184
Networking: A Beginner's Guide
TIP
You can combine RAID levels 0 and 1 to achieve the performance benefit of RAID 0 with
the high level of redundancy of RAID 1. Imagine a series of RAID 1 arrays with two disks each.
Combine each of these RAID 1 arrays so that data is striped across them, and you have what
is called a RAID 10 array (with 10 referring to a combination of RAID 1 and RAID 0). This is
sometimes also called RAID 0 + 1 or RAID 1 + 0.
RAID 2
You probably won't see RAID 2 implemented in the real world.
RAID 2 is a technical specification that stripes data across multiple disks and
then uses a Hamming Code ECC that is written to a set of ECC disks. The ratio
of ECC disks to data disks is quite high with RAID 2: There are three ECC disks
for every four data disks. RAID 2 isn't used because of its inefficiencies.
RAID 3
This is where RAID starts to get interesting. RAID 3 implementations
used to be fairly common, although these days you see RAID 5 used much
more often than RAID 3. RAID 3 stripes data across multiple data disks and
then uses an exclusive OR (XOR) bit-wise operation against all the stored
data on each data disk to come up with ECC data, which is written to a
single ECC drive. So, for example, you can have four data drives and one
ECC drive to back them up. Figure 13-3 shows a RAID 3 array. The XOR
data has an interesting mathematical property. If you remove one of the data
drives, you can take the remaining data, plus the data on the ECC drive, and
reconstruct what is missing from the failed drive. RAID disk controllers do this
automatically if a drive fails, although the drives operate at a slower rate than
normal because of the overhead of having to reconstruct the data on the fly. A
more useful technique is to replace the failed drive and then use the ECC data
to rebuild the lost data.
Figure 13-2.
A RAID 1 array mirrors data between two disks
File
Disk 1
Disk 2
Summary :
184 Networking: A Beginner's Guide TIP You can combine RAID levels 0 and 1 to achieve the performance benefit of RAID 0 with the high level of redundancy of RAID 1. Combine each of these RAID 1 arrays so that data is striped across them, and you have what is called a RAID 10 array (with 10 referring to a combination of RAID 1 and RAID 0).
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