped across six disks

A volume set of mirror sets is a multilevel container that is a concatenation of two or more mirror sets. A volume set of mirror sets' data is redundant.

The mirror sets in a volume set of mirror sets do not have to be equally sized.

Figure shows a volume set of mirror sets created from two mirror sets (A and B). Each mirror set is made up of two equal-sized partitions on two separate disks.

The volume set of mirror sets is the top-level container, and mirror sets A and B are the underlying containers.

Figure 10. Volume set of mirror sets created from two
independent mirror sets

A volume set of stripe sets is a multilevel container made up of two or more stripe sets. The volume set of stripe sets is the top-level container, and stripe sets A and B are the underlying containers.

Figure shows a volume set of stripe sets created from two stripe sets (A and B). Each stripe set is made up of three equal-sized partitions on three separate disks.

Figure 11. Volume set of stripe sets created from
two independent stripe sets

A volume set of RAID-5 sets is a multilevel container made up of two or more RAID-5 sets. A volume set of RAID-5 sets' data is redundant.

Figure shows a volume set of RAID-5 sets created from two RAID-5 sets (A and B). Each RAID-5 set is made up of three equal-sized partitions on three separate disks.

Figure 12. Volume set of RAID-5 sets created from two
independent RAID-5 sets

The volume set of RAID-5 sets is the top-level container, and RAID-5 sets A and B are the underlying containers.

A file system is a layer of software that manages a collection of files within a directory structure. The file system relieves the user or application of the burden of knowing the exact location of user data. Rather, the user or application accesses file data by the file's name. The file system indexes and stores the data that corresponds to the locations of the file's name and data. This location information is generally referred to as metadata.

In addition to supporting containers with no file systems, the controller management software supports three types of file systems for Windows NT and Windows 2000: FAT, FAT32, and NTFS. These file systems are native to Windows NT and Windows 2000 and are maintained in full by the host server processor.

On Windows NT systems, the controller initializes the specified file system as soon as the container³ has been configured. This process makes the container immediately available to users. Other RAID controllers cannot initialize the file system until the array has been fully created. This creation process, which often takes several hours, significantly delays users' access to the container. (On Windows 2000 systems, the file system for a container must be created using the Microsoft Disk Management utility.)

¹ If a disk in a redundant container fails, one or more assigned failover disks automatically come on line (and become part of the container) to allow reconstruction of the data from the failed disk.

² The logical distance from the beginning of the disk to the start of the partition.

³ For any RAID-5 set created using the Clear command, the file system is not initialized until after the clear operation has completed.

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Storage Concepts and Terminology: Dell™ Flexible Array Storage Tool User's Guide

Partitions

Containers

Multilevel Containers

File Systems