<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
storage	B-Application
virtualization	I-Application
is	O
"	O
the	O
process	O
of	O
presenting	O
a	O
logical	O
view	O
of	O
the	O
physical	B-Device
storage	I-Device
resources	O
to	O
"	O
a	O
host	O
computer	O
system	O
,	O
"	O
treating	O
all	O
storage	B-General_Concept
media	O
(	O
hard	B-Device
disk	I-Device
,	O
optical	O
disk	B-Device
,	O
tape	O
,	O
etc	O
.	O
)	O
</s>
<s>
in	O
the	O
enterprise	O
as	O
a	O
single	O
pool	O
of	O
storage.	O
"	O
</s>
<s>
A	O
"	O
storage	B-General_Concept
system	I-General_Concept
"	O
is	O
also	O
known	O
as	O
a	O
storage	B-General_Concept
array	I-General_Concept
,	O
disk	B-General_Concept
array	I-General_Concept
,	O
or	O
filer	O
.	O
</s>
<s>
Storage	B-General_Concept
systems	I-General_Concept
typically	O
use	O
special	O
hardware	O
and	O
software	O
along	O
with	O
disk	B-Device
drives	I-Device
in	O
order	O
to	O
provide	O
very	O
fast	O
and	O
reliable	O
storage	B-General_Concept
for	O
computing	O
and	O
data	O
processing	O
.	O
</s>
<s>
Storage	B-General_Concept
systems	I-General_Concept
are	O
complex	O
,	O
and	O
may	O
be	O
thought	O
of	O
as	O
a	O
special	O
purpose	O
computer	O
designed	O
to	O
provide	O
storage	B-General_Concept
capacity	I-General_Concept
along	O
with	O
advanced	O
data	O
protection	O
features	O
.	O
</s>
<s>
Disk	B-Device
drives	I-Device
are	O
only	O
one	O
element	O
within	O
a	O
storage	B-General_Concept
system	I-General_Concept
,	O
along	O
with	O
hardware	O
and	O
special	O
purpose	O
embedded	O
software	O
within	O
the	O
system	O
.	O
</s>
<s>
Storage	B-General_Concept
systems	I-General_Concept
can	O
provide	O
either	O
block	O
accessed	O
storage	B-General_Concept
,	O
or	O
file	O
accessed	O
storage	B-General_Concept
.	O
</s>
<s>
Block	O
access	O
is	O
typically	O
delivered	O
over	O
Fibre	B-Architecture
Channel	I-Architecture
,	O
iSCSI	B-Protocol
,	O
SAS	O
,	O
FICON	B-Architecture
or	O
other	O
protocols	O
.	O
</s>
<s>
File	B-Application
access	I-Application
is	O
often	O
provided	O
using	O
NFS	B-Protocol
or	O
SMB	B-Operating_System
protocols	I-Operating_System
.	O
</s>
<s>
Within	O
the	O
context	O
of	O
a	O
storage	B-General_Concept
system	I-General_Concept
,	O
there	O
are	O
two	O
primary	O
types	O
of	O
virtualization	O
that	O
can	O
occur	O
:	O
</s>
<s>
Block	O
virtualization	O
used	O
in	O
this	O
context	O
refers	O
to	O
the	O
abstraction	O
(	O
separation	O
)	O
of	O
logical	B-General_Concept
storage	I-General_Concept
(	O
partition	O
)	O
from	O
physical	B-Device
storage	I-Device
so	O
that	O
it	O
may	O
be	O
accessed	O
without	O
regard	O
to	O
physical	B-Device
storage	I-Device
or	O
heterogeneous	O
structure	O
.	O
</s>
<s>
This	O
separation	O
allows	O
the	O
administrators	O
of	O
the	O
storage	B-General_Concept
system	I-General_Concept
greater	O
flexibility	O
in	O
how	O
they	O
manage	O
storage	B-General_Concept
for	O
end	O
users	O
.	O
</s>
<s>
File	O
virtualization	O
addresses	O
the	O
NAS	B-Application
challenges	O
by	O
eliminating	O
the	O
dependencies	O
between	O
the	O
data	O
accessed	O
at	O
the	O
file	O
level	O
and	O
the	O
location	O
where	O
the	O
files	O
are	O
physically	O
stored	O
.	O
</s>
<s>
This	O
provides	O
opportunities	O
to	O
optimize	O
storage	B-General_Concept
use	O
and	O
server	O
consolidation	O
and	O
to	O
perform	O
non-disruptive	O
file	O
migrations	O
.	O
</s>
<s>
Virtualization	O
of	O
storage	B-General_Concept
helps	O
achieve	O
location	O
independence	O
by	O
abstracting	O
the	O
physical	O
location	O
of	O
the	O
data	O
.	O
</s>
<s>
The	O
virtualization	O
system	O
presents	O
to	O
the	O
user	O
a	O
logical	O
space	O
for	O
data	O
storage	B-General_Concept
and	O
handles	O
the	O
process	O
of	O
mapping	O
it	O
to	O
the	O
actual	O
physical	O
location	O
.	O
</s>
<s>
Typical	O
granularities	O
range	O
from	O
a	O
single	O
physical	O
disk	B-Device
down	O
to	O
some	O
small	O
subset	O
(	O
multiples	O
of	O
megabytes	O
or	O
gigabytes	O
)	O
of	O
the	O
physical	O
disk	B-Device
.	O
</s>
<s>
In	O
a	O
block-based	O
storage	B-General_Concept
environment	O
,	O
a	O
single	O
block	O
of	O
information	O
is	O
addressed	O
using	O
a	O
LUN	O
identifier	O
and	O
an	O
offset	O
within	O
that	O
LUN	O
known	O
as	O
a	O
logical	B-Device
block	I-Device
addressing	I-Device
(	O
LBA	O
)	O
.	O
</s>
<s>
The	O
virtualization	O
software	O
or	O
device	O
is	O
responsible	O
for	O
maintaining	O
a	O
consistent	O
view	O
of	O
all	O
the	O
mapping	O
information	O
for	O
the	O
virtualized	O
storage	B-General_Concept
.	O
</s>
<s>
In	O
several	O
storage	B-General_Concept
systems	I-General_Concept
deployed	O
today	O
,	O
it	O
is	O
common	O
to	O
utilize	O
three	O
layers	O
of	O
virtualization	O
.	O
</s>
<s>
It	O
will	O
receive	O
an	O
incoming	O
I/O	O
request	O
containing	O
information	O
about	O
the	O
location	O
of	O
the	O
data	O
in	O
terms	O
of	O
the	O
logical	B-General_Concept
disk	I-General_Concept
(	O
vdisk	B-General_Concept
)	O
and	O
translates	O
this	O
into	O
a	O
new	O
I/O	O
request	O
to	O
the	O
physical	O
disk	B-Device
location	O
.	O
</s>
<s>
Most	O
implementations	O
allow	O
for	O
heterogeneous	O
management	O
of	O
multi-vendor	O
storage	B-General_Concept
devices	O
within	O
the	O
scope	O
of	O
a	O
given	O
implementation	O
's	O
support	O
matrix	O
.	O
</s>
<s>
This	O
means	O
that	O
the	O
following	O
capabilities	O
are	O
not	O
limited	O
to	O
a	O
single	O
vendor	O
's	O
device	O
(	O
as	O
with	O
similar	O
capabilities	O
provided	O
by	O
specific	O
storage	B-General_Concept
controllers	O
)	O
and	O
are	O
in	O
fact	O
possible	O
across	O
different	O
vendors	O
 '	O
devices	O
.	O
</s>
<s>
When	O
storage	B-General_Concept
is	O
virtualized	O
,	O
replication	O
services	O
must	O
be	O
implemented	O
above	O
the	O
software	O
or	O
device	O
that	O
is	O
performing	O
the	O
virtualization	O
.	O
</s>
<s>
This	O
is	O
true	O
because	O
it	O
is	O
only	O
above	O
the	O
virtualization	O
layer	O
that	O
a	O
true	O
and	O
consistent	O
image	O
of	O
the	O
logical	B-General_Concept
disk	I-General_Concept
(	O
vdisk	B-General_Concept
)	O
can	O
be	O
copied	O
.	O
</s>
<s>
If	O
the	O
virtualization	O
is	O
implemented	O
in	O
the	O
network	O
or	O
higher	O
,	O
this	O
renders	O
any	O
replication	O
services	O
provided	O
by	O
the	O
underlying	O
storage	B-General_Concept
controllers	O
useless	O
.	O
</s>
<s>
The	O
physical	B-Device
storage	I-Device
resources	O
are	O
aggregated	O
into	O
storage	B-General_Concept
pools	O
,	O
from	O
which	O
the	O
logical	B-General_Concept
storage	I-General_Concept
is	O
created	O
.	O
</s>
<s>
More	O
storage	B-General_Concept
systems	I-General_Concept
,	O
which	O
may	O
be	O
heterogeneous	O
in	O
nature	O
,	O
can	O
be	O
added	O
as	O
and	O
when	O
needed	O
,	O
and	O
the	O
virtual	O
storage	B-General_Concept
space	O
will	O
scale	O
up	O
by	O
the	O
same	O
amount	O
.	O
</s>
<s>
This	O
process	O
is	O
fully	O
transparent	O
to	O
the	O
applications	O
using	O
the	O
storage	B-General_Concept
infrastructure	O
.	O
</s>
<s>
The	O
software	O
or	O
device	O
providing	O
storage	B-Application
virtualization	I-Application
becomes	O
a	O
common	O
disk	B-Device
manager	O
in	O
the	O
virtualized	O
environment	O
.	O
</s>
<s>
Logical	B-General_Concept
disks	I-General_Concept
(	O
vdisks	B-General_Concept
)	O
are	O
created	O
by	O
the	O
virtualization	O
software	O
or	O
device	O
and	O
are	O
mapped	O
(	O
made	O
visible	O
)	O
to	O
the	O
required	O
host	O
or	O
server	O
,	O
thus	O
providing	O
a	O
common	O
place	O
or	O
way	O
for	O
managing	O
all	O
volumes	O
in	O
the	O
environment	O
.	O
</s>
<s>
This	O
is	O
relatively	O
easy	O
to	O
implement	O
as	O
physical	B-Device
storage	I-Device
is	O
only	O
allocated	O
in	O
the	O
mapping	O
table	O
when	O
it	O
is	O
used	O
.	O
</s>
<s>
One	O
of	O
the	O
major	O
benefits	O
of	O
abstracting	O
the	O
host	O
or	O
server	O
from	O
the	O
actual	O
storage	B-General_Concept
is	O
the	O
ability	O
to	O
migrate	O
data	O
while	O
maintaining	O
concurrent	O
I/O	O
access	O
.	O
</s>
<s>
The	O
host	O
only	O
knows	O
about	O
the	O
logical	B-General_Concept
disk	I-General_Concept
(	O
the	O
mapped	O
LUN	O
)	O
and	O
so	O
any	O
changes	O
to	O
the	O
meta-data	O
mapping	O
is	O
transparent	O
to	O
the	O
host	O
.	O
</s>
<s>
When	O
the	O
data	O
has	O
been	O
copied	O
or	O
moved	O
,	O
the	O
meta-data	O
can	O
simply	O
be	O
updated	O
to	O
point	O
to	O
the	O
new	O
location	O
,	O
therefore	O
freeing	O
up	O
the	O
physical	B-Device
storage	I-Device
at	O
the	O
old	O
location	O
.	O
</s>
<s>
The	O
process	O
of	O
moving	O
the	O
physical	O
location	O
is	O
known	O
as	O
data	B-General_Concept
migration	I-General_Concept
.	O
</s>
<s>
Most	O
implementations	O
allow	O
for	O
this	O
to	O
be	O
done	O
in	O
a	O
non-disruptive	O
manner	O
,	O
that	O
is	O
concurrently	O
while	O
the	O
host	O
continues	O
to	O
perform	O
I/O	O
to	O
the	O
logical	B-General_Concept
disk	I-General_Concept
(	O
or	O
LUN	O
)	O
.	O
</s>
<s>
The	O
smaller	O
the	O
granularity	O
the	O
faster	O
the	O
update	O
,	O
less	O
space	O
required	O
and	O
quicker	O
the	O
old	O
storage	B-General_Concept
can	O
be	O
freed	O
up	O
.	O
</s>
<s>
There	O
are	O
many	O
day	O
to	O
day	O
tasks	O
a	O
storage	B-General_Concept
administrator	O
has	O
to	O
perform	O
that	O
can	O
be	O
simply	O
and	O
concurrently	O
performed	O
using	O
data	B-General_Concept
migration	I-General_Concept
techniques	O
.	O
</s>
<s>
Moving	O
data	O
off	O
an	O
over-utilized	O
storage	B-General_Concept
device	O
.	O
</s>
<s>
Utilization	O
can	O
be	O
increased	O
by	O
virtue	O
of	O
the	O
pooling	O
,	O
migration	O
,	O
and	O
thin	B-General_Concept
provisioning	I-General_Concept
services	O
.	O
</s>
<s>
This	O
allows	O
users	O
to	O
avoid	O
over-buying	O
and	O
over-provisioning	O
storage	B-General_Concept
solutions	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
this	O
kind	O
of	O
utilization	O
through	O
a	O
shared	O
pool	O
of	O
storage	B-General_Concept
can	O
be	O
easily	O
and	O
quickly	O
allocated	O
as	O
it	O
is	O
needed	O
to	O
avoid	O
constraints	O
on	O
storage	B-General_Concept
capacity	I-General_Concept
that	O
often	O
hinder	O
application	O
performance	O
.	O
</s>
<s>
When	O
all	O
available	O
storage	B-General_Concept
capacity	I-General_Concept
is	O
pooled	O
,	O
system	O
administrators	O
no	O
longer	O
have	O
to	O
search	O
for	O
disks	B-Device
that	O
have	O
free	O
space	O
to	O
allocate	O
to	O
a	O
particular	O
host	O
or	O
server	O
.	O
</s>
<s>
A	O
new	O
logical	B-General_Concept
disk	I-General_Concept
can	O
be	O
simply	O
allocated	O
from	O
the	O
available	O
pool	O
,	O
or	O
an	O
existing	O
disk	B-Device
can	O
be	O
expanded	O
.	O
</s>
<s>
Pooling	O
also	O
means	O
that	O
all	O
the	O
available	O
storage	B-General_Concept
capacity	I-General_Concept
can	O
potentially	O
be	O
used	O
.	O
</s>
<s>
In	O
a	O
traditional	O
environment	O
,	O
an	O
entire	O
disk	B-Device
would	O
be	O
mapped	O
to	O
a	O
host	O
.	O
</s>
<s>
In	O
a	O
virtual	O
environment	O
,	O
the	O
logical	B-General_Concept
disk	I-General_Concept
(	O
LUN	O
)	O
is	O
assigned	O
the	O
capacity	O
required	O
by	O
the	O
using	O
host	O
.	O
</s>
<s>
Storage	B-General_Concept
can	O
be	O
assigned	O
where	O
it	O
is	O
needed	O
at	O
that	O
point	O
in	O
time	O
,	O
reducing	O
the	O
need	O
to	O
guess	O
how	O
much	O
a	O
given	O
host	O
will	O
need	O
in	O
the	O
future	O
.	O
</s>
<s>
Using	O
Thin	B-General_Concept
Provisioning	I-General_Concept
,	O
the	O
administrator	O
can	O
create	O
a	O
very	O
large	O
thin	O
provisioned	O
logical	B-General_Concept
disk	I-General_Concept
,	O
thus	O
the	O
using	O
system	O
thinks	O
it	O
has	O
a	O
very	O
large	O
disk	B-Device
from	O
day	O
one	O
.	O
</s>
<s>
With	O
storage	B-Application
virtualization	I-Application
,	O
multiple	O
independent	O
storage	B-General_Concept
devices	O
,	O
even	O
if	O
scattered	O
across	O
a	O
network	O
,	O
appear	O
to	O
be	O
a	O
single	O
monolithic	O
storage	B-General_Concept
device	O
and	O
can	O
be	O
managed	O
centrally	O
.	O
</s>
<s>
However	O
,	O
traditional	O
storage	B-General_Concept
controller	O
management	O
is	O
still	O
required	O
.	O
</s>
<s>
That	O
is	O
,	O
the	O
creation	O
and	O
maintenance	O
of	O
RAID	B-Architecture
arrays	I-Architecture
,	O
including	O
error	O
and	O
fault	O
management	O
.	O
</s>
<s>
Backing	O
out	O
of	O
a	O
virtual	O
storage	B-General_Concept
environment	O
therefore	O
requires	O
the	O
reconstruction	O
of	O
the	O
logical	B-General_Concept
disks	I-General_Concept
as	O
contiguous	O
disks	B-Device
that	O
can	O
be	O
used	O
in	O
a	O
traditional	O
manner	O
.	O
</s>
<s>
Most	O
implementations	O
will	O
provide	O
some	O
form	O
of	O
back-out	O
procedure	O
and	O
with	O
the	O
data	B-General_Concept
migration	I-General_Concept
services	O
it	O
is	O
at	O
least	O
possible	O
,	O
but	O
time	O
consuming	O
.	O
</s>
<s>
It	O
applies	O
to	O
the	O
actual	O
physical	B-Device
storage	I-Device
controllers	O
and	O
the	O
hosts	O
,	O
their	O
operating	O
systems	O
,	O
multi-pathing	O
software	O
and	O
connectivity	O
hardware	O
.	O
</s>
<s>
For	O
example	O
,	O
virtualization	O
implemented	O
within	O
a	O
storage	B-General_Concept
controller	O
adds	O
no	O
extra	O
overhead	O
to	O
host	O
based	O
interoperability	O
,	O
but	O
will	O
require	O
additional	O
support	O
of	O
other	O
storage	B-General_Concept
controllers	O
if	O
they	O
are	O
to	O
be	O
virtualized	O
by	O
the	O
same	O
software	O
.	O
</s>
<s>
Switch	B-Architecture
based	O
virtualization	O
may	O
not	O
require	O
specific	O
host	O
interoperability	O
—	O
if	O
it	O
uses	O
packet	O
cracking	O
techniques	O
to	O
redirect	O
the	O
I/O	O
.	O
</s>
<s>
Network	O
based	O
appliances	O
have	O
the	O
highest	O
level	O
of	O
interoperability	O
requirements	O
as	O
they	O
have	O
to	O
interoperate	O
with	O
all	O
devices	O
,	O
storage	B-General_Concept
and	O
hosts	O
.	O
</s>
<s>
Management	O
of	O
environment	O
:	O
Although	O
a	O
virtual	O
storage	B-General_Concept
infrastructure	O
benefits	O
from	O
a	O
single	O
point	O
of	O
logical	B-General_Concept
disk	I-General_Concept
and	O
replication	O
service	O
management	O
,	O
the	O
physical	B-Device
storage	I-Device
must	O
still	O
be	O
managed	O
.	O
</s>
<s>
Any	O
implementation	O
must	O
ensure	O
its	O
protection	O
with	O
appropriate	O
levels	O
of	O
back-ups	B-Protocol
and	O
replicas	O
.	O
</s>
<s>
In	O
some	O
implementations	O
the	O
performance	O
of	O
the	O
physical	B-Device
storage	I-Device
can	O
actually	O
be	O
improved	O
,	O
mainly	O
due	O
to	O
caching	O
.	O
</s>
<s>
Assuming	O
the	O
software	O
or	O
device	O
is	O
efficiently	O
designed	O
this	O
impact	O
should	O
be	O
minimal	O
when	O
compared	O
with	O
the	O
latency	O
associated	O
with	O
physical	O
disk	B-Device
accesses	O
.	O
</s>
<s>
In	O
some	O
cases	O
volume	B-Application
management	I-Application
is	O
built	O
into	O
the	O
operating	O
system	O
,	O
and	O
in	O
other	O
instances	O
it	O
is	O
offered	O
as	O
a	O
separate	O
product	O
.	O
</s>
<s>
However	O
,	O
a	O
software	O
layer	O
(	O
the	O
volume	B-Application
manager	I-Application
)	O
resides	O
above	O
the	O
disk	B-Device
device	O
driver	O
intercepts	O
the	O
I/O	O
requests	O
,	O
and	O
provides	O
the	O
meta-data	O
lookup	O
and	O
I/O	O
mapping	O
.	O
</s>
<s>
Most	O
modern	O
operating	O
systems	O
have	O
some	O
form	O
of	O
logical	B-Application
volume	I-Application
management	I-Application
built-in	O
(	O
in	O
Linux	O
called	O
Logical	B-Application
Volume	I-Application
Manager	I-Application
or	O
LVM	O
;	O
in	O
Solaris	O
and	O
FreeBSD	O
,	O
ZFS	B-Application
's	O
zpool	O
layer	O
;	O
in	O
Windows	O
called	O
Logical	B-General_Concept
Disk	I-General_Concept
Manager	O
or	O
LDM	O
)	O
,	O
that	O
performs	O
virtualization	O
tasks	O
.	O
</s>
<s>
Note	O
:	O
Host	O
based	O
volume	B-Application
managers	I-Application
were	O
in	O
use	O
long	O
before	O
the	O
term	O
storage	B-Application
virtualization	I-Application
had	O
been	O
coined	O
.	O
</s>
<s>
Simple	O
data	O
storage	B-General_Concept
devices	O
,	O
like	O
single	O
hard	B-Device
disk	I-Device
drives	I-Device
,	O
do	O
not	O
provide	O
any	O
virtualization	O
.	O
</s>
<s>
But	O
even	O
the	O
simplest	O
disk	B-General_Concept
arrays	I-General_Concept
provide	O
a	O
logical	O
to	O
physical	O
abstraction	O
,	O
as	O
they	O
use	O
RAID	B-Architecture
schemes	O
to	O
join	O
multiple	O
disks	B-Device
in	O
a	O
single	O
array	O
(	O
and	O
possibly	O
later	O
divide	O
the	O
array	O
it	O
into	O
smaller	O
volumes	O
)	O
.	O
</s>
<s>
Advanced	O
disk	B-General_Concept
arrays	I-General_Concept
often	O
feature	O
cloning	O
,	O
snapshots	O
and	O
remote	O
replication	O
.	O
</s>
<s>
Generally	O
these	O
devices	O
do	O
not	O
provide	O
the	O
benefits	O
of	O
data	B-General_Concept
migration	I-General_Concept
or	O
replication	O
across	O
heterogeneous	O
storage	B-General_Concept
,	O
as	O
each	O
vendor	O
tends	O
to	O
use	O
their	O
own	O
proprietary	O
protocols	O
.	O
</s>
<s>
A	O
new	O
breed	O
of	O
disk	B-General_Concept
array	I-General_Concept
controllers	O
allows	O
the	O
downstream	O
attachment	O
of	O
other	O
storage	B-General_Concept
devices	O
.	O
</s>
<s>
For	O
the	O
purposes	O
of	O
this	O
article	O
we	O
will	O
only	O
discuss	O
the	O
later	O
style	O
which	O
do	O
actually	O
virtualize	O
other	O
storage	B-General_Concept
devices	O
.	O
</s>
<s>
A	O
primary	O
storage	B-General_Concept
controller	O
provides	O
the	O
services	O
and	O
allows	O
the	O
direct	O
attachment	O
of	O
other	O
storage	B-General_Concept
controllers	O
.	O
</s>
<s>
Storage	B-Application
virtualization	I-Application
operating	O
on	O
a	O
network	O
based	O
device	O
(	O
typically	O
a	O
standard	O
server	O
or	O
smart	O
switch	B-Architecture
)	O
and	O
using	O
iSCSI	B-Protocol
or	O
FC	O
Fibre	B-Architecture
channel	I-Architecture
networks	O
to	O
connect	O
as	O
a	O
SAN	O
.	O
</s>
<s>
The	O
virtualization	O
device	O
sits	O
in	O
the	O
SAN	O
and	O
provides	O
the	O
layer	O
of	O
abstraction	O
between	O
the	O
hosts	O
performing	O
the	O
I/O	O
and	O
the	O
storage	B-General_Concept
controllers	O
providing	O
the	O
storage	B-General_Concept
capacity	I-General_Concept
.	O
</s>
<s>
There	O
are	O
two	O
commonly	O
available	O
implementations	O
of	O
network-based	O
storage	B-Application
virtualization	I-Application
,	O
appliance-based	O
and	O
switch-based	O
.	O
</s>
<s>
Both	O
models	O
can	O
provide	O
the	O
same	O
services	O
,	O
disk	B-Device
management	O
,	O
metadata	O
lookup	O
,	O
data	B-General_Concept
migration	I-General_Concept
and	O
replication	O
.	O
</s>
<s>
Appliance	B-Architecture
based	O
devices	O
are	O
dedicated	O
hardware	O
devices	O
that	O
provide	O
SAN	O
connectivity	O
of	O
one	O
form	O
or	O
another	O
.	O
</s>
<s>
These	O
sit	O
between	O
the	O
hosts	O
and	O
storage	B-General_Concept
and	O
in	O
the	O
case	O
of	O
in-band	O
(	O
symmetric	O
)	O
appliances	O
can	O
provide	O
all	O
of	O
the	O
benefits	O
and	O
services	O
discussed	O
in	O
this	O
article	O
.	O
</s>
<s>
I/O	O
requests	O
are	O
targeted	O
at	O
the	O
appliance	B-Architecture
itself	O
,	O
which	O
performs	O
the	O
meta-data	O
mapping	O
before	O
redirecting	O
the	O
I/O	O
by	O
sending	O
its	O
own	O
I/O	O
request	O
to	O
the	O
underlying	O
storage	B-General_Concept
.	O
</s>
<s>
The	O
in-band	O
appliance	B-Architecture
can	O
also	O
provide	O
caching	O
of	O
data	O
,	O
and	O
most	O
implementations	O
provide	O
some	O
form	O
of	O
clustering	O
of	O
individual	O
appliances	O
to	O
maintain	O
an	O
atomic	O
view	O
of	O
the	O
metadata	O
as	O
well	O
as	O
cache	O
data	O
.	O
</s>
<s>
Switch	B-Architecture
based	O
devices	O
,	O
as	O
the	O
name	O
suggests	O
,	O
reside	O
in	O
the	O
physical	O
switch	B-Architecture
hardware	O
used	O
to	O
connect	O
the	O
SAN	O
devices	O
.	O
</s>
<s>
These	O
also	O
sit	O
between	O
the	O
hosts	O
and	O
storage	B-General_Concept
but	O
may	O
use	O
different	O
techniques	O
to	O
provide	O
the	O
metadata	O
mapping	O
,	O
such	O
as	O
packet	O
cracking	O
to	O
snoop	O
on	O
incoming	O
I/O	O
requests	O
and	O
perform	O
the	O
I/O	O
redirection	O
.	O
</s>
<s>
In-band	O
,	O
also	O
known	O
as	O
symmetric	O
,	O
virtualization	O
devices	O
actually	O
sit	O
in	O
the	O
data	O
path	O
between	O
the	O
host	O
and	O
storage	B-General_Concept
.	O
</s>
<s>
Hosts	O
perform	O
I/O	O
to	O
the	O
virtualization	O
device	O
and	O
never	O
interact	O
with	O
the	O
actual	O
storage	B-General_Concept
device	O
.	O
</s>
<s>
The	O
virtualization	O
device	O
in	O
turn	O
performs	O
I/O	O
to	O
the	O
storage	B-General_Concept
device	O
.	O
</s>
<s>
Caching	O
of	O
data	O
,	O
statistics	O
about	O
data	O
usage	O
,	O
replications	O
services	O
,	O
data	B-General_Concept
migration	I-General_Concept
and	O
thin	B-General_Concept
provisioning	I-General_Concept
are	O
all	O
easily	O
implemented	O
in	O
an	O
in-band	O
device	O
.	O
</s>
<s>
The	O
information	O
is	O
then	O
retrieved	O
through	O
an	O
actual	O
I/O	O
request	O
to	O
the	O
storage	B-General_Concept
.	O
</s>
<s>
File-based	O
virtualization	O
is	O
a	O
type	O
of	O
storage	B-Application
virtualization	I-Application
that	O
uses	O
files	O
as	O
the	O
basic	O
unit	O
of	O
storage	B-General_Concept
.	O
</s>
<s>
This	O
is	O
in	O
contrast	O
to	O
block-based	O
storage	B-Application
virtualization	I-Application
,	O
which	O
uses	O
blocks	O
as	O
the	O
basic	O
unit	O
.	O
</s>
<s>
It	O
is	O
a	O
way	O
to	O
abstract	O
away	O
the	O
physical	O
details	O
of	O
storage	B-General_Concept
and	O
allow	O
files	O
to	O
be	O
stored	O
on	O
any	O
type	O
of	O
storage	B-General_Concept
device	O
,	O
without	O
the	O
need	O
for	O
specific	O
drivers	O
or	O
other	O
low-level	O
configuration	O
.	O
</s>
<s>
File-based	O
virtualization	O
can	O
be	O
used	O
for	O
a	O
variety	O
of	O
purposes	O
,	O
including	O
storage	B-General_Concept
consolidation	O
,	O
improved	O
storage	B-General_Concept
utilization	O
,	O
and	O
disaster	B-General_Concept
recovery	I-General_Concept
.	O
</s>
<s>
This	O
can	O
simplify	O
storage	B-General_Concept
administration	O
and	O
reduce	O
the	O
overall	O
number	O
of	O
storage	B-General_Concept
devices	O
that	O
need	O
to	O
be	O
managed	O
.	O
</s>
<s>
File-based	O
virtualization	O
can	O
also	O
improve	O
storage	B-General_Concept
utilization	O
by	O
allowing	O
files	O
to	O
be	O
stored	O
on	O
devices	O
that	O
are	O
not	O
being	O
used	O
to	O
their	O
full	O
capacity	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
a	O
file	O
server	O
has	O
a	O
number	O
of	O
hard	B-Device
drives	I-Device
that	O
are	O
only	O
partially	O
filled	O
,	O
file-based	O
virtualization	O
can	O
be	O
used	O
to	O
store	O
files	O
on	O
those	O
drives	O
,	O
thereby	O
increasing	O
the	O
utilization	O
of	O
the	O
storage	B-General_Concept
devices	O
.	O
</s>
<s>
Finally	O
,	O
file-based	O
virtualization	O
can	O
be	O
used	O
for	O
disaster	B-General_Concept
recovery	I-General_Concept
purposes	O
.	O
</s>
<s>
By	O
replicating	O
files	O
across	O
a	O
variety	O
of	O
storage	B-General_Concept
devices	O
,	O
it	O
is	O
possible	O
to	O
recover	O
in	O
case	O
of	O
a	O
storage	B-General_Concept
device	O
failure	O
.	O
</s>
<s>
File-based	O
virtualization	O
can	O
be	O
used	O
to	O
create	O
a	O
virtual	O
file	O
server	O
(	O
or	O
virtual	O
NAS	B-Application
device	O
)	O
,	O
which	O
is	O
a	O
storage	B-General_Concept
system	I-General_Concept
that	O
appears	O
to	O
the	O
user	O
as	O
a	O
single	O
file	O
server	O
but	O
which	O
is	O
actually	O
implemented	O
as	O
a	O
set	O
of	O
files	O
stored	O
on	O
a	O
number	O
of	O
physical	O
file	O
servers	O
.	O
</s>
