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The main goals of using redundant arrays of inexpensive disks (RAID) are to improve disk data performance and provide data redundancy. RAID can be handled either by the operating system software or it may be implemented via a purpose built RAID disk controller card without having to configure the operating system at all. The aim of Linear RAID is to accommodate large filesystems spread over multiple devices with no data redundancy. With RAID 0, the RAID controller tries to evenly distribute data across all disks in the RAID set.
Like Linear RAID, RAID 0 aims to accommodate large filesystems spread over multiple devices with no data redundancy.
A limitation of RAID 1 is that the total RAID size in gigabytes is equal to that of the smallest disk in the RAID set. RAID 4 operates likes RAID 0 but inserts a special error-correcting or parity chunk on an additional disk dedicated to this purpose. RAID 4 requires at least three disks in the RAID set and can survive the loss of a single drive only.
RAID 5 improves on RAID 4 by striping the parity data between all the disks in the RAID set. Specially built hardware-based RAID disk controllers are available for both IDE and SCSI drives. If hardware RAID isn't available, then you should be aware of these basic guidelines to follow when setting up software RAID. To save costs, many small business systems will probably use IDE disks, but they do have some limitations. The total length of an IDE cable can be only a few feet long, which generally limits IDE drives to small home systems. The performance of the IDE bus can be degraded by the presence of a second device on the cable.
The failure of one drive on an IDE bus often causes the malfunctioning of the second device.
For these reasons, I recommend you use only one IDE drive per controller when using RAID, especially in a corporate environment.
There is only one drive per cable which makes hot swapping, or the capability to replace components while the system is still running, possible without the fear of affecting other devices on the data cable.
There are no jumpers to set on Serial ATA drives to make it a master or slave which makes them simpler to configure.
SCSI hard disks have a number of features that make them more attractive for RAID use than either IDE or Serial ATA drives. SCSI cables can be up to 25 meters long, making them suitable for data center applications. Some models of SCSI devices support "hot swapping" which allows you to replace them while the system is running. SCSI drives tend to be more expensive than IDE drives, however, which may make them less attractive for home use. It is generally a not a good idea to share RAID-configured partitions with non-RAID partitions.

If you decide to use RAID, all the partitions on each RAID disk should be part of a RAID set.
Software RAID creates the equivalent of a single RAID virtual disk drive made up of all the underlying regular partitions used to create it. As you will be modifying the disk structure of your system, you should also consider configuring RAID while your system is running in single-user mode from the VGA console. Configuring RAID using Fedora Linux requires a number of steps that need to be followed carefully. First use the fdisk -l command to view all the mounted and unmounted filesystems available on your system.
You don't want anyone else accessing these partitions while you are creating the RAID set, so you need to make sure they are unmounted. You have to change each partition in the RAID set to be of type FD (Linux raid autodetect), and you can do this with fdisk. Now that the partitions have been prepared, we have to merge them into a new RAID partition that we'll then have to format and mount. Linux software RAID provides redundancy across partitions and hard disks, but it tends to be slower and less reliable than RAID provided by a hardware-based RAID disk controller.
Hardware RAID configuration is usually done via the system BIOS when the server boots up, and once configured, it is absolutely transparent to Linux. Remember to take these factors into consideration when determining the right solution for your needs and research the topic thoroughly before proceeding. Although it isn't your question, it may be useful to consult RAID Boot for more information on using initramfs to start a system booting from md volumes.
Not the answer you're looking for?Browse other questions tagged linux raid software-raid raid-1 or ask your own question. When RAID runs out of striping space on the smallest device, it then continues the striping using the available space on the remaining drives.
When the failed disk is replaced, the data is automatically cloned to the new disk from the surviving disk. A disadvantage of software-based RAID 1 is that the server has to send data twice to be written to each of the mirror disks. When this occurs, the data in it can be recreated on the fly with the aid of the information on the RAID set's parity disk. This avoids the parity disk bottleneck, while maintaining many of the speed features of RAID 0 and the redundancy of RAID 1. They usually have their own BIOS, so you can configure them right after your system's the power on self test (POST).
This can be fatal if you have two IDE drives of the same RAID set attached to the same cable. Many people simplify this problem by filling each disk of a RAID set with only one partition. This makes sure that most applications and networking are shutdown and that no other users can access the system, reducing the risk of data corruption during the exercise.
In the tutorial example, you'll be configuring RAID 5 using a system with three pre-partitioned hard disks.

If you are doing RAID 0 or RAID 5, the partitions should be of approximately the same size, as in this scenario. You may then also want to use the df -k command, which shows only mounted filesystems but has the big advantage of giving you the mount points too. Confirm that the initialization is finished by inspecting the file and making sure that there are no initialization related messages. Unlike software RAID, hardware RAID requires entire disks to be dedicated to the purpose and when combined with the fact that it usually requires faster SCSI hard disks and an additional controller card, it tends to be expensive. The initialization process of RAID 0, divides the cakes and distributes the slices across all the plates. When this occurs, the data access speed is lower for this portion of data, because the total number of RAID drives available is reduced. Think of telling two people the same story so that if one forgets some of the details you can ask the other one to remind you.
RAID 1 also offers the possibility of using a hot standby spare disk that will be automatically cloned in the event of a disk failure on any of the primary RAID devices.
When the failed disk is replaced, it is repopulated with the lost data with the help of the parity disk's information. In other words, any data written to any section of the data portion of the RAID set must be followed by an update of the parity disk. Hardware-based RAID is transparent to your operating system; the hardware does all the work. RAID limits the extent of data access on each partition to an area no larger than that of the smallest partition in the RAID set. The formatting can be tricky, but fortunately the output of the mdadm --detail --scan command provides you with it. Make sure that all references to them in this file are commented with a # at the beginning of the line or deleted entirely. The RAID 0 drivers make the operating system feel that the cakes are intact and placed on one large plate.
With a hardware-based solution, the server CPU sends the data to the RAID disk controller once, and the disk controller then duplicates the data to the mirror disks. It is best to backup the data on these and any other partitions on the disk drive on which you want implement RAID.
The script would not automatically start the RAID set driver for the RAID set if it didn't find a match. For example, four 9GB hard disks configured in a RAID 0 set are seen by the operating system to be one 36GB disk. Mounting a RAID device that doesn't have a loaded driver can corrupt your data and produce this error.

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