Top Forums UNIX for Dummies Questions & Answers RAID software vs hardware RAID Post 302438726 by Corona688 on Tuesday 20th of July 2010 02:09:23 PM
Hardware RAID is handled by a specialized RAID controller card which does its own processing to make many devices act like one. This card is either configured through BIOS extensions(you may get an extra 'hit ESC to setup' message on boot) or through proprietary utilities. Once configured, the separate drives act like one device to the OS that you could do anything to you'd do with a normal drive, including boot from.

Software RAID on the other hand takes several devices already apparent to the OS and just accesses them through special routines, so they're treated as one device. The computer has to do all its own processing, so there may be some performance cost there. It may not be able to combine reads the way a hardware RAID could, i.e. you might not get double-speed reading from a mirrored array, but implementations vary. Because the drives still present themselves as many seperate drives to the kernel, it's harder to boot properly from a software RAID. I won't say impossible, but the kernel may need special configuration to properly autodetect and mount the array.
 
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ATARAID(4)						   BSD Kernel Interfaces Manual 						ATARAID(4)

NAME
ataraid -- ATA software RAID support SYNOPSIS
device ata device ataraid DESCRIPTION
The ataraid driver provides support for so-called software RAID (sometimes referred to as fake RAID or pseudo RAID). When a controller that supports software RAID is instructed to create a RAID array, its BIOS writes data structures in a specific metadata format to the disks. These data structures are picked up by the ataraid driver, so that FreeBSD can work with the array. The ataraid driver has to understand the specific metadata format of a controller BIOS in order to support its RAID capabilities. Read-only support for a metadata format means that FreeBSD can use the given RAID array for normal read/write operations. Creation and rebuild of such arrays has to be done from the controller BIOS. Read and write support for a metadata format means that FreeBSD can use the given RAID array for normal read/write operations. Additionally, the atacontrol(8) utility can be used to create, rebuild, update and fail such RAID arrays. The ataraid driver can read the following metadata formats: o Adaptec HostRAID o Highpoint V2 RocketRAID o Highpoint V3 RocketRAID o Intel MatrixRAID o Integrated Technology Express (ITE) o JMicron o LSI Logic V2 MegaRAID o LSI Logic V3 MegaRAID o NVIDIA MediaShield o Promise FastTrak o Silicon Image Medley o Silicon Integrated Systems (SiS) o VIA Tech V-RAID o FreeBSD PseudoRAID The ataraid driver can write the following metadata formats: o Highpoint V2 RocketRAID o Intel MatrixRAID o JMicron o Promise FastTrak o Silicon Integrated Systems (SiS) o VIA Tech V-RAID o FreeBSD PseudoRAID It is also possible to use software RAID on controllers that do not have special software RAID capabilities. See atacontrol(8) for details. FILES
/dev/ar* ATA RAID device nodes SEE ALSO
ata(4), atacontrol(8) CAVEATS
RAID5 is not supported at this time. Code exists, but it neither uses nor maintains parity information. AUTHORS
The ataraid driver was written by Soren Schmidt <sos@FreeBSD.org>. This manual page was written by Christian Brueffer <brueffer@FreeBSD.org>. BSD
February 17, 2006 BSD

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