Basics: Data Mirroring

Mirroring – also known as RAID 1 – consists of automatically copying the entire contents of a hard disk to another one. In other words, if you equip your computer such a system, the second hard disk will be the spitting image of the first. If your main hard disk goes up in smoke, the second will automatically activate.

It is amazing: mirroring is automatic backing up via hardware, enhancing your computer’s safety factor. It goes without saying that this system eliminates the need to backup (seeing it is possible that both hard disks collapse together – an extremely remote, but real probability) but it really provides a feeling of safety to people that cannot, under any circumstances whatsoever, lose data stored on their hard disks. The best thing about mirroring is that it is carried out automatically by hardware on the motherboard or controller card, not requiring any operating system set up for backing up (as the system believes that the computer has a single hard disk).

And better still: mirroring does not have to be implemented at the time you format your hard disk and install the operating system. You can take a disk with years-old data and start mirroring it. Upon configuring – done by a self set up – the contents of the main hard disk will be copied to the backup hard disk (a procedure that takes some time, of course).

Data splitting and mirroring can be set to work at the same time through a set-up usually called RAID 0+1. This set-up requires at least four hard disks. Data splitting will be used on two disks, to increase the speed, while the other two disks will provide backup the first pair. If one of the disks goes down, the system starts acting like a RAID0 system, i.e., just data stripping. Another system for putting RAID0 and RAID1 together is called RAID10. It works like RAID0+1 but if a had disk fails, the RAID10 the system becomes a RAID1 system, i.e., just mirroring.

Modern systems allows the use of RAID0+1 using just two hard disks. This setup is called JBOD (Just a Bunch of Disks) and works using only half of each hard disk capacity, thus simulating four hard drives. For example, using two 40 GB hard disks with JBOD RAID configuration, the total available space will be 20 GB (the other 20 GB space will be used for backing up the data from the first half of the disk). Of course this system is slower than RAID0+1.


Gabriel Torres is a Brazilian best-selling ICT expert, with 24 books published. He started his online career in 1996, when he launched Clube do Hardware, which is one of the oldest and largest websites about technology in Brazil. He created Hardware Secrets in 1999 to expand his knowledge outside his home country.