UNIX for Dummies Questions & Answers

Hi

Can someone tell me what are the differences between software and hardware raid ?

thx for help.

Read this. Basically: software RAID is done in software (within the OS or a driver) and hardware RAID is done on chip.

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.