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™ 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.