Raids

THE 3 MAIN CONCEPTS IN RAID
When you talk RAID, 3 terms are important and relevant. One is striping, another is mirroring and the third is parity.
WHAT IS STRIPING?
Striping is the process of breaking down data into pieces and distributing it across multiple disks that support a logical volume – “Divide, Conquer & Rule”.  This often results in a “logical volume” that is larger and hasgreater I-O bandwidth than a single disk. It is purely based on the linear power of incrementally adding disks to a volume to increase the size and I-O bandwidth of the logical volume.  The increase in bandwidth is a result of how read/write operations are done on a striped volume.
Imagine that you are in a grocery store. With you are about two hundred of your closest friends and neighbors allshopping for the week’s groceries. Now consider what it’s like when you get to the checkout area and find that only one checkout line is open. That poor clerk can only deal with a limited number of customers per hour. The line starts to grow progressively.  The same is true of your I-O sub-system. A given disk can process a specific number of I-O operations per second. Anything more than that and therequests start to queue up. Now stop and think about how great it feels when you get to the front of the store and find that all 20 lines are open. You find your way to the shortest line and your headed out the door in no time.
Striping has a similar effect to your I-O system. By creating a single volume from pieces of data on several disks, we can increase the capacity to handle I-O requests in alinear fashion, by combining each disk’s I-O bandwidth. Now, when multiple I-O requests for a file on a striped volume is processed, they can be serviced by multiple drives in the volume, as the requests are sub-divided across several disks. This way all drives in the striped volume can engage and service multiple I/O requests in a more efficient manner.  This “cohesive and independent”functioning of all the drives in a logical volume is relevant for both read and write operations.  It must be noted that striping by itself, does not reduce “response time” for servicing I-O requests.  However, it does provide predictable response times and facilitates the notion of better performance, by balancing I-O requests across multiple drives in the striped volume.
Figure 2 depicts a 4-way stripedvolume (v1) with 4 disks (1-4).  A given stripe of data (Data1) in a file on v1 will be split/striped across the 4 disks, into 4 pieces (Data11-Data14).
Disk1 | Disk2 | Disk3 | Disk4 |
Data11 | Data12 | Data13 | Data14 |
Data21 | Data22 | Data23 | Data24 |
Figure 2
WHAT IS MIRRORING?
Mirroring is the process of writing the same data, to another “member” of the same volume simultaneously.Mirroring provides protection for data by writing exactly the same information to every member in the volume. Additionally, mirroring can provide enhanced read operations because the read requests can be serviced from either “member” of the volume. If you have ever made a photocopy of a document before mailing the original then you have mirrored data.  One of the common myths with mirroring, isthat it takes “twice as long” to write.  But in many performance measurements and benchmarks, the overhead has been observed to be around 15-20%.
Figure 3 illustrates a 4-way striped mirrored volume (v1) with 8 Disks (1-8).  A given stripe of data (Data1) in a file on v1 will be split/striped across the Disks (1-4) and then mirrored across Disks (5-8).  Disks (1-4) and (5-8) are called “MirrorMembers” of the volume v1.
Disk1 | Disk2 | Disk3 | Disk4 | Disk5 | Disk6 | Disk7 | Disk8 |
Data11 | Data12 | Data13 | Data14 | Data11 | Data12 | Data13 | Data14 |
Data21 | Data22 | Data23 | Data24 | Data21 | Data22 | Data23 | Data24 |
Figure 3
WHAT IS PARITY?
Parity is the term for error checking. Some levels of RAID, perform calculations when reading and writing data.  The calculations are...