<s>
DRBD	B-Application
is	O
a	O
distributed	O
replicated	O
storage	B-General_Concept
system	I-General_Concept
for	O
the	O
Linux	B-Application
platform	O
.	O
</s>
<s>
It	O
is	O
implemented	O
as	O
a	O
kernel	O
driver	O
,	O
several	O
userspace	B-Operating_System
management	O
applications	O
,	O
and	O
some	O
shell	B-Language
scripts	I-Language
.	O
</s>
<s>
DRBD	B-Application
is	O
traditionally	O
used	O
in	O
high	O
availability	O
(	O
HA	O
)	O
computer	B-Architecture
clusters	I-Architecture
,	O
but	O
beginning	O
with	O
DRBD	B-Application
version	O
9	O
,	O
it	O
can	O
also	O
be	O
used	O
to	O
create	O
larger	O
software	O
defined	O
storage	O
pools	O
with	O
a	O
focus	O
on	O
cloud	O
integration	O
.	O
</s>
<s>
A	O
DRBD	B-Application
device	O
is	O
a	O
DRBD	B-Application
block	O
device	O
that	O
refers	O
to	O
a	O
logical	O
block	O
device	O
in	O
a	O
logical	O
volume	O
schema	O
.	O
</s>
<s>
The	O
DRBD	B-Application
software	O
is	O
free	B-Application
software	I-Application
released	O
under	O
the	O
terms	O
of	O
the	O
GNU	B-License
General	I-License
Public	I-License
License	I-License
version	O
2	O
.	O
</s>
<s>
DRBD	B-Application
is	O
part	O
of	O
the	O
Lisog	O
open	O
source	O
stack	O
initiative	O
.	O
</s>
<s>
DRBD	B-Application
layers	O
logical	O
block	O
devices	O
(	O
conventionally	O
named	O
/dev/drbdX	O
,	O
where	O
X	O
is	O
the	O
device	O
minor	O
number	O
)	O
over	O
existing	O
local	O
block	O
devices	O
on	O
participating	O
cluster	O
nodes	O
.	O
</s>
<s>
All	O
read	O
I/O	B-General_Concept
is	O
performed	O
locally	O
unless	O
read-balancing	O
is	O
configured	O
.	O
</s>
<s>
Should	O
the	O
primary	O
node	O
fail	O
,	O
a	O
cluster	B-Operating_System
management	I-Operating_System
process	I-Operating_System
promotes	O
the	O
secondary	O
node	O
to	O
a	O
primary	O
state	O
.	O
</s>
<s>
This	O
transition	O
may	O
require	O
a	O
subsequent	O
verification	O
of	O
the	O
integrity	O
of	O
the	O
file	B-Application
system	I-Application
stacked	O
on	O
top	O
of	O
DRBD	B-Application
,	O
by	O
way	O
of	O
a	O
filesystem	B-Device
check	I-Device
or	O
a	O
journal	B-Application
replay	O
.	O
</s>
<s>
DRBD	B-Application
's	O
synchronization	O
algorithm	O
is	O
efficient	O
in	O
the	O
sense	O
that	O
only	O
those	O
blocks	O
that	O
were	O
changed	O
during	O
the	O
outage	O
must	O
be	O
resynchronized	O
,	O
rather	O
than	O
the	O
device	O
in	O
its	O
entirety	O
.	O
</s>
<s>
DRBD	B-Application
is	O
often	O
deployed	O
together	O
with	O
the	O
Pacemaker	B-Operating_System
or	O
Heartbeat	B-Operating_System
cluster	O
resource	O
managers	O
,	O
although	O
it	O
does	O
integrate	O
with	O
other	O
cluster	B-Operating_System
management	I-Operating_System
frameworks	O
.	O
</s>
<s>
It	O
integrates	O
with	O
virtualization	B-General_Concept
solutions	O
such	O
as	O
Xen	B-Operating_System
,	O
and	O
may	O
be	O
used	O
both	O
below	O
and	O
on	O
top	O
of	O
the	O
Linux	B-Application
LVM	B-Application
stack	O
.	O
</s>
<s>
DRBD	B-Application
allows	O
for	O
load-balancing	B-Application
configurations	O
,	O
allowing	O
both	O
nodes	O
to	O
access	O
a	O
particular	O
DRBD	B-Application
in	O
read/write	O
mode	O
with	O
shared	B-General_Concept
storage	I-General_Concept
semantics	O
.	O
</s>
<s>
A	O
multiple	O
primary	O
(	O
multiple	O
read/write	O
nodes	O
)	O
configuration	O
requires	O
the	O
use	O
of	O
a	O
distributed	B-Operating_System
lock	I-Operating_System
manager	I-Operating_System
.	O
</s>
<s>
Conventional	O
computer	B-Architecture
cluster	I-Architecture
systems	O
typically	O
use	O
some	O
sort	O
of	O
shared	B-General_Concept
storage	I-General_Concept
for	O
data	O
being	O
used	O
by	O
cluster	O
resources	O
.	O
</s>
<s>
This	O
approach	O
has	O
a	O
number	O
of	O
disadvantages	O
,	O
which	O
DRBD	B-Application
may	O
help	O
offset	O
:	O
</s>
<s>
Shared	B-General_Concept
storage	I-General_Concept
resources	O
must	O
typically	O
be	O
accessed	O
over	O
a	O
storage	O
area	O
network	O
or	O
on	O
a	O
network	B-Application
attached	I-Application
storage	I-Application
server	O
,	O
which	O
creates	O
some	O
overhead	O
in	O
read	O
I/O	B-General_Concept
.	O
</s>
<s>
In	O
DRBD	B-Application
that	O
overhead	O
is	O
reduced	O
as	O
all	O
read	O
operations	O
are	O
carried	O
out	O
locally	O
.	O
</s>
<s>
Shared	B-General_Concept
storage	I-General_Concept
is	O
usually	O
expensive	O
and	O
consumes	O
more	O
space	O
(	O
2U	O
and	O
more	O
)	O
and	O
power	O
.	O
</s>
<s>
DRBD	B-Application
allows	O
for	O
an	O
HA	O
setup	O
with	O
only	O
2	O
machines	O
.	O
</s>
<s>
Shared	B-General_Concept
storage	I-General_Concept
is	O
not	O
necessarily	O
highly	O
available	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
single	O
storage	O
area	O
network	O
accessed	O
by	O
multiple	O
virtualization	B-General_Concept
hosts	O
is	O
considered	O
shared	B-General_Concept
storage	I-General_Concept
,	O
but	O
is	O
not	O
considered	O
highly	O
available	O
at	O
the	O
storage	O
level	O
-	O
if	O
that	O
single	O
storage	O
area	O
network	O
fails	O
,	O
neither	O
host	O
within	O
the	O
cluster	O
can	O
access	O
the	O
shared	B-General_Concept
storage	I-General_Concept
.	O
</s>
<s>
DRBD	B-Application
allows	O
for	O
a	O
storage	O
target	O
that	O
is	O
both	O
shared	O
and	O
highly	O
available	O
.	O
</s>
<s>
A	O
disadvantage	O
is	O
the	O
lower	O
time	O
required	O
to	O
write	O
directly	O
to	O
a	O
shared	B-General_Concept
storage	I-General_Concept
device	O
than	O
to	O
route	O
the	O
write	O
through	O
the	O
other	O
node	O
.	O
</s>
<s>
DRBD	B-Application
bears	O
a	O
superficial	O
similarity	O
to	O
RAID-1	O
in	O
that	O
it	O
involves	O
a	O
copy	O
of	O
data	O
on	O
two	O
storage	O
devices	O
,	O
such	O
that	O
if	O
one	O
fails	O
,	O
the	O
data	O
on	O
the	O
other	O
can	O
be	O
used	O
.	O
</s>
<s>
In	O
contrast	O
,	O
with	O
DRBD	B-Application
there	O
are	O
two	O
instances	O
of	O
the	O
application	O
,	O
and	O
each	O
can	O
read	O
only	O
from	O
one	O
of	O
the	O
two	O
storage	O
devices	O
.	O
</s>
<s>
Conversely	O
,	O
in	O
RAID	O
,	O
if	O
the	O
single	O
application	O
instance	O
fails	O
,	O
the	O
information	O
on	O
the	O
two	O
storage	O
devices	O
is	O
effectively	O
unusable	O
,	O
but	O
in	O
DRBD	B-Application
,	O
the	O
other	O
application	O
instance	O
can	O
take	O
over	O
.	O
</s>
<s>
Operating	O
within	O
the	O
Linux	B-Application
kernel	O
's	O
block	O
layer	O
,	O
DRBD	B-Application
is	O
essentially	O
workload	O
agnostic	O
.	O
</s>
<s>
A	O
DRBD	B-Application
can	O
be	O
used	O
as	O
the	O
basis	O
of	O
:	O
</s>
<s>
A	O
conventional	O
file	B-Application
system	I-Application
(	O
this	O
is	O
the	O
canonical	O
example	O
)	O
,	O
</s>
<s>
a	O
shared	O
disk	B-Application
file	I-Application
system	I-Application
such	O
as	O
GFS2	B-Application
or	O
OCFS2	B-Application
,	O
</s>
<s>
another	O
logical	O
block	O
device	O
(	O
as	O
used	O
in	O
LVM	B-Application
,	O
for	O
example	O
)	O
,	O
</s>
<s>
DRBD-based	O
clusters	O
are	O
often	O
employed	O
for	O
adding	O
synchronous	O
replication	O
and	O
high	O
availability	O
to	O
file	B-Application
servers	I-Application
,	O
relational	O
databases	O
(	O
such	O
as	O
MySQL	B-Application
)	O
,	O
and	O
many	O
other	O
workloads	O
.	O
</s>
<s>
DRBD	B-Application
's	O
authors	O
originally	O
submitted	O
the	O
software	O
to	O
the	O
Linux	B-Application
kernel	O
community	O
in	O
July	O
2007	O
,	O
for	O
possible	O
inclusion	O
in	O
the	O
canonical	O
kernel.org	O
version	O
of	O
the	O
Linux	B-Application
kernel	O
.	O
</s>
<s>
After	O
a	O
lengthy	O
review	O
and	O
several	O
discussions	O
,	O
Linus	O
Torvalds	O
agreed	O
to	O
have	O
DRBD	B-Application
as	O
part	O
of	O
the	O
official	O
Linux	B-Application
kernel	O
.	O
</s>
<s>
DRBD	B-Application
was	O
merged	O
on	O
8	O
December	O
2009	O
during	O
the	O
"	O
merge	O
window	O
"	O
for	O
Linux	B-Application
kernel	O
version	O
2.6.33	O
.	O
</s>
