<s>
In	O
computer	B-General_Concept
storage	I-General_Concept
,	O
logical	B-Application
volume	I-Application
management	I-Application
or	O
LVM	O
provides	O
a	O
method	O
of	O
allocating	O
space	O
on	O
mass-storage	B-Device
devices	I-Device
that	O
is	O
more	O
flexible	O
than	O
conventional	O
partitioning	B-Application
schemes	O
to	O
store	O
volumes	O
.	O
</s>
<s>
In	O
particular	O
,	O
a	O
volume	B-Application
manager	I-Application
can	O
concatenate	O
,	O
stripe	B-General_Concept
together	O
or	O
otherwise	O
combine	O
partitions	B-Application
(	O
or	O
block	O
devices	O
in	O
general	O
)	O
into	O
larger	O
virtual	O
partitions	B-Application
that	O
administrators	O
can	O
re-size	O
or	O
move	O
,	O
potentially	O
without	O
interrupting	O
system	O
use	O
.	O
</s>
<s>
Volume	B-Application
management	I-Application
represents	O
just	O
one	O
of	O
many	O
forms	O
of	O
storage	B-Application
virtualization	I-Application
;	O
its	O
implementation	O
takes	O
place	O
in	O
a	O
layer	O
in	O
the	O
device-driver	B-Application
stack	O
of	O
an	O
operating	B-General_Concept
system	I-General_Concept
(	O
OS	O
)	O
(	O
as	O
opposed	O
to	O
within	O
storage	O
devices	O
or	O
in	O
a	O
network	O
)	O
.	O
</s>
<s>
They	O
start	O
with	O
physical	O
volumes	O
(	O
PVs	O
)	O
,	O
which	O
can	O
be	O
either	O
hard	B-Device
disks	I-Device
,	O
hard	B-Application
disk	I-Application
partitions	I-Application
,	O
or	O
Logical	O
Unit	O
Numbers	O
(	O
LUNs	O
)	O
of	O
an	O
external	O
storage	O
device	O
.	O
</s>
<s>
Volume	B-Application
management	I-Application
treats	O
each	O
PV	O
as	O
being	O
composed	O
of	O
a	O
sequence	O
of	O
chunks	O
called	O
physical	O
extents	O
(	O
PEs	O
)	O
.	O
</s>
<s>
Some	O
volume	B-Application
managers	I-Application
(	O
such	O
as	O
that	O
in	O
HP-UX	B-Application
and	O
Linux	B-Application
)	O
have	O
PEs	O
of	O
a	O
uniform	O
size	O
;	O
others	O
(	O
such	O
as	O
that	O
in	O
Veritas	B-Application
)	O
have	O
variably-sized	O
PEs	O
that	O
can	O
be	O
split	O
and	O
merged	O
at	O
will	O
.	O
</s>
<s>
These	O
PEs	O
are	O
drawn	O
from	O
a	O
physical	O
volume	B-Application
group	I-Application
(	O
PVG	O
)	O
,	O
a	O
set	O
of	O
same-sized	O
PVs	O
which	O
act	O
similarly	O
to	O
hard	B-Device
disks	I-Device
in	O
a	O
RAID1	O
array	O
.	O
</s>
<s>
PVGs	O
are	O
usually	O
laid	O
out	O
so	O
that	O
they	O
reside	O
on	O
different	O
disks	B-Device
or	O
data	B-General_Concept
buses	I-General_Concept
for	O
maximum	O
redundancy	O
.	O
</s>
<s>
The	O
system	O
pools	O
LEs	O
into	O
a	O
volume	B-Application
group	I-Application
(	O
VG	O
)	O
.	O
</s>
<s>
The	O
pooled	O
LEs	O
can	O
then	O
be	O
concatenated	O
together	O
into	O
virtual	O
disk	B-Application
partitions	I-Application
called	O
logical	B-Application
volumes	I-Application
or	O
LVs	B-Application
.	O
</s>
<s>
Systems	O
can	O
use	O
LVs	B-Application
as	O
raw	O
block	O
devices	O
just	O
like	O
disk	B-Application
partitions	I-Application
:	O
creating	O
mountable	O
file	B-Application
systems	I-Application
on	O
them	O
,	O
or	O
using	O
them	O
as	O
swap	B-Architecture
storage	O
.	O
</s>
<s>
Striped	O
LVs	B-Application
allocate	O
each	O
successive	O
LE	O
from	O
a	O
different	O
PV	O
;	O
depending	O
on	O
the	O
size	O
of	O
the	O
LE	O
,	O
this	O
can	O
improve	O
performance	O
on	O
large	O
sequential	O
reads	O
by	O
bringing	O
to	O
bear	O
the	O
combined	O
read-throughput	O
of	O
multiple	O
PVs	O
.	O
</s>
<s>
Administrators	O
can	O
grow	O
LVs	B-Application
(	O
by	O
concatenating	O
more	O
LEs	O
)	O
or	O
shrink	O
them	O
(	O
by	O
returning	O
LEs	O
to	O
the	O
pool	O
)	O
.	O
</s>
<s>
This	O
allows	O
LVs	B-Application
to	O
grow	O
without	O
having	O
to	O
move	O
already-allocated	O
LEs	O
.	O
</s>
<s>
Some	O
volume	B-Application
managers	I-Application
allow	O
the	O
re-sizing	O
of	O
LVs	B-Application
in	O
either	O
direction	O
while	O
online	O
.	O
</s>
<s>
Changing	O
the	O
size	O
of	O
the	O
LV	O
does	O
not	O
necessarily	O
change	O
the	O
size	O
of	O
a	O
file	B-Application
system	I-Application
on	O
it	O
;	O
it	O
merely	O
changes	O
the	O
size	O
of	O
its	O
containing	O
space	O
.	O
</s>
<s>
A	O
file	B-Application
system	I-Application
that	O
can	O
be	O
resized	O
online	O
is	O
recommended	O
in	O
that	O
it	O
allows	O
the	O
system	O
to	O
adjust	O
its	O
storage	O
on-the-fly	O
without	O
interrupting	O
applications	O
.	O
</s>
<s>
PVs	O
and	O
LVs	B-Application
cannot	O
be	O
shared	O
between	O
or	O
span	O
different	O
VGs	O
(	O
although	O
some	O
volume	B-Application
managers	I-Application
may	O
allow	O
moving	O
them	O
at	O
will	O
between	O
VGs	O
on	O
the	O
same	O
host	O
)	O
.	O
</s>
<s>
Most	O
volume	B-Application
managers	I-Application
can	O
perform	O
this	O
movement	O
online	O
;	O
if	O
the	O
underlying	O
hardware	O
is	O
hot-pluggable	O
this	O
allows	O
engineers	O
to	O
upgrade	O
or	O
replace	O
storage	O
without	O
system	O
downtime	O
.	O
</s>
<s>
On	O
Linux	B-Application
,	O
bcache	B-Application
or	O
dm-cache	B-Application
may	O
be	O
used	O
for	O
this	O
purpose	O
,	O
while	O
Fusion	B-Device
Drive	I-Device
may	O
be	O
used	O
on	O
OSX	O
.	O
</s>
<s>
ZFS	B-Application
also	O
implements	O
this	O
functionality	O
at	O
the	O
file	B-Application
system	I-Application
level	O
,	O
by	O
allowing	O
administrators	O
to	O
configure	O
multi-level	O
read/write	O
caching	O
.	O
</s>
<s>
Hybrid	O
volumes	O
present	O
a	O
similar	O
concept	O
as	O
hybrid	B-Device
drives	I-Device
,	O
which	O
also	O
combine	O
solid-state	O
storage	O
and	O
rotational	O
magnetic	O
media	O
.	O
</s>
<s>
Some	O
volume	B-Application
managers	I-Application
also	O
implement	O
snapshots	B-Application
by	O
applying	O
copy-on-write	B-Language
to	O
each	O
LE	O
.	O
</s>
<s>
In	O
this	O
scheme	O
,	O
the	O
volume	B-Application
manager	I-Application
will	O
copy	O
the	O
LE	O
to	O
a	O
copy-on-write	B-Language
table	O
just	O
before	O
it	O
is	O
written	O
to	O
.	O
</s>
<s>
This	O
preserves	O
an	O
old	O
version	O
of	O
the	O
LV	O
,	O
the	O
snapshot	B-Application
,	O
which	O
may	O
be	O
later	O
reconstructed	O
by	O
overlaying	O
the	O
copy-on-write	B-Language
table	O
atop	O
the	O
current	O
LV	O
.	O
</s>
<s>
Unless	O
the	O
volume	B-Application
management	I-Application
supports	O
both	O
thin	B-General_Concept
provisioning	I-General_Concept
and	O
discard	O
,	O
once	O
an	O
LE	O
in	O
the	O
origin	O
volume	O
is	O
written	O
to	O
,	O
it	O
is	O
permanently	O
stored	O
in	O
the	O
snapshot	B-Application
volume	O
.	O
</s>
<s>
If	O
the	O
snapshot	B-Application
volume	O
was	O
made	O
smaller	O
than	O
its	O
origin	O
,	O
which	O
is	O
a	O
common	O
practice	O
,	O
this	O
may	O
render	O
the	O
snapshot	B-Application
inoperable	O
.	O
</s>
<s>
Snapshots	B-Application
can	O
be	O
useful	O
for	O
backing	O
up	O
self-consistent	O
versions	O
of	O
volatile	O
data	O
such	O
as	O
table	O
files	O
from	O
a	O
busy	O
database	O
,	O
or	O
for	O
rolling	O
back	O
large	O
changes	O
(	O
such	O
as	O
an	O
operating	B-General_Concept
system	I-General_Concept
upgrade	O
)	O
in	O
a	O
single	O
operation	O
.	O
</s>
<s>
Snapshots	B-Application
have	O
a	O
similar	O
effect	O
as	O
rendering	O
storage	O
quiescent	O
,	O
and	O
are	O
similar	O
to	O
the	O
shadow	O
copy	O
(	O
VSS	O
)	O
service	O
in	O
Microsoft	O
Windows	O
.	O
</s>
<s>
Some	O
Linux-based	O
Live	O
CDs	O
also	O
use	O
snapshots	B-Application
to	O
simulate	O
read-write	O
access	O
to	O
a	O
read-only	O
optical	B-Device
disc	I-Device
.	O
</s>
<s>
Vendor	O
Introduced	O
in	O
Volume	B-Application
manager	I-Application
Allocate	O
anywhere	O
Snapshots	B-Application
RAID	O
0	O
RAID	O
1	O
RAID	O
5	O
RAID	O
10	O
Thin	B-General_Concept
provisioning	I-General_Concept
Notes	O
IBM	B-Application
AIX	I-Application
3.0	O
(	O
1989	O
)	O
Logical	B-Application
Volume	I-Application
Manager	I-Application
Refers	O
to	O
PEs	O
as	O
PPs	O
(	O
physical	O
partitions	B-Application
)	O
,	O
and	O
to	O
LEs	O
as	O
LPs	O
(	O
logical	O
partitions	B-Application
)	O
.	O
</s>
<s>
Does	O
not	O
have	O
a	O
copy-on-write	B-Language
snapshot	B-Application
mechanism	O
;	O
creates	O
snapshots	B-Application
by	O
freezing	O
one	O
volume	O
of	O
a	O
mirror	O
pair	O
.	O
</s>
<s>
Hewlett-Packard	O
HP-UX	B-Application
9.0	O
HP	O
Logical	B-Application
Volume	I-Application
Manager	I-Application
FreeBSD	B-Operating_System
Vinum	B-Application
Volume	I-Application
Manager	I-Application
The	O
FreeBSD	B-Operating_System
fast	O
file	B-Application
system	I-Application
(	O
UFS	O
)	O
supports	O
snapshots	B-Application
.	O
</s>
<s>
FreeBSD	B-Operating_System
ZFS	B-Application
A	O
file	B-Application
system	I-Application
with	O
integrated	O
volume	O
managementThe	O
NetBSD	B-Device
Foundation	O
,	O
Inc	O
.	O
NetBSD	B-Device
Logical	B-Application
Volume	I-Application
Manager	I-Application
NetBSD	B-Device
from	O
version	O
6.0	O
supports	O
its	O
own	O
re-implementation	O
of	O
Linux	B-Application
LVM	I-Application
.	O
</s>
<s>
Re-implementation	O
is	O
based	O
on	O
a	O
BSD	O
licensed	O
device-mapper	B-Application
driver	O
and	O
uses	O
a	O
port	O
of	O
Linux	B-Application
lvm	I-Application
tools	O
as	O
the	O
userspace	O
part	O
of	O
LVM	O
.	O
</s>
<s>
There	O
is	O
no	O
need	O
to	O
support	O
RAID5	O
in	O
LVM	O
because	O
of	O
NetBSD	B-Device
superior	O
RAIDFrame	O
subsystem	O
.	O
</s>
<s>
NetBSD	B-Device
ZFS	B-Application
A	O
file	B-Application
system	I-Application
with	O
integrated	O
volume	B-Application
management	I-Application
(	O
2009	O
)	O
bioctl	B-Device
arcmsr	O
bioctl	B-Device
on	O
NetBSD	B-Device
can	O
be	O
used	O
for	O
both	O
maintenance	O
and	O
initialisation	O
of	O
hardware	O
RAID	O
,	O
although	O
initialisation	O
(	O
through	O
BIOCVOLOPS	O
ioctl	B-Operating_System
)	O
is	O
only	O
supported	O
by	O
a	O
single	O
driver	O
as	O
of	O
2019	O
—	O
arcmsr(4 )	O
;	O
software	O
RAID	O
is	O
supported	O
separately	O
through	O
RAIDframe	O
and	O
ZFS	B-Application
The	O
OpenBSD	B-Operating_System
Project	I-Operating_System
OpenBSD	B-Operating_System
4.2	O
(	O
2007	O
)	O
bioctl	B-Device
softraid	O
bioctl	B-Device
on	O
OpenBSD	B-Operating_System
can	O
be	O
used	O
for	O
maintenance	O
of	O
hardware	O
RAID	O
,	O
as	O
well	O
as	O
for	O
both	O
initialisation	O
and	O
maintenance	O
of	O
software	O
RAID	O
Linux	B-Application
2.2	O
Logical	B-Application
Volume	I-Application
Manager	I-Application
version	I-Application
1	I-Application
Linux	B-Application
2.4	O
Enterprise	B-Application
Volume	I-Application
Management	I-Application
System	I-Application
Linux	B-Application
2.6	O
and	O
above	O
Logical	B-Application
Volume	I-Application
Manager	I-Application
version	I-Application
2	I-Application
Linux	B-Application
2.6	O
and	O
above	O
Btrfs	B-Operating_System
(	O
not	O
stable	O
)	O
A	O
file	B-Application
system	I-Application
with	O
integrated	O
volume	B-Application
management	I-Application
Silicon	O
Graphics	O
IRIX	B-Operating_System
or	O
Linux	B-Application
XVM	O
Volume	B-Application
Manager	I-Application
Sun	O
Microsystems	O
SunOS	B-Operating_System
Solaris	B-Application
Volume	I-Application
Manager	I-Application
(	O
was	O
Solstice	B-Application
DiskSuite	I-Application
)	O
.	O
</s>
<s>
Refers	O
to	O
PVs	O
as	O
volumes	O
(	O
which	O
can	O
be	O
combined	O
with	O
RAID0	O
,	O
RAID1	O
or	O
RAID5	O
primitives	O
into	O
larger	O
volumes	O
)	O
,	O
to	O
LVs	B-Application
as	O
soft	O
partitions	B-Application
(	O
which	O
are	O
contiguous	O
extents	O
placeable	O
anywhere	O
on	O
volumes	O
,	O
but	O
which	O
cannot	O
span	O
multiple	O
volumes	O
)	O
,	O
and	O
to	O
VGs	O
as	O
disk	O
sets	O
.	O
</s>
<s>
Solaris	B-Application
10	I-Application
ZFS	B-Application
A	O
file	B-Application
system	I-Application
with	O
integrated	O
volume	B-Application
management	I-Application
illumos	B-Application
ZFS	B-Application
A	O
file	B-Application
system	I-Application
with	O
integrated	O
volume	B-Application
management	I-Application
Veritas	B-Application
Cross-OS	O
Veritas	B-Application
Volume	I-Application
Manager	I-Application
(	O
VxVM	B-Application
)	O
Refers	O
to	O
LVs	B-Application
as	O
volumes	O
,	O
to	O
VGs	O
as	O
disk	O
groups	O
;	O
has	O
variably-sized	O
PEs	O
called	O
subdisks	O
and	O
LEs	O
called	O
plexes	O
.	O
</s>
<s>
Microsoft	B-Application
Windows	I-Application
2000	I-Application
and	O
later	O
NT-based	O
operating	B-General_Concept
systems	I-General_Concept
Logical	O
Disk	O
Manager	O
Does	O
not	O
have	O
a	O
concept	O
of	O
PEs	O
or	O
LEs	O
;	O
can	O
only	O
RAID0	O
,	O
RAID1	O
,	O
RAID5	O
or	O
concatenate	O
disk	B-Application
partitions	I-Application
into	O
larger	O
volumes	O
;	O
file	B-Application
systems	I-Application
must	O
span	O
whole	O
volumes	O
.	O
</s>
<s>
Windows	B-Application
8	I-Application
Storage	O
Spaces	O
Higher-level	O
logic	O
than	O
RAID1	O
and	O
RAID5	O
-	O
multiple	O
storage	O
spaces	O
span	O
multiple	O
disks	B-Device
of	O
different	O
size	O
,	O
storage	O
spaces	O
are	O
resilient	O
from	O
physical	O
failure	O
with	O
either	O
mirroring	O
(	O
at	O
least	O
2	O
disks	B-Device
)	O
or	O
striped	O
parity	O
(	O
at	O
least	O
3	O
disks	B-Device
)	O
,	O
disk	O
management	O
and	O
data	O
recovery	O
is	O
fully	O
automatic	O
Windows	B-Operating_System
10	I-Operating_System
Storage	O
Spaces	O
RAID	O
10	O
is	O
called	O
disk	O
mirroring	O
Red	O
Hat	O
Linux	B-Application
4.14	O
and	O
above	O
Stratis	B-Application
RAID	O
support	O
planned	O
in	O
2.0	O
version	O
Apple	O
Mac	B-Application
OS	I-Application
X	I-Application
Lion	I-Application
Core	B-Operating_System
Storage	I-Operating_System
Currently	O
,	O
it	O
is	O
used	O
in	O
Lion	B-Application
's	O
implementation	O
of	O
FileVault	O
,	O
in	O
order	O
to	O
allow	O
for	O
full	O
disk	O
encryption	O
,	O
as	O
well	O
as	O
Fusion	B-Device
Drive	I-Device
,	O
which	O
is	O
merely	O
a	O
multi-PV	O
LVG	O
.	O
</s>
<s>
Snapshots	B-Application
are	O
handled	O
by	O
Time	B-Operating_System
Machine	I-Operating_System
;	O
Software-based	O
RAID	O
is	O
provided	O
by	O
AppleRAID	O
.	O
</s>
<s>
Both	O
are	O
separate	O
from	O
Core	B-Operating_System
Storage	I-Operating_System
.	O
</s>
<s>
Logical	B-Application
volumes	I-Application
can	O
suffer	O
from	O
external	O
fragmentation	O
when	O
the	O
underlying	O
storage	O
devices	O
do	O
not	O
allocate	O
their	O
PEs	O
contiguously	O
.	O
</s>
<s>
This	O
can	O
reduce	O
I/O	O
performance	O
on	O
slow-seeking	O
media	O
such	O
as	O
magnetic	O
disks	B-Device
and	O
other	O
rotational	B-Device
media	I-Device
.	O
</s>
<s>
Volume	B-Application
managers	I-Application
that	O
use	O
fixed-size	O
PEs	O
,	O
however	O
,	O
typically	O
make	O
PEs	O
relatively	O
large	O
(	O
for	O
example	O
,	O
Linux	B-Application
LVM	I-Application
uses	O
4MB	O
by	O
default	O
)	O
in	O
order	O
to	O
amortize	O
the	O
cost	O
of	O
these	O
seeks	O
.	O
</s>
<s>
With	O
implementations	O
that	O
are	O
solely	O
volume	B-Application
management	I-Application
,	O
such	O
as	O
Core	B-Operating_System
Storage	I-Operating_System
and	O
Linux	B-Application
LVM	I-Application
,	O
separating	O
and	O
abstracting	O
away	O
volume	B-Application
management	I-Application
from	O
the	O
file	B-Application
system	I-Application
loses	O
the	O
ability	O
to	O
easily	O
make	O
storage	O
decisions	O
for	O
particular	O
files	O
or	O
directories	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
a	O
certain	O
directory	O
(	O
but	O
not	O
the	O
entire	O
file	B-Application
system	I-Application
)	O
is	O
to	O
be	O
permanently	O
moved	O
to	O
faster	O
storage	O
,	O
both	O
the	O
file	B-Application
system	I-Application
layout	O
and	O
the	O
underlying	O
volume	B-Application
management	I-Application
layer	O
need	O
to	O
be	O
traversed	O
.	O
</s>
<s>
For	O
example	O
,	O
on	O
Linux	B-Application
it	O
would	O
be	O
needed	O
to	O
manually	O
determine	O
the	O
offset	O
of	O
a	O
file	O
's	O
contents	O
within	O
a	O
file	B-Application
system	I-Application
and	O
then	O
manually	O
the	O
extents	O
(	O
along	O
with	O
data	O
not	O
related	O
to	O
that	O
file	O
)	O
to	O
the	O
faster	O
storage	O
.	O
</s>
