Few Definitions !!!

http://www.pctechguide.com/glossary/disk-storage

1) Disk Overhead :

 disk overhead - (computer science) the disk space required for information that is not data but is used for location and timing .

Disk drive overhead is not solely applied to RAID system (although this case, you have 50% overhead because of RAID 1 mirror). What about a single IDE drive, does it have hard drive overhead? The answer is "Yes". Disk drive overhead is some spaces in the hard drive has been utilized for system file (FAT, NTFS, FHS....) and RAID array (if you have RAID configured). Hard drive overhead cannot be reduced in single drive. However, in RAID (multiple drives) you can reduce the overhead (in each drive) by adding more drives in your array (the total overhead is unchanged but it spread out to multi drives so each drive has smaller overhead usage).

Command overhead refers to the processing time required by the controller, host adapter, or drive prior to the execution of a command. Lower command overhead yields higher drive performance. Disk overhead refers to the space required for non- data information such as location and timing. Disk overhead often accounts for about ten percent of drive capacity. Lower disk overhead yields greater disk capacity.

RAID concept

User data in RAID can be stored using various data storage techniques:

• striping
• mirroring
• parity
• combination of the above

Striping is a way of writing data to member disks in which the data flow is split onto the blocks of a certain size and then written to the disks in turn.

Mirroring - stores identical data copies on the array member disks.

Parity is a type of data organization where splitting data onto the blocks (striping) is used along with calculation of a certain checksum which then is written to the member disks.

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There are the following methods to write data to RAID.

• Mirroring: It is required to use no less than two disks. The same data is written on two of the array member disks. The example of such an array is RAID 1 (sometimes called Mirror).
• Striping: It is required to use no less than two disks. The data blocks are written on each of the array member disks in turn. The example of such an array is RAID 0.
• Parity: It is required to use no less than three disks. Along with data blocks, some checksum (called parity) is written to provide array redundancy. The examples are RAID 5 and RAID 6.

RAID levels

Depending on the way the data in RAID are organized, the following RAID types (so calledlevels) can be distinguished.

RAID 0 - sometimes, this RAID level is called stripe set since it is based on a striping technique. This is a non-redundant array and it is created mainly to increase system performance because N-disks array provides N times read and write speed improvement as compared to a single disk. For a controller failure, RAID 0 recovery is relatively simple and straightforward, However, if one of the disks is unreadable, almost nothing can be recovered.

RAID 1 is based on a mirroring technique. It is used where system reliability is critical. RAID 1 is very easy to recover - just attach the disk to a known-good PC.

RAID 0+1, same as RAID 1+0 is based on mirroring and striping techniques. The array inherits RAID 1 fault tolerance and RAID 0 speed efficiency. There is a 100% disk space overhead in RAID 0+1 implementations. Such arrays are able to survive a single disk failure and, in some cases, multiple simultaneous disk failures as well. This is one of the most expensive RAID types, because the capacity overhead increases with a number of disks.

RAID 5 - sometimes called stripe set with parity - utilizes striping and parity techniques. Parity technique provides fault tolerance - in case of a single disk failure the missing data is reconstructed using parity data and data from other member disks. Read speed for the array with one failed disk degrades dramatically. This is relatively inexpensive RAID type because disk space overhead equals the capacity of one member disk and doesn't increase with a number of disks. RAID 5 recovery is more complicated than for RAID 0, but the data can still be recovered if one of the disks had failed. There are many variations of RAID 5 layout: left and right, synchronous and asyncronous, plus an optional delayed parity.

RAID 6 is similar to RAID 5 but utilizes two different parity functions. Usually one of them is the same as in RAID 5 (XOR), but the second is more complex. On the one hand, using the two parity functions enables RAID 6 to survive a dual hard drive failure, on the other hand, the need to store parity data of the two functions results in that the disk space overhead equals the capacity of two member disks. RAID 6 recovery is very complex.

>>youtube : https://www.youtube.com/watch?v=Rn5LOtMBj_8