Solaris Volume Manager Administration Guide

Chapter 2 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

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 unchanged (persistent) even after unexpected events (hardware failure, software failure, or other similar event).

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^TM Operating Environment.

RAID Levels

RAID is an acronym for Redundant Array of Inexpensive (or Independent) Disks. Basically, this term refers to a set of disks (called an array, or, more commonly, a volume) that appears to the user as a single large disk drive. This array provides, depending on the configuration, improved reliability, response time, and/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 because it is the basis for the majority of RAID configurations in use.) Very few storage environments support RAID levels 2, 3, and 4, so they are not described here.

Solaris Volume Manager supports the following RAID levels:

RAID Level 0–Although they do not provide redundancy, stripes and concatenations 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 availability than a single disk
RAID Level 1–Mirroring uses equal amounts of disk capacity to store data and a copy (mirror) of it. Data is duplicated, or mirrored, over two or more physical disks. Data can be read from both drives simultaneously (either drive can service any request), providing 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, and, if used in conjunction with hot spares, can withstand multiple failures, albeit with 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, reducing the write time for large independent writes because the writes do not have to wait until a single 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 Solaris Volume Manager RAID 0 (concatenation and stripe) volumes, RAID 1 (mirror) volumes, RAID 5 volumes, soft partitions, transactional (logging) volumes, and file systems that are constructed on 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 among them. 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 listed are not mutually exclusive. You can use them in combination to meet multiple goals. For example, you could create a RAID 1 volume for redundancy, then create soft partitions on it to increase the number of discrete file systems that are possible.

Table 2–1 Choosing Storage Mechanisms


Requirements	RAID 0 (Concatenation)	RAID 0 (Stripe)	RAID 1 (Mirror)	RAID 5	Soft Partitions
Redundant data	No	No	Yes	Yes	No
Improved read performance	No	Yes	Depends on underlying device	Yes	No
Improved write performance	No	Yes	No	No	No
More than 8 slices/device	No	No	No	No	Yes
Larger available storage space	Yes	Yes	No	Yes	No

Table 2–2 Optimizing Redundant Storage


	RAID 1 (Mirror)	RAID 5
Write operations	Faster	Slower
Random read	Faster	Slower
Hardware cost	Higher	Lower

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; write performance is always degraded.
Because of the read-modify-write nature of RAID 5 volumes, volumes with greater than about 20 percent writes should probably not be RAID 5. If redundancy is required, consider mirroring.
RAID 5 writes will never be as fast as mirrored writes, which in turn will never be as fast as unprotected writes.
Soft partitions are useful for managing very large storage devices.

Note –

In addition to these generic storage options, see Hot Spare Pools for more information about using Solaris Volume Manager to support redundant devices.

Performance Issues

General Performance Guidelines

When you design your storage configuration, consider the following performance guidelines:

Striping generally has the best performance, but it offers no data redundancy. For write-intensive applications, RAID 1 volumes generally have better performance than RAID 5 volumes.
RAID 1 and RAID 5 volumes both increase data availability, but they both generally have lower performance, especially for write operations. Mirroring does improve random read performance.
RAID 5 volumes have a lower hardware cost than RAID 1 volumes, while RAID 0 volumes have no additional hardware cost.
Identify the most frequently accessed data, and increase access bandwidth to that data with mirroring or striping.
Both stripes and RAID 5 volumes distribute data across multiple disk drives and help balance the I/O load.
Use available performance monitoring capabilities and generic tools such as the iostat command to identify the most frequently accessed data. Once identified, the access bandwidth to this data can be increased using striping, RAID 1 volumes or RAID 5 volumes.
The performance of soft partitions can degrade when the soft partition size is changed multiple times.

RAID 5 volume performance is lower than stripe performance for write operations, because the RAID 5 volume requires multiple I/O operations to calculate and store the parity.
For raw random I/O reads, the stripe and the RAID 5 volume are comparable. Both the stripe and RAID 5 volume split the data across multiple disks, and the RAID 5 volume parity calculations aren't a factor in reads except after a slice failure.
For raw random I/O writes, the stripe is superior to RAID 5 volumes.

Optimizing for Random I/O and Sequential I/O

This section explains Solaris Volume Manager strategies for optimizing your particular configuration.

In general, if you do not know if sequential I/O or random I/O predominates on file systems you will be implementing on Solaris Volume Manager volumes, do not implement these performance tuning tips. These tips can degrade performance if they are improperly implemented.

The following optimization suggestions assume that you are optimizing a RAID 0 volume. In general, you would want to optimize a RAID 0 volume, then mirror that volume to provide both optimal performance and data redundancy.

Random I/O

If you have a random I/O environment, such as an environment used for databases and general-purpose file servers, you want all disk spindles to be approximately equal amounts of time servicing I/O requests.

For example, assume that you have 40 Gbytes of storage for a database application. If you stripe across four 10 Gbyte disk spindles, and if the I/O load is truly random and evenly dispersed across the entire range of the table space, then each of the four spindles will tend to be equally busy, which will generally improve performance.

The target for maximum random I/O performance on a disk is 35 percent or lower usage, as reported by the iostat command. Disk use in excess of 65 percent on a typical basis is a problem. Disk use in excess of 90 percent is a significant problem. The solution to having disk use values that are too high is to create a new RAID 0 volume with more disks (spindles).

Note –

Simply attaching additional disks to an existing volume will not improve performance. You must create a new volume with the ideal parameters to optimize performance.

The interlace size of the stripe doesn't matter because you just want to spread the data across all the disks. Any interlace value greater than the typical I/O request will do.

Sequential Access I/O

You can optimize the performance of your configuration to take advantage of a sequential I/O environment, such as DBMS servers that are dominated by full table scans and NFS servers in very data-intensive environments, by setting the interlace value low relative to the size of the typical I/O request.

For example, assume a typical I/O request size of 256 Kbyte and striping across 4 spindles. A good choice for stripe unit size in this example would be: 256 Kbyte / 4 = 64 Kbyte, or smaller.

This strategy ensures that the typical I/O request is spread across multiple disk spindles, thus increasing the sequential bandwidth.

Note –

Seek time and rotation time are practically zero in the sequential case. When you optimize sequential I/O, the internal transfer rate of a disk is most important.

In sequential applications, the typical I/O size is usually large (greater than 128 Kbytes, often greater than 1 Mbytes). Assume an application with a typical I/O request size of 256 Kbytes and assume striping across 4 disk spindles. 256 Kbytes / 4 = 64 Kbytes. So, a good choice for the interlace size would be 32 to 64 Kbyte.