<s>
A	O
Redundant	B-Application
Array	I-Application
of	I-Application
Inexpensive	I-Application
Servers	I-Application
(	O
RAIS	O
)	O
or	O
Redundant	B-Application
Array	I-Application
of	I-Application
Independent	I-Application
Nodes	I-Application
(	O
RAIN	O
)	O
is	O
the	O
use	O
of	O
multiple	O
servers	B-Application
to	O
maintain	O
service	O
if	O
one	O
server	O
fails	O
.	O
</s>
<s>
This	O
is	O
similar	O
in	O
concept	O
to	O
how	O
RAID	B-Architecture
turns	O
a	O
cluster	O
of	O
ordinary	O
disks	O
into	O
a	O
single	O
block	O
device	O
.	O
</s>
<s>
RAIS	O
was	O
designed	O
to	O
provide	O
the	O
benefits	O
of	O
a	O
symmetric	B-Operating_System
multiprocessor	I-Operating_System
system	I-Operating_System
(	O
SMP	O
)	O
at	O
the	O
entry	O
cost	O
of	O
computer	B-Architecture
clusters	I-Architecture
.	O
</s>
<s>
The	O
term	O
may	O
imply	O
some	O
kind	O
of	O
load	B-Application
balancing	I-Application
between	O
the	O
servers	B-Application
.	O
</s>
<s>
It	O
turns	O
an	O
array	O
of	O
ordinary	O
servers	B-Application
into	O
a	O
single	O
virtual	O
machine	O
,	O
similar	O
in	O
concept	O
to	O
how	O
RAID	B-Architecture
turns	O
a	O
cluster	O
of	O
ordinary	O
disks	O
into	O
a	O
single	O
block	O
device	O
.	O
</s>
<s>
RAIS	O
stripes	O
and	O
mirrors	O
application	O
code	O
and	O
memory	O
across	O
an	O
array	O
of	O
ordinary	O
servers	B-Application
using	O
the	O
standard	O
RAID	B-Architecture
schemata	O
of	O
level	O
0	O
,	O
level	O
1	O
,	O
level	O
5	O
,	O
level	O
1+0	O
.	O
</s>
<s>
Data	O
blocks	O
of	O
each	O
stream	O
are	O
striped	O
across	O
the	O
array	O
servers	B-Application
.	O
</s>
<s>
The	O
server	O
capacity	O
increases	O
with	O
the	O
number	O
of	O
servers	B-Application
.	O
</s>
<s>
In	O
a	O
manner	O
more	O
usually	O
associated	O
with	O
high	O
cost	O
SMP	O
architectures	O
,	O
RAIS	O
achieves	O
this	O
by	O
turning	O
a	O
cluster	O
of	O
independent	O
servers	B-Application
into	O
a	O
single	O
large	O
server	O
running	O
applications	O
across	O
a	O
virtualised	O
network	O
of	O
nodes	O
.	O
</s>
<s>
The	O
concept	O
is	O
similar	O
to	O
how	O
RAID	B-Architecture
stripes	O
and	O
mirrors	O
data	O
across	O
multiple	O
independent	O
disks	O
and	O
code	O
of	O
an	O
application	O
program	O
across	O
multiple	O
independent	O
nodes	O
of	O
a	O
cluster	O
.	O
</s>
