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is a data redundancy technique
used by some RAID levels, in particular ,
to provide data protection on a RAID array. While mirroring has some advantages and is
well-suited for certain RAID implementations, it also has some limitations. It has a high
overhead cost, because fully 50% of the drives in the array are reserved for duplicate
and it doesn't improve performance as much as data striping does for many
applications. For this reason, a different way of protecting data is provided as an
alternate to mirroring. It involves the use of parity information, which is
redundancy information calculated from the actual data values.
You may have heard the term &parity& before, use in fact, the
parity used in RAID is very similar in concept to parity RAM. The principle behind parity
is simple: take &N& pieces of data, and from them, compute an extra piece of
data. Take the &N+1& pieces of data and store them on &N+1& drives. If
you lose any one of the &N+1& pieces of data, you can recreate it from
the &N& that remain, regardless of which piece is lost. Parity protection is
used with , and the &N&
pieces of data are typically the blocks or bytes distributed across the drives in the
array. The parity information can either be stored on a separate, dedicated drive, or be
mixed with the data across all the drives in the array.
The parity calculation is typically performed using a logical operation called
&exclusive OR& or &XOR&. As you may know, the &OR& logical
operator is &true& (1) if either of its operands is true, and false (0) if
neither is true. The exclusive OR operator is &true& if and only if one
it differs from &OR& in that if both operands are true,
&XOR& is false. This truth table for the two operators will illustrate:
Uh huh. So what, right? Well, the interesting thing about &XOR& is that it is
a logical operation that if performed twice in a row, &undoes itself&. If you
calculate &A XOR B& and then take that result and do another &XOR B&
on it, you get back A, the value you started with. That is to say, &A XOR B XOR B =
A&. This property is exploited for parity calculation under RAID. If we have four
data elements, D1, D2, D3 and D4, we can calculate the parity data, &DP& as
&D1 XOR D2 XOR D3 XOR D4&. Then, if we know any four of D1, D2, D3, D4 and DP,
we can XOR those four together and it will yield the missing element.
Let's take an example to you can do this yourself easily on a
sheet of paper. Suppose we have the following four bytes of data: D1=,
D2=, D3=, and D4=. We can &XOR& them together as
follows, one step at a time:
D1 XOR D2 XOR D3 XOR D4
= ( (D1 XOR D2) XOR D3) XOR D4
= ( ( XOR ) XOR ) XOR
= (.XOR ) XOR
So && becomes the parity byte, DP. Now let's say we store these five
values on five hard disks, and hard disk #3, containing value &&, goes
el-muncho. We can retrieve the missing byte simply by XOR'ing together the other three
original data pieces, and the parity byte we calculated earlier, as so:
D1 XOR D2 XOR D4 XOR DP
= ( (D1 XOR D2) XOR D4) XOR DP
= ( ( XOR ) XOR ) XOR
= ( XOR ) XOR
Which is D3, the missing value. Pretty neat, huh? :^) This operation can be done on any
number of bits, I just used eight bits for simplicity. It's also a very
simple binary calculation--which is a good thing, because it has to be done for every bit
stored in a parity-enabled RAID array.
Compared to mirroring, parity (used with striping) has some advantages and
disadvantages. The most obvious advantage is that parity protects data against any single
drive in the array failing without requiring the 50% &waste& only
one of the &N+1& drives contains redundancy information. (The overhead of parity
is equal to (100/N)% where N is the total number of drives in the array.) Striping with
parity also allows you to take advantage of the performance advantages of . The chief disadvantages of striping
with parity relate to complexity: all those parity bytes have to be computed--millions of
them per second!--and that takes computing power. This means a
that performs these calculations is required for high performance--if you
do software RAID with striping and parity the system CPU will be dragged down
doing all these computations. Also, while you can recover from a lost drive under parity,
the missing data all has to be rebuilt, which has i recovering from a
lost mirrored drive is comparatively simple.
All of the RAID levels from RAID 3 to RAID 7 the most popular of these
today is . RAID 2 uses a concept similar
to parity but not exactly the same.
The PC Guide
Site Version: 2.2.0 - Version Date: April 17, 2001 Charles M. Kozierok.&All Rights
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Custom Search英语词根---par, peer   【希腊】=equal,表示“平等”
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英语词根---par, peer   【希腊】=equal,表示“平等”
par, peer & 【希腊】=equal,表示“平等”
同根词 & comparison | peer | pair | comparative | peerage | par | parity |
comparison
com一起+par相等=放在一起，看是否相等=相比
释义： n. 比较
释义： n. 比较
释义： vi. 凝视，盯着看；窥视 vt. 封为贵族；与…同等 n. 贵族；同等的人
释义： n. 双,对,副 vt.& vi. 使...成对,配对
comparative
com一起+par相等=放在一起，看是否相等=相比
释义： adj. 比较的,相当的 n. 对手
释义： n. 贵族, 贵族阶级, 贵族地位
释义： n. 贵族, 贵族阶级, 贵族地位
释义： n. 标准, 票面价值, 平均数量 adj. 票面的, 平常的, 标准的
释义： n. 相等,势均力敌,等值
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