7.
RAID 5
•RAID 5 is organized in a similar fashion to RAID
4.The difference is that RAID 5
•distributes the parity strips across all disks.
•RAID 5 comprises block-level striping with
distributed parity. It requires that all drives but one
be present to operate. Upon failure of a single
drive, subsequent reads can
•be calculated from the distributed parity such that
no data is lost. RAID 5 requires at least three
disks.
8.
Following are the key points to remember for RAID
level 5.
Minimum 3 disks.
Good performance ( as blocks are striped ).
Good redundancy ( distributed parity ).
9.
Working
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First we need to remind you XOR definition:
XOR function result is equal 1 if both arguments are different.
XOR (0, 1) = 1
XOR (1, 0) = 1
XOR function output is equal 0 if both arguments are same.
XOR (0, 0) = 0
XOR (1, 1) = 0
Now let us assume we have 3 drives with the following bits:
| 101 | 010 | 011 |
And we calculate XOR of those data and place it on 4th drive
XOR (101, 010, 011) = 100 (XOR (101,010) = 111 and then XOR (111, 011) = 100
So the data on the four drives looks like this below:
| 101 | 010 | 011 | 100 |
Now let’s see how the XOR MAGIC works. Let’s assume the second drive has failed. When we
calculate XOR all the remaining data will be present from the missing drive.
| 101 | 010 | 011 | 100 |
XOR (101, 011, 100) = 010
You can check the missing other drives and XOR of the remaining data will always give you
exactly the data of your missing drive.
| 101 | 010 | 011 | 100 |
XOR (101, 010, 100) = 011
What works for 3 bits and 4 drives only, works for any number of bits and any number of drives.
Real RAID 5 has the most common stripe size of 64k (65536 * 8 = 524288 bits )
10.
•So the real XOR engine only needs to deal with
524288 bits and not 3 bits as in our exercise.
This is why the RAID 5 needs a very efficient
XOR engine in order to calculate it fast.
•So when adding one drive for parity you will be
able to rebuild the missing data in case of any
drive failure.
11.
Advantages
• Cost effective - only 1 extra disk is required.
• Read data transactions are very fast while write data
transaction are somewhat slower (due to the parity that
has to be calculated).
Disadvantages
• Individual block data transfer rate same as a single disk.
• Like RAID 3, this is complex technology.
13.
RAID 6
Very similar to RAID 5, but adds an additional
parity block.
Raid 6 creates two parity blocks for each data
block.
Can handle two disk failure
This RAID configuration is complex to
implement
in a RAID controller, as it has to
calculate two parity data for each data block
14.
RAID 6
Raid 6 is of extremely high help in systems having a very high
capacity and when the rebuilding of a RAID 5 volume takes a long
time or when there is significant chance of failure of another drive
before the rebuild process is completed.
RAID 6 increases the data reliability by using the two parity
stripes. These stripes allow for the failure of the two disks which
can occur within the RAID disk drive set. Major application of
RAID 6 is in SATA, IDE and the SCSI environments.
15.
Independent Data Disks with Two Independent
Parity Schemes
16.
Advantages of RAID 6
•Raid 6 is highly reliable compared to the other raid
levels.
•It is the most suited raid level for small as well as
moderate size database systems.
•Raid 6 provides protection against multiple bad
block failures while non-degraded.
•It allows for the failure of two disks simultaneously
with no data loss.
17.
Disadvantages of RAID 6
•The raid 6 controller design is very complex
compared to the other level controllers.
•The overhead of the controller for computing
parity addresses is high.
•RAID 6 needs N+2 drives in order to implement,
the reason being the dual parity scheme.
•Write performance can be the same as that for
RAID 5.
•Two drives are used for parity calculations.
18.
Applications recommended for RAID 6
•Web and email servers.
•Database servers.
•Intranet servers.
•Excellent fault tolerance.
•File and application servers.
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