Storage Management Concepts

This chapter provides a brief introduction to some common storage management concepts. If you are already familiar with storage management concepts, you can proceed directly to Chapter 3, Solaris Volume Manager Overview.

This chapter contains the following information:

• Introduction to Storage Management

• Configuration Planning Guidelines

• Performance Issues

• Optimizing for Random I/O and Sequential I/O

Introduction to Storage Management

Storage management is the means by which you control the devices on which the active data on your system is kept. To be useful, active data must be available and remain persistent even after unexpected events like hardware or software failure.

Storage Hardware

There are many different devices on which data can be stored. The selection of devices to best meet your storage needs depends primarily on three factors:

• Performance

• Availability

• Cost

You can use Solaris Volume Manager to help manage the tradeoffs in performance, availability and cost. You can often mitigate many of the tradeoffs completely with Solaris Volume Manager.

Solaris Volume Manager works well with any supported storage on any system that runs the Solaris™ Operating Environment.

RAID Levels

RAID is an acronym for Redundant Array of Inexpensive (or Independent) Disks. RAID refers to a set of disks, called an array or a volume, that appears to the user as a single large disk drive. This array provides, depending on the configuration, improved reliability, response time, or storage capacity.

Technically, there are six RAID levels, 0-5,. Each level refers to a method of distributing data while ensuring data redundancy. (RAID level 0 does not provide data redundancy, but is usually included as a RAID classification anyway. RAID level 0 provides the basis for the majority of RAID configurations in use.) Very few storage environments support RAID levels 2, 3, and 4, so those environments are not described here.

Solaris Volume Manager supports the following RAID levels:

• RAID Level 0-Although stripes and concatenations do not provide redundancy, these constructions are often referred to as RAID 0. Basically, data are spread across relatively small, equally-sized fragments that are allocated alternately and evenly across multiple physical disks. Any single drive failure can cause data loss. RAID 0 offers a high data transfer rate and high I/O throughput, but suffers lower reliability and lower availability than a single disk

• RAID Level 1-Mirroring uses equal amounts of disk capacity to store data and a copy (mirror) of the data. Data is duplicated, or mirrored, over two or more physical disks. Data can be read from both drives simultaneously, meaning that either drive can service any request, which provides improved performance. If one physical disk fails, you can continue to use the mirror with no loss in performance or loss of data.

  Solaris Volume Manager supports both RAID 0+1 and (transparently) RAID 1+0 mirroring, depending on the underlying devices. See Providing RAID 1+0 and RAID 0+1 for details.

• RAID Level 5-RAID 5 uses striping to spread the data over the disks in an array. RAID 5 also records parity information to provide some data redundancy. A RAID level 5 volume can withstand the failure of an underlying device without failing. If a RAID level 5 volume is used in conjunction with hot spares, the volume can withstand multiple failures without failing. A RAID level 5 volume will have a substantial performance degradation when operating with a failed device.

  In the RAID 5 model, every device has one area that contains a parity stripe and others that contain data. The parity is spread over all of the disks in the array, which reduces the write time. Write time is reduced because writes do not have to wait until a dedicated parity disk can accept the data.

Configuration Planning Guidelines

When you are planning your storage management configuration, keep in mind that for any given application there are trade-offs in performance, availability, and hardware costs. You might need to experiment with the different variables to determine what works best for your configuration.

This section provides guidelines for working with the following types of volumes:

• Solaris Volume Manager RAID 0 (concatenation and stripe) volumes

• RAID 1 (mirror) volumes

• RAID 5 volumes

• Soft partitions

• Transactional (logging) volumes

• File systems that are constructed on Solaris Volume Manager volumes

Choosing Storage Mechanisms

Before you implement your storage management approach, you need to decide what kinds of storage devices to use. This set of guidelines compares the various storage mechanisms to help you choose. Additional sets of guidelines apply to specific storage mechanisms as implemented in Solaris Volume Manager. See specific chapters about each volume type for details.

Note - The storage mechanisms that are listed here are not mutually exclusive. You can use these volumes in combination to meet multiple goals. For example, you could first create a RAID 1 volume for redundancy. Next, you could create soft partitions on the RAID 1 volume to increase the number of discrete file systems that are possible.

Table 2-1 Choosing Storage Mechanisms

Table 2-2 Optimizing Redundant Storage

• RAID 0 devices (stripes and concatenations), and soft partitions do not provide any redundancy of data.

• Concatenation works well for small random I/O.

• Striping performs well for large sequential I/O and for random I/O distributions.

• Mirroring might improve read performance, but write performance is always degraded in mirrors.

• Because of the read-modify-write nature of RAID 5 volumes, volumes with over about 20 percent writes should not be RAID 5. If redundancy is required, consider mirroring.

• RAID 5 writes cannot be as fast as mirrored writes, which in turn cannot be as fast as unprotected writes.

• Soft partitions are useful for managing very large storage devices.