<s>
Block-level	B-General_Concept
storage	I-General_Concept
is	O
a	O
concept	O
in	O
cloud-hosted	B-Architecture
data	B-Application
persistence	I-Application
where	O
cloud	B-Architecture
services	I-Architecture
emulate	O
the	O
behaviour	O
of	O
a	O
traditional	O
block	B-General_Concept
device	O
,	O
such	O
as	O
a	O
physical	O
hard	B-Device
drive	I-Device
.	O
</s>
<s>
This	O
emulates	O
the	O
type	O
of	O
behaviour	O
seen	O
in	O
traditional	O
disks	O
or	O
tape	B-Architecture
storage	I-Architecture
through	O
storage	B-Application
virtualization	I-Application
.	O
</s>
<s>
A	O
file	B-Application
system	I-Application
must	O
be	O
applied	O
on	O
top	O
of	O
the	O
block-level	B-General_Concept
storage	I-General_Concept
to	O
map	O
'	O
files	O
 '	O
onto	O
a	O
sequence	O
of	O
blocks	O
.	O
</s>
<s>
Amazon	O
EBS	O
(	O
elastic	O
block	B-General_Concept
store	O
)	O
is	O
an	O
example	O
of	O
a	O
cloud	O
block	B-General_Concept
store	O
.	O
</s>
<s>
Cloud	O
block-level	B-General_Concept
storage	I-General_Concept
will	O
usually	O
offer	O
facilities	O
such	O
as	O
replication	O
for	O
reliability	O
,	O
or	O
backup	O
services	O
.	O
</s>
<s>
Block-level	B-General_Concept
storage	I-General_Concept
is	O
in	O
contrast	O
to	O
an	O
object	B-Application
store	I-Application
or	O
'	O
bucket	O
store	O
 '	O
,	O
such	O
as	O
Amazon	B-General_Concept
S3	I-General_Concept
(	O
simple	B-General_Concept
storage	I-General_Concept
service	I-General_Concept
)	O
,	O
or	O
to	O
a	O
database	O
.	O
</s>
<s>
Instance	O
stores	O
are	O
another	O
form	O
of	O
cloud-hosted	B-Architecture
block-level	B-General_Concept
storage	I-General_Concept
.	O
</s>
<s>
These	O
are	O
provided	O
as	O
part	O
of	O
an	O
'	O
instance	O
 '	O
,	O
such	O
as	O
an	O
Amazon	B-General_Concept
EC2	I-General_Concept
(	O
elastic	B-General_Concept
compute	I-General_Concept
cloud	I-General_Concept
)	O
service	O
.	O
</s>
<s>
As	O
these	O
stores	O
are	O
part	O
of	O
the	O
instance	O
's	O
virtual	B-Architecture
server	I-Architecture
they	O
offer	O
higher	O
performance	O
and	O
bandwidth	O
to	O
the	O
instance	O
.	O
</s>
<s>
At	O
one	O
time	O
,	O
block-level	B-General_Concept
storage	I-General_Concept
was	O
provided	O
by	O
storage	O
area	O
networks	O
(	O
SAN	O
)	O
and	O
NAS	O
provided	O
file-level	O
storage	O
.	O
</s>
<s>
With	O
the	O
shift	O
from	O
on-premises	O
hosting	O
to	O
cloud	B-Architecture
services	I-Architecture
,	O
this	O
distinction	O
has	O
shifted	O
.	O
</s>
<s>
Even	O
block-storage	O
is	O
now	O
seen	O
as	O
distinct	O
servers	O
(	O
thus	O
NAS	O
)	O
,	O
rather	O
than	O
the	O
previous	O
array	O
of	O
bare	O
discs	O
.	O
</s>
