<s>
In	O
computing	O
(	O
specifically	O
data	O
transmission	O
and	O
data	B-General_Concept
storage	I-General_Concept
)	O
,	O
a	O
block	O
,	O
sometimes	O
called	O
a	O
physical	O
record	O
,	O
is	O
a	O
sequence	O
of	O
bytes	B-Application
or	O
bits	O
,	O
usually	O
containing	O
some	O
whole	O
number	O
of	O
records	O
,	O
having	O
a	O
maximum	O
length	O
;	O
a	O
block	B-General_Concept
size	I-General_Concept
.	O
</s>
<s>
Data	O
thus	O
structured	B-General_Concept
are	O
said	O
to	O
be	O
blocked	O
.	O
</s>
<s>
Blocked	O
data	O
is	O
normally	O
stored	O
in	O
a	O
data	B-General_Concept
buffer	I-General_Concept
,	O
and	O
read	O
or	O
written	O
a	O
whole	O
block	O
at	O
a	O
time	O
.	O
</s>
<s>
Blocking	O
reduces	O
the	O
overhead	O
and	O
speeds	O
up	O
the	O
handling	O
of	O
the	O
data	B-General_Concept
stream	I-General_Concept
.	O
</s>
<s>
For	O
some	O
devices	O
,	O
such	O
as	O
magnetic	O
tape	O
and	O
CKD	B-Application
disk	I-Application
devices	O
,	O
blocking	O
reduces	O
the	O
amount	O
of	O
external	O
storage	O
required	O
for	O
the	O
data	O
.	O
</s>
<s>
Blocking	O
is	O
almost	O
universally	O
employed	O
when	O
storing	O
data	O
to	O
9-track	B-Device
magnetic	O
tape	O
,	O
NAND	O
flash	B-Device
memory	I-Device
,	O
and	O
rotating	O
media	O
such	O
as	O
floppy	B-Device
disks	I-Device
,	O
hard	B-Device
disks	I-Device
,	O
and	O
optical	B-Device
discs	I-Device
.	O
</s>
<s>
Most	O
file	B-Application
systems	I-Application
are	O
based	O
on	O
a	O
block	O
device	O
,	O
which	O
is	O
a	O
level	O
of	O
abstraction	B-Application
for	O
the	O
hardware	O
responsible	O
for	O
storing	O
and	O
retrieving	O
specified	O
blocks	O
of	O
data	O
,	O
though	O
the	O
block	B-General_Concept
size	I-General_Concept
in	O
file	B-Application
systems	I-Application
may	O
be	O
a	O
multiple	O
of	O
the	O
physical	O
block	B-General_Concept
size	I-General_Concept
.	O
</s>
<s>
This	O
leads	O
to	O
space	O
inefficiency	O
due	O
to	O
internal	O
fragmentation	O
,	O
since	O
file	O
lengths	O
are	O
often	O
not	O
integer	O
multiples	O
of	O
block	B-General_Concept
size	I-General_Concept
,	O
and	O
thus	O
the	O
last	O
block	O
of	O
a	O
file	O
may	O
remain	O
partially	O
empty	O
.	O
</s>
<s>
Some	O
newer	O
file	B-Application
systems	I-Application
,	O
such	O
as	O
Btrfs	B-Operating_System
and	O
FreeBSD	B-Operating_System
UFS2	B-Operating_System
,	O
attempt	O
to	O
solve	O
this	O
through	O
techniques	O
called	O
block	B-Application
suballocation	I-Application
and	I-Application
tail	I-Application
merging	I-Application
.	O
</s>
<s>
Other	O
file	B-Application
systems	I-Application
such	O
as	O
ZFS	B-Application
support	O
variable	O
block	B-General_Concept
sizes	I-General_Concept
.	O
</s>
<s>
Block	B-General_Concept
storage	I-General_Concept
is	O
normally	O
abstracted	O
by	O
a	O
file	B-Application
system	I-Application
or	O
database	B-Application
management	I-Application
system	I-Application
(	O
DBMS	O
)	O
for	O
use	O
by	O
applications	O
and	O
end	O
users	O
.	O
</s>
<s>
The	O
physical	O
or	O
logical	O
volumes	O
accessed	O
via	O
block	B-General_Concept
I/O	I-General_Concept
may	O
be	O
devices	O
internal	O
to	O
a	O
server	O
,	O
directly	O
attached	O
via	O
SCSI	B-Architecture
or	O
Fibre	B-Architecture
Channel	I-Architecture
,	O
or	O
distant	O
devices	O
accessed	O
via	O
a	O
storage	O
area	O
network	O
(	O
SAN	O
)	O
using	O
a	O
protocol	O
such	O
as	O
iSCSI	B-Protocol
,	O
or	O
AoE	B-Protocol
.	O
</s>
<s>
DBMSes	O
often	O
use	O
their	O
own	O
block	B-General_Concept
I/O	I-General_Concept
for	O
improved	O
performance	O
and	O
recoverability	O
as	O
compared	O
to	O
layering	O
the	O
DBMS	O
on	O
top	O
of	O
a	O
file	B-Application
system	I-Application
.	O
</s>
