<s>
When	O
a	O
RAID	B-Architecture
array	I-Architecture
experiences	O
the	O
failure	O
of	O
one	O
or	O
more	O
disks	O
,	O
it	O
can	O
enter	O
degraded	B-General_Concept
mode	I-General_Concept
,	O
a	O
fallback	O
mode	O
that	O
generally	O
allows	O
the	O
continued	O
usage	O
of	O
the	O
array	O
,	O
but	O
either	O
loses	O
the	O
performance	O
boosts	O
of	O
the	O
RAID	B-Architecture
technique	O
(	O
such	O
as	O
a	O
RAID-1	O
mirror	O
across	O
two	O
disks	O
when	O
one	O
of	O
them	O
fails	O
;	O
performance	O
will	O
fall	O
back	O
to	O
that	O
of	O
a	O
normal	O
,	O
single	O
drive	O
)	O
or	O
experiences	O
severe	O
performance	O
penalties	O
due	O
to	O
the	O
necessity	O
to	O
reconstruct	O
the	O
damaged	O
data	O
from	O
error	B-Error_Name
correction	I-Error_Name
data	O
.	O
</s>
<s>
Depending	O
on	O
the	O
severity	O
of	O
the	O
problem	O
,	O
the	O
array	O
may	O
be	O
placed	O
into	O
a	O
read-only	O
mode	O
,	O
either	O
automatically	O
or	O
by	O
the	O
system	B-Application
administrator	I-Application
,	O
until	O
it	O
can	O
be	O
corrected	O
.	O
</s>
<s>
Many	O
RAID	B-Architecture
configurations	O
feature	O
spare	O
disks	O
that	O
are	O
already	O
installed	O
and	O
can	O
be	O
automatically	O
added	O
to	O
the	O
array	O
as	O
needed	O
;	O
when	O
this	O
happens	O
,	O
the	O
array	O
may	O
or	O
may	O
not	O
go	O
into	O
degraded	B-General_Concept
mode	I-General_Concept
until	O
the	O
spare	O
is	O
rebuilt	O
,	O
but	O
in	O
any	O
case	O
should	O
not	O
be	O
in	O
degraded	B-General_Concept
mode	I-General_Concept
after	O
the	O
spare	O
is	O
rebuilt	O
.	O
</s>
<s>
If	O
no	O
spares	O
are	O
available	O
,	O
the	O
array	O
will	O
remain	O
in	O
degraded	B-General_Concept
mode	I-General_Concept
until	O
a	O
spare	O
disk	O
is	O
added	O
,	O
or	O
the	O
array	O
is	O
reconfigured	O
(	O
if	O
that	O
is	O
possible	O
for	O
the	O
configuration	O
in	O
question	O
)	O
.	O
</s>
<s>
A	O
typical	O
case	O
where	O
a	O
RAID	B-Architecture
enters	O
degraded	B-General_Concept
mode	I-General_Concept
is	O
a	O
simple	O
two-drive	O
mirror	O
after	O
a	O
power	O
failure	O
–	O
it	O
is	O
unlikely	O
the	O
drives	O
are	O
in	O
sync	O
.	O
</s>
<s>
The	O
RAID	B-Architecture
controller	O
will	O
notice	O
that	O
the	O
storage	O
elements	O
are	O
not	O
in	O
sync	O
,	O
will	O
place	O
the	O
array	O
in	O
degraded	B-General_Concept
mode	I-General_Concept
,	O
and	O
–	O
generally	O
–	O
will	O
start	O
a	O
background	O
resync	O
(	O
rebuild	O
)	O
operation	O
.	O
</s>
