<s>
A	O
VMScluster	B-Operating_System
,	O
originally	O
known	O
as	O
a	O
VAXcluster	B-Operating_System
,	O
is	O
a	O
computer	B-Architecture
cluster	I-Architecture
involving	O
a	O
group	O
of	O
computers	O
running	O
the	O
OpenVMS	B-Operating_System
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Whereas	O
tightly	O
coupled	O
multiprocessor	O
systems	O
run	O
a	O
single	O
copy	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
,	O
a	O
VMScluster	B-Operating_System
is	O
loosely	O
coupled	O
:	O
each	O
machine	O
runs	O
its	O
own	O
copy	O
of	O
OpenVMS	B-Operating_System
,	O
but	O
the	O
disk	O
storage	O
,	O
lock	B-Operating_System
manager	I-Operating_System
,	O
and	O
security	O
domain	O
are	O
all	O
cluster-wide	O
,	O
providing	O
a	O
single	B-Operating_System
system	I-Operating_System
image	I-Operating_System
abstraction	O
.	O
</s>
<s>
Machines	O
can	O
join	O
or	O
leave	O
a	O
VMScluster	B-Operating_System
without	O
affecting	O
the	O
rest	O
of	O
the	O
cluster	O
.	O
</s>
<s>
For	O
enhanced	O
availability	O
,	O
VMSclusters	B-Operating_System
support	O
the	O
use	O
of	O
dual-ported	O
disks	O
connected	O
to	O
two	O
machines	O
or	O
storage	O
controllers	O
simultaneously	O
.	O
</s>
<s>
Digital	O
Equipment	O
Corporation	O
(	O
DEC	O
)	O
first	O
announced	O
VAXclusters	B-Operating_System
in	O
May	O
1983	O
.	O
</s>
<s>
At	O
this	O
stage	O
,	O
clustering	O
required	O
specialised	O
communications	O
hardware	O
,	O
as	O
well	O
as	O
some	O
major	O
changes	O
to	O
low-level	O
subsystems	O
in	O
VMS	B-Operating_System
.	O
</s>
<s>
VAXcluster	B-Operating_System
support	O
was	O
first	O
added	O
in	O
VAX/VMS	B-Operating_System
V4.0	O
,	O
which	O
was	O
released	O
in	O
1984	O
.	O
</s>
<s>
At	O
the	O
center	O
of	O
each	O
cluster	O
was	O
a	O
star	O
coupler	O
,	O
to	O
which	O
every	O
node	O
(	O
computer	O
)	O
and	O
data	B-Architecture
storage	I-Architecture
device	I-Architecture
in	O
the	O
cluster	O
was	O
connected	O
by	O
one	O
or	O
two	O
pairs	O
of	O
CI	O
cables	O
.	O
</s>
<s>
Each	O
CI	O
cable	O
connected	O
to	O
its	O
computer	O
via	O
a	O
CI	O
Port	O
,	O
which	O
could	O
send	O
and	O
receive	O
packets	B-Protocol
without	O
any	O
CPU	O
involvement	O
.	O
</s>
<s>
To	O
send	O
a	O
packet	B-Protocol
,	O
a	O
CPU	O
had	O
only	O
to	O
create	O
a	O
small	O
data	O
structure	O
in	O
memory	O
and	O
append	O
it	O
to	O
a	O
"	O
send	O
"	O
queue	O
;	O
similarly	O
,	O
the	O
CI	O
Port	O
would	O
append	O
each	O
incoming	O
message	O
to	O
a	O
"	O
receive	O
"	O
queue	O
.	O
</s>
<s>
The	O
closely	O
related	O
Mass	B-Device
Storage	I-Device
Control	I-Device
Protocol	I-Device
(	O
MSCP	O
)	O
allowed	O
similarly	O
high	O
performance	O
from	O
the	O
mass	O
storage	O
subsystem	O
.	O
</s>
<s>
In	O
addition	O
,	O
MSCP	O
packets	B-Protocol
were	O
very	O
easily	O
transported	O
over	O
the	O
CI	O
allowing	O
remote	O
access	O
to	O
storage	O
devices	O
.	O
</s>
<s>
VAXclustering	B-Operating_System
was	O
the	O
first	O
clustering	O
system	O
to	O
achieve	O
commercial	O
success	O
,	O
and	O
was	O
a	O
major	O
selling	O
point	O
for	O
VAX	O
systems	O
.	O
</s>
<s>
In	O
1986	O
,	O
DEC	O
added	O
VAXclustering	B-Operating_System
support	O
to	O
their	O
MicroVAX	B-Device
minicomputers	O
,	O
running	O
over	O
Ethernet	O
instead	O
of	O
special-purpose	O
hardware	O
.	O
</s>
<s>
While	O
not	O
giving	O
the	O
high-availability	B-General_Concept
advantages	O
of	O
the	O
CI	O
hardware	O
,	O
these	O
Local	O
Area	O
VAXclusters	B-Operating_System
(	O
LAVc	O
)	O
provided	O
an	O
attractive	O
expansion	O
path	O
for	O
buyers	O
of	O
low-end	O
minicomputers	O
.	O
</s>
<s>
LAVc	O
also	O
allowed	O
diskless	B-Device
satellite	O
nodes	O
to	O
bootstrap	O
over	O
the	O
network	O
using	O
the	O
system	O
disk	O
of	O
a	O
bootnode	O
.	O
</s>
<s>
Later	O
versions	O
of	O
OpenVMS	B-Operating_System
(	O
V5.0	O
and	O
later	O
)	O
supported	O
"	O
mixed	O
interconnect	O
"	O
VAXclusters	B-Operating_System
(	O
using	O
both	O
CI	O
and	O
Ethernet	O
)	O
,	O
and	O
VAXclustering	B-Operating_System
over	O
DSSI	O
(	O
Digital	B-Architecture
Systems	I-Architecture
and	I-Architecture
Storage	I-Architecture
Interconnect	I-Architecture
)	O
,	O
SCSI	B-Architecture
and	O
FDDI	B-Protocol
,	O
among	O
other	O
transports	O
.	O
</s>
<s>
This	O
allowed	O
the	O
creation	O
of	O
disaster-tolerant	O
clusters	O
;	O
by	O
locating	O
the	O
single	O
VAXcluster	B-Operating_System
in	O
several	O
diverse	O
geographical	O
areas	O
,	O
the	O
cluster	O
could	O
survive	O
infrastructure	O
failures	O
and	O
natural	O
disasters	O
.	O
</s>
<s>
VAXclustering	B-Operating_System
was	O
greatly	O
aided	O
by	O
the	O
introduction	O
of	O
terminal	B-Device
servers	I-Device
using	O
the	O
LAT	B-General_Concept
protocol	I-General_Concept
.	O
</s>
<s>
This	O
made	O
it	O
much	O
simpler	O
to	O
accomplish	O
fail	B-General_Concept
over	I-General_Concept
of	O
the	O
user	O
terminals	O
from	O
one	O
node	O
of	O
the	O
cluster	O
to	O
another	O
.	O
</s>
<s>
Support	O
for	O
clustering	O
over	O
TCP/IP	O
was	O
added	O
in	O
OpenVMS	B-Operating_System
version	O
8.4	O
,	O
which	O
was	O
released	O
in	O
2010	O
.	O
</s>
<s>
With	O
Gigabit	O
Ethernet	O
now	O
common	O
and	O
10	O
Gigabit	O
Ethernet	O
being	O
introduced	O
,	O
standard	O
networking	O
cables	O
and	O
cards	O
are	O
quite	O
sufficient	O
to	O
support	O
VMSclustering	B-Operating_System
.	O
</s>
<s>
OpenVMS	B-Operating_System
supports	O
up	O
to	O
96	O
nodes	O
in	O
a	O
single	O
cluster	O
,	O
and	O
allows	O
mixed-architecture	O
clusters	O
,	O
where	O
VAX	O
and	O
Alpha	O
systems	O
,	O
or	O
Alpha	O
and	O
Itanium	O
systems	O
can	O
co-exist	O
in	O
a	O
single	O
cluster	O
(	O
Various	O
organizations	O
have	O
demonstrated	O
triple-architecture	O
clusters	O
and	O
cluster	O
configurations	O
with	O
up	O
to	O
150	O
nodes	O
,	O
but	O
these	O
configurations	O
are	O
not	O
officially	O
supported	O
)	O
.	O
</s>
<s>
Unlike	O
many	O
other	O
clustering	O
solutions	O
,	O
VMScluster	B-Operating_System
offers	O
transparent	O
and	O
fully	O
distributed	O
read-write	O
with	O
record-level	O
locking	O
,	O
which	O
means	O
that	O
the	O
same	O
disk	O
and	O
even	O
the	O
same	O
file	O
can	O
be	O
accessed	O
by	O
several	O
cluster	O
nodes	O
at	O
once	O
;	O
the	O
locking	O
occurs	O
only	O
at	O
the	O
level	O
of	O
a	O
single	O
record	O
of	O
a	O
file	O
,	O
which	O
would	O
usually	O
be	O
one	O
line	O
of	O
text	O
or	O
a	O
single	O
record	O
in	O
a	O
database	O
.	O
</s>
<s>
This	O
allows	O
the	O
construction	O
of	O
high-availability	B-General_Concept
multiply	O
redundant	O
database	O
servers	O
.	O
</s>
<s>
Full	O
access	O
into	O
the	O
distributed	B-Operating_System
lock	I-Operating_System
manager	I-Operating_System
(	O
DLM	O
)	O
is	O
available	O
to	O
application	O
programmers	O
,	O
and	O
this	O
allows	O
applications	O
to	O
coordinate	O
arbitrary	O
resources	O
and	O
activities	O
across	O
all	O
cluster	O
nodes	O
.	O
</s>
