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Part III. System Administration
Chapter 19. GEOM: Modular Disk Transformation Framework
GEOM: Modular Disk Transformation Framework
In FreeBSD, the GEOM framework permits access and control to classes, such as Master Boot Records and BSD labels, through the use of providers, or the disk devices in [.filename]#/dev#. By supporting various software RAID configurations, GEOM transparently provides access to the operating system and operating system utilities.
This chapter covers the use of disks under the GEOM framework in FreeBSD. This includes the major RAID control utilities which use the framework for configuration. This chapter is not a definitive guide to RAID configurations and only GEOM-supported RAID classifications are discussed.
What type of RAID support is available through GEOM.
How to use the base utilities to configure, maintain, and manipulate the various RAID levels.
How to mirror, stripe, encrypt, and remotely connect disk devices through GEOM.
How to troubleshoot disks attached to the GEOM framework.
Understand how FreeBSD treats disk devices (crossref:disks[disks,Storage]).
Know how to configure and install a new kernel (crossref:kernelconfig[kernelconfig,Configuring the FreeBSD Kernel]).
RAID0 - Striping
Striping combines several disk drives into a single volume. Striping can be performed through the use of hardware RAID controllers. The GEOM disk subsystem provides software support for disk striping, also known as RAID0, without the need for a RAID disk controller.
In RAID0, data is split into blocks that are written across all the drives in the array. As seen in the following illustration, instead of having to wait on the system to write 256k to one disk, RAID0 can simultaneously write 64k to each of the four disks in the array, offering superior I/O performance. This performance can be enhanced further by using multiple disk controllers.
Disk Striping Illustration
Each disk in a RAID0 stripe must be of the same size, since I/O requests are interleaved to read or write to multiple disks in parallel.
RAID0 does _not_ provide any redundancy. This means that if one disk in the array fails, all of the data on the disks is lost. If the data is important, implement a backup strategy that regularly saves backups to a remote system or device.
The process for creating a software, GEOM-based RAID0 on a FreeBSD system using commodity disks is as follows. Once the stripe is created, refer to man:gstripe[8] for more information on how to control an existing stripe.
*Procedure: Creating a Stripe of Unformatted ATA Disks*