<s>
A	O
hot	B-General_Concept
spare	I-General_Concept
or	O
warm	O
spare	O
or	O
hot	B-General_Concept
standby	I-General_Concept
is	O
used	O
as	O
a	O
failover	B-General_Concept
mechanism	O
to	O
provide	O
reliability	O
in	O
system	O
configurations	O
.	O
</s>
<s>
The	O
hot	B-General_Concept
spare	I-General_Concept
is	O
active	O
and	O
connected	O
as	O
part	O
of	O
a	O
working	O
system	O
.	O
</s>
<s>
When	O
a	O
key	O
component	O
fails	O
,	O
the	O
hot	B-General_Concept
spare	I-General_Concept
is	O
switched	O
into	O
operation	O
.	O
</s>
<s>
More	O
generally	O
,	O
a	O
hot	B-General_Concept
standby	I-General_Concept
can	O
be	O
used	O
to	O
refer	O
to	O
any	O
device	O
or	O
system	O
that	O
is	O
held	O
in	O
readiness	O
to	O
overcome	O
an	O
otherwise	O
significant	O
start-up	O
delay	O
.	O
</s>
<s>
Examples	O
of	O
hot	B-General_Concept
spares	I-General_Concept
are	O
components	O
such	O
as	O
A/V	O
switches	O
,	O
computers	O
,	O
network	O
printers	O
,	O
and	O
hard	B-Device
disks	I-Device
.	O
</s>
<s>
Electrical	O
generators	O
may	O
be	O
held	O
on	O
hot	B-General_Concept
standby	I-General_Concept
,	O
or	O
a	O
steam	O
train	O
may	O
be	O
held	O
at	O
the	O
shed	O
fired	O
up	O
(	O
literally	O
hot	O
)	O
ready	O
to	O
replace	O
a	O
possible	O
failure	O
of	O
an	O
engine	O
in	O
service	O
.	O
</s>
<s>
More	O
often	O
,	O
a	O
hot	B-General_Concept
spare	I-General_Concept
is	O
a	O
single	O
vital	O
component	O
without	O
which	O
the	O
entire	O
system	O
would	O
fail	O
.	O
</s>
<s>
A	O
hot	B-General_Concept
spare	I-General_Concept
does	O
not	O
necessarily	O
give	O
100%	O
availability	O
or	O
protect	O
against	O
temporary	O
loss	O
of	O
the	O
system	O
during	O
the	O
switching	O
process	O
;	O
it	O
is	O
designed	O
to	O
significantly	O
reduce	O
the	O
time	O
that	O
the	O
system	O
is	O
unavailable	O
.	O
</s>
<s>
Hot	B-General_Concept
standby	I-General_Concept
may	O
have	O
a	O
slightly	O
different	O
connotation	O
of	O
being	O
active	O
but	O
not	O
productive	O
to	O
hot	B-General_Concept
spare	I-General_Concept
,	O
that	O
is	O
it	O
is	O
a	O
state	O
rather	O
than	O
object	O
.	O
</s>
<s>
Being	O
in	O
the	O
state	O
of	O
being	O
ready	O
to	O
run	O
is	O
known	O
as	O
hot	B-General_Concept
standby	I-General_Concept
.	O
</s>
<s>
In	O
high	O
availability	O
systems	O
,	O
it	O
is	O
common	O
to	O
design	O
so	O
that	O
not	O
only	O
is	O
there	O
a	O
spare	O
that	O
can	O
quickly	O
be	O
switched	O
in	O
,	O
but	O
also	O
that	O
the	O
failed	O
component	O
can	O
be	O
repaired	O
or	O
replaced	O
without	O
stopping	O
the	O
system	O
-	O
this	O
is	O
known	O
as	O
hot	B-Device
swapping	I-Device
.	O
</s>
<s>
there	O
are	O
two	O
types	O
of	O
hot	B-General_Concept
standby	I-General_Concept
:	O
</s>
<s>
A	O
hot	B-General_Concept
spare	I-General_Concept
disk	I-General_Concept
is	O
a	O
disk	O
or	O
group	O
of	O
disks	O
used	O
to	O
automatically	O
or	O
manually	O
,	O
depending	O
upon	O
the	O
hot	B-General_Concept
spare	I-General_Concept
policy	O
,	O
replace	O
a	O
failing	O
or	O
failed	O
disk	O
in	O
a	O
RAID	B-Architecture
configuration	O
.	O
</s>
<s>
The	O
hot	B-General_Concept
spare	I-General_Concept
disk	I-General_Concept
reduces	O
the	O
mean	O
time	O
to	O
recovery	O
(	O
MTTR	O
)	O
for	O
the	O
RAID	B-Architecture
redundancy	O
group	O
,	O
thus	O
reducing	O
the	O
probability	O
of	O
a	O
second	O
disk	O
failure	O
and	O
the	O
resultant	O
data	O
loss	O
that	O
would	O
occur	O
in	O
any	O
singly	O
redundant	O
RAID	B-Architecture
(	O
e.g.	O
,	O
RAID-1	O
,	O
RAID-5	O
,	O
RAID-10	O
)	O
.	O
</s>
<s>
Typically	O
,	O
a	O
hot	B-General_Concept
spare	I-General_Concept
is	O
available	O
to	O
replace	O
a	O
number	O
of	O
different	O
disks	O
and	O
systems	O
employing	O
a	O
hot	B-General_Concept
spare	I-General_Concept
normally	O
require	O
a	O
redundant	O
group	O
to	O
allow	O
time	O
for	O
the	O
data	O
to	O
be	O
generated	O
onto	O
the	O
spare	O
disk	O
.	O
</s>
<s>
The	O
concept	O
of	O
hot	B-General_Concept
spares	I-General_Concept
is	O
not	O
limited	O
to	O
hardware	O
,	O
but	O
also	O
software	O
systems	O
can	O
be	O
held	O
in	O
a	O
state	O
of	O
readiness	O
,	O
for	O
example	O
a	O
database	O
server	O
may	O
have	O
a	O
software	O
copy	O
on	O
hot	B-General_Concept
standby	I-General_Concept
,	O
possibly	O
even	O
on	O
the	O
same	O
machine	O
to	O
cope	O
with	O
the	O
various	O
factors	O
that	O
make	O
a	O
database	O
unreliable	O
,	O
such	O
as	O
the	O
impact	O
of	O
disc	O
failure	O
,	O
poorly	O
written	O
queries	O
or	O
database	O
software	O
errors	O
.	O
</s>
<s>
The	O
other	O
unit	O
is	O
a	O
hot-standby	O
or	O
a	O
hot	B-General_Concept
spare	I-General_Concept
unit	O
,	O
ready	O
to	O
take	O
over	O
if	O
the	O
master	O
unit	O
fails	O
.	O
</s>
<s>
When	O
the	O
master	O
unit	O
fails	O
,	O
an	O
automatic	O
failover	B-General_Concept
to	O
the	O
hot	B-General_Concept
spare	I-General_Concept
occurs	O
within	O
a	O
very	O
short	O
time	O
and	O
the	O
outputs	O
from	O
the	O
hot	B-General_Concept
spare	I-General_Concept
,	O
now	O
the	O
master	O
unit	O
,	O
are	O
delivered	O
to	O
the	O
controlled	O
devices	O
and	O
displays	O
.	O
</s>
<s>
The	O
controlled	O
devices	O
and	O
displays	O
may	O
experience	O
a	O
short	O
blip	O
or	O
disturbance	O
during	O
the	O
failover	B-General_Concept
time	O
.	O
</s>
