<s>
Although	O
all	O
RAID	B-Architecture
implementations	O
differ	O
from	O
the	O
specification	O
to	O
some	O
extent	O
,	O
some	O
companies	O
and	O
open-source	O
projects	O
have	O
developed	O
non-standard	O
RAID	B-Architecture
implementations	O
that	O
differ	O
substantially	O
from	O
the	O
standard	O
.	O
</s>
<s>
Additionally	O
,	O
there	O
are	O
non-RAID	B-General_Concept
drive	I-General_Concept
architectures	I-General_Concept
,	O
providing	O
configurations	O
of	O
multiple	O
hard	O
drives	O
not	O
referred	O
to	O
by	O
RAID	B-Architecture
acronyms	O
.	O
</s>
<s>
Now	O
part	O
of	O
RAID6	O
,	O
double	O
parity	B-Error_Name
(	O
sometimes	O
known	O
as	O
row	O
diagonal	O
parity	B-Error_Name
)	O
features	O
two	O
sets	O
of	O
parity	B-Error_Name
checks	I-Error_Name
,	O
like	O
traditional	O
RAID6	O
.	O
</s>
<s>
Rather	O
,	O
double	O
parity	B-Error_Name
calculates	O
the	O
extra	O
parity	B-Error_Name
against	O
a	O
different	O
group	O
of	O
blocks	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
our	O
graph	O
both	O
RAID5	O
and	O
6	O
consider	O
all	O
A-labeled	O
blocks	O
to	O
produce	O
one	O
or	O
more	O
parity	B-Error_Name
blocks	I-Error_Name
.	O
</s>
<s>
However	O
,	O
it	O
is	O
fairly	O
easy	O
to	O
calculate	O
parity	B-Error_Name
against	O
multiple	O
groups	O
of	O
blocks	O
,	O
one	O
can	O
calculate	O
all	O
A	O
blocks	O
and	O
a	O
permuted	O
group	O
of	O
blocks	O
.	O
</s>
<s>
RAID-DP	O
is	O
proprietary	O
NetApp	O
RAID	B-Architecture
implementation	O
available	O
only	O
in	O
ONTAP	B-Operating_System
systems	O
.	O
</s>
<s>
RAID	B-Architecture
DP	O
implements	O
RAID	B-Architecture
4	O
,	O
except	O
with	O
an	O
additional	O
disk	O
that	O
is	O
used	O
for	O
a	O
second	O
parity	B-Error_Name
,	O
so	O
it	O
has	O
the	O
same	O
failure	O
characteristics	O
of	O
a	O
RAID	B-Architecture
6	O
.	O
</s>
<s>
The	O
performance	O
penalty	O
of	O
RAID-DP	O
is	O
typically	O
under	O
2%	O
when	O
compared	O
to	O
a	O
similar	O
RAID4	O
configuration	O
.	O
</s>
<s>
RAID5E	O
,	O
RAID5EE	O
,	O
and	O
RAID6E	O
(	O
with	O
the	O
added	O
E	O
standing	O
for	O
Enhanced	O
)	O
generally	O
refer	O
to	O
variants	O
of	O
RAID5	O
or	O
6	O
with	O
an	O
integrated	O
hot-spare	B-General_Concept
drive	O
,	O
where	O
the	O
spare	O
drive	O
is	O
an	O
active	O
part	O
of	O
the	O
block	O
rotation	O
scheme	O
.	O
</s>
<s>
Intel	O
Matrix	O
RAID	B-Architecture
(	O
a	O
feature	O
of	O
Intel	O
Rapid	O
Storage	O
Technology	O
)	O
is	O
a	O
feature	O
(	O
not	O
a	O
RAID	B-Architecture
level	O
)	O
present	O
in	O
the	O
ICH6R	O
and	O
subsequent	O
Southbridge	O
chipsets	O
from	O
Intel	O
,	O
accessible	O
and	O
configurable	O
via	O
the	O
RAID	B-Architecture
BIOS	B-Operating_System
setup	O
utility	O
.	O
</s>
<s>
Matrix	O
RAID	B-Architecture
supports	O
as	O
few	O
as	O
two	O
physical	O
disks	O
or	O
as	O
many	O
as	O
the	O
controller	O
supports	O
.	O
</s>
<s>
The	O
distinguishing	O
feature	O
of	O
Matrix	O
RAID	B-Architecture
is	O
that	O
it	O
allows	O
any	O
assortment	O
of	O
RAID0	O
,	O
1	O
,	O
5	O
,	O
or	O
10	O
volumes	O
in	O
the	O
array	O
,	O
to	O
which	O
a	O
controllable	O
(	O
and	O
identical	O
)	O
portion	O
of	O
each	O
disk	O
is	O
allocated	O
.	O
</s>
<s>
As	O
such	O
,	O
a	O
Matrix	O
RAID	B-Architecture
array	I-Architecture
can	O
improve	O
both	O
performance	O
and	O
data	O
integrity	O
.	O
</s>
<s>
A	O
practical	O
instance	O
of	O
this	O
would	O
use	O
a	O
small	O
RAID0	O
(	O
stripe	O
)	O
volume	O
for	O
the	O
operating	B-General_Concept
system	I-General_Concept
,	O
program	O
,	O
and	O
paging	O
files	O
;	O
second	O
larger	O
RAID1	O
(	O
mirror	O
)	O
volume	O
would	O
store	O
critical	O
data	O
.	O
</s>
<s>
Linux	B-Application
MD	I-Application
RAID	I-Application
is	O
also	O
capable	O
of	O
this	O
.	O
</s>
<s>
The	O
software	O
RAID	B-Architecture
subsystem	O
provided	O
by	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
,	O
called	O
,	O
supports	O
the	O
creation	O
of	O
both	O
classic	O
(	O
nested	O
)	O
RAID	B-Architecture
1+0	O
arrays	O
,	O
and	O
non-standard	O
RAID	B-Architecture
arrays	I-Architecture
that	O
use	O
a	O
single-level	O
RAID	B-Architecture
layout	O
with	O
some	O
additional	O
features	O
.	O
</s>
<s>
Random	O
reads	O
are	O
somewhat	O
faster	O
,	O
while	O
sequential	O
and	O
random	O
writes	O
offer	O
about	O
equal	O
speed	O
to	O
other	O
mirrored	O
RAID	B-Architecture
configurations	O
.	O
</s>
<s>
For	O
a	O
comparison	O
,	O
regular	O
RAID1	O
as	O
provided	O
by	O
Linux	B-Application
software	I-Application
RAID	I-Application
,	O
does	O
not	O
stripe	O
reads	O
,	O
but	O
can	O
perform	O
reads	O
in	O
parallel	O
.	O
</s>
<s>
Linux	B-Application
software	I-Application
RAID	I-Application
(	O
Linux	B-Operating_System
kernel	I-Operating_System
's	O
driver	O
)	O
also	O
supports	O
creation	O
of	O
standard	O
RAID0	O
,	O
1	O
,	O
4	O
,	O
5	O
,	O
and	O
6	O
configurations	O
.	O
</s>
<s>
Some	O
RAID	B-Architecture
1	O
implementations	O
treat	O
arrays	O
with	O
more	O
than	O
two	O
disks	O
differently	O
,	O
creating	O
a	O
non-standard	O
RAID	B-Architecture
level	O
known	O
as	O
RAID	B-Architecture
1E	O
.	O
</s>
<s>
The	O
ZFS	B-Application
filesystem	B-Application
provides	O
RAID-Z	O
,	O
a	O
data/parity	O
distribution	O
scheme	O
similar	O
to	O
RAID	B-Architecture
5	O
,	O
but	O
using	O
dynamic	O
stripe	O
width	O
:	O
every	O
block	O
is	O
its	O
own	O
RAID	B-Architecture
stripe	O
,	O
regardless	O
of	O
blocksize	O
,	O
resulting	O
in	O
every	O
RAID-Z	O
write	O
being	O
a	O
full-stripe	O
write	O
.	O
</s>
<s>
This	O
,	O
when	O
combined	O
with	O
the	O
copy-on-write	O
transactional	O
semantics	O
of	O
ZFS	B-Application
,	O
eliminates	O
the	O
write	O
hole	O
error	O
.	O
</s>
<s>
RAID-Z	O
is	O
also	O
faster	O
than	O
traditional	O
RAID5	O
because	O
it	O
does	O
not	O
need	O
to	O
perform	O
the	O
usual	O
read-modify-write	B-Operating_System
sequence	O
.	O
</s>
<s>
RAID-Z	O
does	O
not	O
require	O
any	O
special	O
hardware	O
,	O
such	O
as	O
NVRAM	O
for	O
reliability	O
,	O
or	O
write	O
buffering	O
for	O
performance	O
.	O
</s>
<s>
Given	O
the	O
dynamic	O
nature	O
of	O
RAID-Z	O
'	O
s	O
stripe	O
width	O
,	O
RAID-Z	O
reconstruction	O
must	O
traverse	O
the	O
filesystem	B-Application
metadata	O
to	O
determine	O
the	O
actual	O
RAID-Z	O
geometry	O
.	O
</s>
<s>
This	O
would	O
be	O
impossible	O
if	O
the	O
filesystem	B-Application
and	O
the	O
RAID	B-Architecture
array	I-Architecture
were	O
separate	O
products	O
,	O
whereas	O
it	O
becomes	O
feasible	O
when	O
there	O
is	O
an	O
integrated	O
view	O
of	O
the	O
logical	O
and	O
physical	O
structure	O
of	O
the	O
data	O
.	O
</s>
<s>
Going	O
through	O
the	O
metadata	O
means	O
that	O
ZFS	B-Application
can	O
validate	O
every	O
block	O
against	O
its	O
256-bit	O
checksum	O
as	O
it	O
goes	O
,	O
whereas	O
traditional	O
RAID	B-Architecture
products	O
usually	O
cannot	O
do	O
this	O
.	O
</s>
<s>
In	O
addition	O
to	O
handling	O
whole-disk	O
failures	O
,	O
RAID-Z	O
can	O
also	O
detect	O
and	O
correct	O
silent	O
data	O
corruption	O
,	O
offering	O
"	O
self-healing	O
data	O
"	O
:	O
when	O
reading	O
a	O
RAID-Z	O
block	O
,	O
ZFS	B-Application
compares	O
it	O
against	O
its	O
checksum	O
,	O
and	O
if	O
the	O
data	O
disks	O
did	O
not	O
return	O
the	O
right	O
answer	O
,	O
ZFS	B-Application
reads	O
the	O
parity	B-Error_Name
and	O
then	O
figures	O
out	O
which	O
disk	O
returned	O
bad	O
data	O
.	O
</s>
<s>
There	O
are	O
five	O
different	O
RAID-Z	O
modes	O
:	O
RAID-Z0	O
(	O
similar	O
to	O
RAID0	O
,	O
offers	O
no	O
redundancy	O
)	O
,	O
RAID-Z1	O
(	O
similar	O
to	O
RAID5	O
,	O
allows	O
one	O
disk	O
to	O
fail	O
)	O
,	O
RAID-Z2	O
(	O
similar	O
to	O
RAID6	O
,	O
allows	O
two	O
disks	O
to	O
fail	O
)	O
,	O
RAID-Z3	B-General_Concept
(	O
a	O
RAID7	O
configuration	O
,	O
allows	O
three	O
disks	O
to	O
fail	O
)	O
,	O
and	O
mirror	O
(	O
similar	O
to	O
RAID1	O
,	O
allows	O
all	O
but	O
one	O
of	O
the	O
disks	O
to	O
fail	O
)	O
.	O
</s>
<s>
Windows	B-Application
Home	I-Application
Server	I-Application
Drive	O
Extender	O
is	O
a	O
specialized	O
case	O
of	O
JBOD	O
RAID1	O
implemented	O
at	O
the	O
file	B-Application
system	I-Application
level	O
.	O
</s>
<s>
Microsoft	O
announced	O
in	O
2011	O
that	O
Drive	O
Extender	O
would	O
no	O
longer	O
be	O
included	O
as	O
part	O
of	O
Windows	B-Application
Home	I-Application
Server	I-Application
Version	O
2	O
,	O
Windows	B-Application
Home	I-Application
Server	I-Application
2011	O
(	O
codename	O
VAIL	O
)	O
.	O
</s>
<s>
BeyondRAID	O
is	O
not	O
a	O
true	O
RAID	B-Architecture
extension	O
,	O
but	O
consolidates	O
up	O
to	O
12	O
SATA	O
hard	O
drives	O
into	O
one	O
pool	O
of	O
storage	O
.	O
</s>
<s>
BeyondRaid	O
offers	O
a	O
RAID6	O
–	O
like	O
feature	O
and	O
can	O
perform	O
hash-based	O
compression	O
using	O
160-bit	O
SHA-1	B-Algorithm
hashes	I-Algorithm
to	O
maximize	O
storage	O
efficiency	O
.	O
</s>
<s>
Unraid	O
is	O
a	O
proprietary	O
Linux-based	O
operating	B-General_Concept
system	I-General_Concept
optimized	O
for	O
media	O
file	B-Application
storage	I-Application
.	O
</s>
<s>
Unfortunately	O
Unraid	O
does	O
n't	O
provide	O
information	O
about	O
is	O
storage	O
technology	O
,	O
but	O
some	O
say	O
its	O
parity	B-Error_Name
array	O
is	O
a	O
rewrite	O
of	O
the	O
mdadm	B-Application
module	O
.	O
</s>
<s>
Cache	O
pool	O
data	O
can	O
be	O
temporarily	O
protected	O
using	O
Btrfs	B-Operating_System
RAID1	O
until	O
Unraid	O
moves	O
it	O
to	O
the	O
array	O
based	O
on	O
a	O
schedule	O
set	O
within	O
the	O
software	O
.	O
</s>
<s>
Advantages	O
include	O
lower	O
power	O
consumption	O
than	O
standard	B-General_Concept
RAID	I-General_Concept
levels	I-General_Concept
,	O
the	O
ability	O
to	O
use	O
multiple	O
hard	O
drives	O
with	O
differing	O
sizes	O
to	O
their	O
full	O
capacity	O
and	O
in	O
the	O
event	O
of	O
multiple	O
concurrent	O
hard	O
drive	O
failures	O
(	O
exceeding	O
the	O
redundancy	O
)	O
,	O
only	O
losing	O
the	O
data	O
stored	O
on	O
the	O
failed	O
hard	O
drives	O
compared	O
to	O
standard	B-General_Concept
RAID	I-General_Concept
levels	I-General_Concept
which	O
offer	O
striping	O
in	O
which	O
case	O
all	O
of	O
the	O
data	O
on	O
the	O
array	O
is	O
lost	O
when	O
more	O
hard	O
drives	O
fail	O
than	O
the	O
redundancy	O
can	O
handle	O
.	O
</s>
<s>
In	O
OpenBSD	B-Operating_System
,	O
CRYPTO	O
is	O
an	O
encrypting	O
discipline	O
for	O
the	O
softraid	O
subsystem	O
.	O
</s>
<s>
RAID	B-Architecture
1C	O
provides	O
both	O
redundancy	O
and	O
encryption	O
.	O
</s>
<s>
Some	O
filesystems	B-Application
,	O
such	O
as	O
Btrfs	B-Operating_System
,	O
and	O
ZFS/OpenZFS	O
(	O
with	O
per-dataset	O
copies	O
=	O
1|2|3	O
property	O
)	O
,	O
support	O
creating	O
multiple	O
copies	O
of	O
the	O
same	O
data	O
on	O
a	O
single	O
drive	O
or	O
disks	O
pool	O
,	O
protecting	O
from	O
individual	O
bad	O
sectors	O
,	O
but	O
not	O
from	O
large	O
numbers	O
of	O
bad	O
sectors	O
or	O
complete	O
drive	O
failure	O
.	O
</s>
<s>
This	O
allows	O
some	O
of	O
the	O
benefits	O
of	O
RAID	B-Architecture
on	O
computers	O
that	O
can	O
only	O
accept	O
a	O
single	O
drive	O
,	O
such	O
as	O
laptops	O
.	O
</s>
<s>
Declustered	O
RAID	B-Architecture
allows	O
for	O
arbitrarily	O
sized	O
disk	O
arrays	O
while	O
reducing	O
the	O
overhead	O
to	O
clients	O
when	O
recovering	O
from	O
disk	O
failures	O
.	O
</s>
<s>
Under	O
traditional	O
RAID	B-Architecture
,	O
an	O
entire	O
disk	O
storage	O
system	O
of	O
,	O
say	O
,	O
100	O
disks	O
would	O
be	O
split	O
into	O
multiple	O
arrays	O
each	O
of	O
,	O
say	O
,	O
10	O
disks	O
.	O
</s>
<s>
By	O
contrast	O
,	O
under	O
declustered	O
RAID	B-Architecture
,	O
the	O
entire	O
storage	O
system	O
is	O
used	O
to	O
make	O
one	O
array	O
.	O
</s>
<s>
When	O
a	O
disk	O
fails	O
,	O
erased	O
data	O
is	O
rebuilt	O
using	O
all	O
the	O
operational	O
disks	O
in	O
the	O
array	O
,	O
the	O
bandwidth	O
of	O
which	O
is	O
greater	O
than	O
that	O
of	O
the	O
fewer	O
disks	O
of	O
a	O
conventional	O
RAID	B-Architecture
group	O
.	O
</s>
<s>
The	O
decrease	O
in	O
declustered	O
rebuild	O
impact	O
and	O
client	O
overhead	O
can	O
be	O
a	O
factor	O
of	O
three	O
to	O
four	O
times	O
less	O
than	O
a	O
conventional	O
RAID	B-Architecture
.	O
</s>
<s>
File	B-Application
system	I-Application
performance	O
becomes	O
less	O
dependent	O
upon	O
the	O
speed	O
of	O
any	O
single	O
rebuilding	O
storage	O
array	O
.	O
</s>
<s>
Dynamic	O
disk	O
pooling	O
(	O
DDP	O
)	O
,	O
also	O
known	O
as	O
D-RAID	O
,	O
maintains	O
performance	O
even	O
when	O
up	O
to	O
2	O
drives	O
fail	O
simultaneously	O
.	O
</s>
<s>
DDP	O
is	O
a	O
high	O
performance	O
type	O
of	O
declustered	O
RAID	B-Architecture
.	O
</s>
