Dell Perc 6i Manual

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DELL PERC H700 and H800 Technical Guide   21 
5 RAID Overview 
5.1 About RAID 
RAID is a way of storing data on multiple independent physical disks for the purpose of enhanced 
performance and/or fault tolerance. The physical disks combine to make up what is called a virtual 
disk. This virtual disk appears to the host system as a single logical unit or drive. For example, if you 
have physical disk 1 and physical disk 2 forming a RAID virtual disk, those two disks appear to the 
host system as one disk. 
Virtual Disks are sometimes called volumes, containers, or arrays. 
There are several different RAID types or levels, which determine how the data is placed in the 
virtual disk. Each RAID level has specific data protection and system performance characteristics. 
The following are commonly used RAID levels: 
• RAID 0: Striping without parity, improved performance, additional storage, no fault tolerance 
• RAID 1: Mirroring without parity, fault tolerance for disk errors, and single disk failures 
• RAID 5: Striping with distributed parity, improved performance, fault tolerance for disk 
errors, and single disk failures 
• RAID 6: Striping with dual parity, fault tolerance for dual drive failures 
• RAID 10: Mirroring combined with striping, better performance, fault tolerance for disk 
errors, and multiple drive failure (one drive failure per mirror set) 
• RAID 50: Combines multiple RAID 5 sets with striping, improved performance, fault disk 
errors, and multiple drive failures (one drive failure per span) 
• RAID 60: Combines multiple RAID 6 sets with striping, improved performance, fault disk 
errors, and multiple drive failures (two drive failures per span) 
These RAID levels are discussed in more detail later in this document. You can manage RAID virtual 
disks with a RAID controller (hardware RAID) or with software (software RAID). 
5.2 Advantages of RAID 
Depending on how you implement RAID, the benefits include one or both of the following: 
• Faster performance—In RAID 0, 10, 50, or 60 virtual disks, the host system can access 
simultaneously. This improves performance because each disk in a virtual disk has to handle 
the request. For example, in a two-disk virtual disk, each disk needs to provide only its 
requested data.  
• Data protection—In RAID 1, 10, 5, 6, 50, and 60 virtual disks, the data is backed up on disk 
(mirror). In the RAID 5, 50, 6, or 60 virtual disks, the data is also parity protected. RAID 10, 
50, and 60 also allow the host to access disks simultaneously. 
5.3 Supported RAID Levels 
Dell servers that use RAID controllers may support RAID 0, 1, 5, 6, 10, 50, and 60 depending upon the 
controller. The following is a brief explanation of these levels. 
5.3.1 RAID 0 (Striped Virtual Disk without Fault Tolerance) 
RAID 0, also known as striping, maps data across the physical drives to create a large virtual disk. 
The data is divided into consecutive segments or stripes that are written sequentially across the 
drives in the virtual disk. See Figure 4. Each stripe has a defined size or depth in blocks.  
  
						

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DELL PERC H700 and H800 Technical Guide   22 
For example, a four-drive virtual disk may be configured with 16 stripes (four stripes of designated 
space per drive). Stripes A, B, C and D are located on corresponding hard drives 0, 1, 2, and 3. Stripe 
E, however, appears on a segment of drive 0 in a different location than stripe A; stripes F through H 
appear accordingly on drives 1, 2 and 3. The remaining eight stripes are allocated in the same even 
fashion across the drives. 
 
RAID 0 provides improved performance because each drive in the virtual disk needs to handle only 
part of a read or write request. However, because none of the data is mirrored or backed up on 
parity drives, one drive failure makes the virtual disk inaccessible and the data is lost permanently. 
 
                        
 
 
 
 
 
 
 
 
 
 
Figure 4. Example of RAID 0 
Advantages of RAID 0 
 I/O performance is greatly improved by spreading the I/O load across many channels and 
drives (best performance is achieved when data is striped across multiple channels with only 
one drive per channel) 
 No parity calculation overhead is involved 
 Very simple design 
 Easy to implement  
Disadvantages of RAID 0 
 Not a true RAID because the failure of just one drive will result in all data in a virtual disk 
being lost 
 Should not be used for critical data unless another form of data redundancy is deployed 
5.3.2 RAID 1 (Mirroring) 
RAID 1 is achieved through disk mirroring to ensure data reliability or a high degree of fault 
tolerance. In a RAID 1 configuration, the RAID management software instructs the subsystems 
controller to store data redundantly across a number of the drives (mirrored set) in the virtual disk. 
See Figure 5.  
In other words, if a disk fails, the mirrored drive takes over and functions as the primary drive. 
 
Drive 0 Drive 1 Drive 2 Drive 3 
Data 1 
Data 16 
Data 2 Data 3 Data 4 
Data 5 Data 6 Data 7 Data 8 
Data 9 Data 10 Data 11 Data 12 
Data 13 Data 14 Data 15      
						

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DELL PERC H700 and H800 Technical Guide   23 
                                               
 
 
 
 
 
 
 
 
 
 
 
Figure 5. Example of RAID 1 (Mirroring) 
Advantages of RAID 1 
 High performance up to twice the read transaction rate of single disks, and the same write 
transaction rate as single disks  
 100 percent redundancy of data means no rebuild of data is necessary in case of disk failure, 
just a copy to the replacement disk 
 Typically supports hot-swap disks  
 Simplest RAID storage subsystem design 
 Fastest recovery of data after a drive failure, no data has to be recreated from parity codes 
on retrieval 
Disadvantages of RAID 1 
 Highest disk overhead of all RAID types (100 percent) results in inefficient use of drive 
capacity 
 Limited capacity since the virtual disk can only include two disk drives 
5.3.3 RAID 5 (Striping With Distributed Parity) 
RAID 5 maps the data across the drives and stores parity information for each data stripe on different 
drives in the virtual disk. Data redundancy is maintained with a technique called parity checking. 
With this technique, the RAID controller writes information called parity bits on the disks. Parity data 
is distributed across disks in the RAID 5 virtual disk such that any 1 disk failure within the virtual disk 
allows data to be recreated from the remaining disks. 
Parity is used to maintain data integrity and to rebuild lost data in case of drive failures. Parity bit 
data can be written on a single drive (this is RAID Level 3), but during periods of high write activity, 
the parity disk can become saturated with writes. This reduces the server’s write throughput. 
However, RAID Level 5 reduces parity write bottlenecks by allowing all of the drives in the virtual 
disk to assume part of the parity responsibilities. This alleviates the single drive bottleneck, 
improving overall subsystem throughput. Figure 6 shows how the parity data is distributed among five 
hard drives.  
A RAID 5 virtual disk can preserve data if one drive fails. However, if two drives fail, the virtual disk 
will fail. 
 
 
Drive 0 Drive 1 
= 
Data 1 Data 1 
Data 2 Data 2 
Data 3 Data 3 
Data 4 Data 4    
						

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DELL PERC H700 and H800 Technical Guide   24 
                                                             
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 6. Example of RAID 5 (Single Virtual Disk with 5 drives) 
Advantages of RAID 5 
 Most efficient use of drive capacity of all the redundant RAID configurations 
 High read transaction rate 
 Medium-to-high write transaction rate 
Disadvantages of RAID 5  
 Disk failure has a medium impact on throughput 
 Most complex controller design 
 Retrieval of parity information after a drive failure takes longer than with mirroring 
5.3.4 RAID 6 (Striping With Dual Distributed Parity) 
RAID 6 provides data redundancy by using data striping in combination with parity information. See 
Figure 7. Similar to RAID 5, the parity is distributed within each stripe. RAID 6, however, uses an 
additional physical disk to maintain parity, such that each stripe in the disk group maintains two disk 
blocks with parity information. The additional parity provides data protection in the event of two 
disk failures. 
Figure 7 depicts the RAID 6 data layout. The second set of parity drives are denoted by Q. The P 
drives follow the RAID 5 parity scheme. The parity blocks on Q drives are computed using Galois Field 
mathematics. There is no performance hit on read operations. However, as a second independent 
parity data needs to be generated for each write operation, there is a performance hit during write. 
Due to dual data protection, a RAID 6 VD can survive the loss of two drives or the loss of a drive when 
one of its drives is being rebuilt.  
 
 
 
Drive 1 Drive 2 Drive 3 Drive 4 Drive 5 
Parity  
Generation 1 parity 
0 parity 
4 parity 
3 parity 
2 parity 
Data 1 Data 2 Data 3 Data 4 
Data 5 Data 6 Data 7 Data 8 
Data 9 Data 10 Data 11 Data 12 
Data 13 Data 14 Data 16 Data 15 
Data 18 Data 20 Data 17 Data 19       
						

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DELL PERC H700 and H800 Technical Guide   25 
                                             
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 7. Example of RAID 6 (Single Virtual Disk with 5 drives) 
Advantages of RAID 6 
 Can survive the loss of two disks without losing data 
 Data redundancy, high read rates, and good performance 
Disadvantages of RAID 6  
 Requires two sets of parity data for each write operation, resulting in significant decrease in 
write performance 
 Additional costs because of the extra capacity required by using two parity blocks per stripe 
 Retrieval of parity information after a drive failure takes longer than with mirroring 
5.3.5 RAID 10 (Striping over Mirrored Sets) 
RAID 10 combines striping and mirroring to produce large virtual disks with high performance and 
fault-tolerance. The performance gain comes from striping across mirror sets without the need for 
parity calculations. See Figure 8. 
Although this delivers the highest performance, the drive storage overhead is 100 percent because 
the entire virtual disk is mirrored. This is an excellent solution for sites that require the highest level 
of performance and redundancy, as well as the fastest recovery of data after a drive failure. 
 
  
Drive 1 Drive 2 Drive 3 Drive 4 Drive 5 
Parity  
Generation Q parity 
Q parity 
Q parity 
Q parity 
Q parity 
Data 1 Data 2 Data 3 P parity 
Data 5 Data 6 P parity Data 8 
Data 9 P parity Data 11 Data 12 
P parity Data 14 Data 16 Data 15 
Data 18 P parity Data 17 Data 19       
						

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DELL PERC H700 and H800 Technical Guide   26 
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 8. Example of RAID 10 (1 + 0) 
Advantages of RAID 10 
 RAID 10 has the same redundancy as RAID level 1 
 High I/O rates are achieved by striping RAID 1 segments 
 Allows creation of largest RAID group with up to 192 drives connected to PERC H800 
Disadvantages of RAID 10 
 Most expensive RAID solution 
 Requires 2n where n > 1 disks 
5.3.6 RAID 50 (Striping Across RAID 5) 
RAID 50 is a variation of RAID 5 that maps data across two or more RAID 5 virtual disks. The RAID 5 
subset must have at least three disks. Figure 9 illustrates how the parity data is stored. RAID 50 
stripes data across each RAID 5 subset. RAID 50 provides a higher degree of fault tolerance since 1 
drive per RAID 5 set may fail without data being lost.  
A performance increase over RAID 5 may be realized depending on the configuration due to fewer 
disks reads per parity calculation.  
For example, if a comparison of a RAID 5 virtual disk with 6 disks were made to a RAID 50 virtual disk 
with two 3 disk RAID 5 virtual disks, the parity calculation on the RAID 10 virtual disk would require 
reading all 6 disks each time, where the parity calculation on the RAID 50 may require only reading 
3. This may vary depending on several factors such as cache and data block sizes. 
 
 
Drive 0 Drive 1 Drive 2 Drive 3 
Data 1 
= = 
RAID 1 RAID 1 
RAID 0 
Data 1 Data 2 Data 2 
Data 3 Data 3 Data 4 Data 4 
Data 5 Data 5 Data 6 Data 6 
Data 7 Data 7 Data 8 Data 8      
						

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DELL PERC H700 and H800 Technical Guide   27 
 
 
 
Figure 9. Example of RAID 50 (5 + 0) 
Advantages of RAID 50 
 Allows creation of largest RAID groups, up to 256 drives (theoretical; large RAID volumes are 
allowed for up to 192 drives connected to PERC H800) 
 High read transaction rate 
 Higher degree of fault tolerance due to parity calculation being done for each RAID 5 subset 
 Potential for faster read transaction rates over large RAID 5 virtual disks 
 Medium-to-high write transaction rate 
Disadvantages of RAID 50  
 Disk failure has a medium impact on throughput 
 One of the more complex RAID implementations 
 Less space efficient than RAID 5 since separate parity calculations are done for each RAID 5 
subset 
 Retrieval of parity information after a drive failure takes longer than using a mirrored solution 
5.3.7 RAID 60 (Striping Across RAID 6) 
RAID 60 is striping over more than one span of physical disks that are configured as a RAID 6. The 
RAID 6 subset must have at least four disks. For example, a RAID 6 disk group that is implemented 
with four physical disks and then continues on with a disk group of four more physical disks would be 
a RAID 60. See Figure 10. 
RAID 60 stripes data across each RAID 6 subset. RAID 60 provides a higher degree of fault tolerance 
since 2 drives per RAID 6 set may fail without data being lost. A performance increase over RAID 6 
may be realized depending on the configuration due to fewer disks reads per parity calculation.  
For example, if a comparison of a RAID 6 virtual disk with 8 disks were made to a RAID 60 virtual disk 
with two 4 disk RAID 6 virtual disks, the parity calculation on the RAID 10 virtual disk would require 
reading all 6 disks each time, where the parity calculation on the RAID 60 may require only reading 
4. This may vary depending on several factors such as cache and data block sizes. 
 
   
      
Drive 1 Drive 2 Drive 3 Drive 4 Drive 5 Drive 6 
  
    
   
   
   
   
   
 
  
 
 Parity  
Generation    
   
   Data 1 
   
   
   
     
   
    0 parity 0 parity 
 
2 parity 
 
 
1 parity  
 
 0 parity 0 parity 
  
   
   
    
2 parity 
 1 parity 
 
  
  
 
 Parity  
Generation 
 RAID 0 
 
RAID 5 
 
RAID 5   
Data 2 Data 3 Data 4 
Data 5 Data 6 Data 7 Data 8 
Data9 Data 10 Data 11 Data 12        
						

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Figure 10. Example of RAID 60 (6 + 0) 
Advantages of RAID 60 
 Allows creation of largest RAID groups, up to 256 drives (theoretical); large RAID volumes are 
allowed for up to 192 drives connected to PERC H800 
 High degree of fault tolerance due to 2 parity calculations being done for each RAID 6 subset 
 Medium-to-high write transaction rate 
Disadvantages of RAID 60  
 One of the more complex RAID implementations 
 Less space-efficient than RAID 6 since separate parity calculations are done for each RAID 6 
subset 
 Retrieval of parity information after a drive failure takes longer than using a mirrored solution 
 
 
 
 
 
 
 
 
   
   
   
      
 
      
   
   
   
   
   
      
    
Drive 1 Drive 2 Drive 3 Drive 5 Drive 6 Drive 7 
  
    
   
   
   
   
   
 
  
 
 Parity  
Generation    
   
    Data 1 
   
   
       Q parity 
 
P parity 
 
P parity 
 
   
   
   
P parity 
P parity 
 
  
  
 
 Parity  
Generation 
 RAID 0 
RAID 6 
 
RAID 6   
   
Q parity 
Q parity 
P parity 
Q parity 
P parity 
Drive 4 
Q parity 
   P parity 
Q parity 
Q parity 
Q parity 
P parity 
Drive 8 
Data 2 Data 3 Data 4 
Data 5 Data 6 Data 8 
Data 10 
Data 7 
Data 15 
Data 9 
Data 16 
Data 11 Data 12 
Data 13 Data 14          
						

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DELL PERC H700 and H800 Technical Guide   29 
Appendix A. Additional Resources 
Table 9. Resource Contact Information and Descriptions 
Type Of Information URL Description 
PERC Resources http://www.dell.com/PERC   
 
Support matrix, whitepapers, 
and important links 
 
PERC Hardware 
Owner’s Manual 
http://support.dell.com/manuals Specifications, feature 
overview and descriptions, 
installation, management, and 
troubleshooting 
 
OpenManage User 
Documentation 
http://www.support.dell.com/ 
manuals  
User Guide for OMSS questions 
and support 
Break/Fix 
First contact for 
product issues.  
www.support.dell.com (Customer 
tab) 
Troubleshooting issues