<s>
NonStop	B-Operating_System
Clusters	I-Operating_System
(	O
NSC	O
)	O
was	O
an	O
add-on	O
package	O
for	O
SCO	B-Operating_System
UnixWare	I-Operating_System
that	O
allowed	O
creation	O
of	O
fault-tolerant	B-General_Concept
single-system	B-Operating_System
image	I-Operating_System
clusters	O
of	O
machines	O
running	O
UnixWare	B-Operating_System
.	O
</s>
<s>
NSC	O
was	O
one	O
of	O
the	O
first	O
commercially	O
available	O
highly	B-General_Concept
available	I-General_Concept
clustering	O
solutions	O
for	O
commodity	B-Device
hardware	I-Device
.	O
</s>
<s>
NSC	O
provided	O
a	O
full	O
single-system	B-Operating_System
image	I-Operating_System
cluster	B-Architecture
:	O
</s>
<s>
Process	O
migrationProcesses	O
started	O
on	O
any	O
node	O
in	O
the	O
cluster	B-Architecture
could	O
be	O
migrated	O
to	O
any	O
other	O
node	O
.	O
</s>
<s>
Migration	O
could	O
be	O
either	O
manual	O
or	O
automatic	O
(	O
for	O
load	B-Application
balancing	I-Application
)	O
.	O
</s>
<s>
Single	O
process	O
spaceAll	O
processes	O
were	O
visible	O
from	O
all	O
nodes	O
in	O
the	O
cluster	B-Architecture
.	O
</s>
<s>
Single	O
I/O	O
spaceAll	O
I/O	O
devices	O
were	O
available	O
from	O
any	O
node	O
in	O
the	O
cluster	B-Architecture
.	O
</s>
<s>
A	O
partition	O
on	O
a	O
SCSI	B-Architecture
disk	O
on	O
node	O
2	O
might	O
be	O
/dev/rdsk/n2c3b0t4d0s3	O
.	O
</s>
<s>
Single	O
IPC	O
spaceThe	O
standard	O
UnixWare	B-Operating_System
IPC	O
mechanisms	O
(	O
shared	B-Operating_System
memory	I-Operating_System
,	O
semaphores	B-Operating_System
,	O
message	O
queues	O
,	O
Unix	B-Protocol
domain	I-Protocol
sockets	I-Protocol
)	O
were	O
all	O
available	O
for	O
communication	O
between	O
processes	O
running	O
on	O
any	O
node	O
.	O
</s>
<s>
Cluster	B-Architecture
virtual	O
IP	O
addressNSC	O
provided	O
a	O
single	O
IP	O
address	O
for	O
access	O
to	O
the	O
cluster	B-Architecture
from	O
other	O
systems	O
.	O
</s>
<s>
Incoming	O
connections	O
were	O
load-balanced	B-Application
between	O
the	O
available	O
cluster	B-Architecture
nodes	O
.	O
</s>
<s>
The	O
NSC	O
system	O
was	O
designed	O
for	O
high	B-General_Concept
availability	I-General_Concept
—	O
all	O
system	O
services	O
were	O
either	O
redundant	O
or	O
would	O
fail-over	O
from	O
one	O
node	O
to	O
another	O
in	O
the	O
advent	O
of	O
a	O
node	O
crash	O
.	O
</s>
<s>
The	O
disk	O
subsystem	O
was	O
either	O
accessible	O
from	O
multiple	O
nodes	O
(	O
using	O
a	O
Fibre	B-Architecture
Channel	I-Architecture
SAN	O
or	O
dual-ported	O
SCSI	B-Architecture
)	O
or	O
used	O
cross-node	O
mirroring	B-General_Concept
in	O
a	O
similar	O
fashion	O
to	O
DRBD	B-Application
.	O
</s>
<s>
Initially	O
NSC	O
was	O
developed	O
for	O
the	O
Compaq	O
Integrity	O
XC	O
packaged	O
cluster	B-Architecture
,	O
consisting	O
of	O
between	O
two	O
and	O
six	O
Compaq	B-Device
ProLiant	I-Device
servers	O
and	O
one	O
or	O
two	O
Compaq	O
ServerNet	B-General_Concept
switches	O
to	O
provide	O
the	O
cluster	B-Architecture
interconnect	O
inter-node	O
communication	O
path	O
.	O
</s>
<s>
In	O
2000	O
NSC	O
was	O
modified	O
to	O
allow	O
standard	O
Fast	B-Protocol
Ethernet	I-Protocol
and	O
later	O
Gigabit	O
Ethernet	O
switches	O
as	O
the	O
cluster	B-Architecture
interconnect	O
and	O
commercialized	O
by	O
SCO	O
as	O
UnixWare	B-Operating_System
NonStop	I-Operating_System
Clusters	I-Operating_System
7.1.1	O
+IP	O
.	O
</s>
<s>
This	O
release	O
of	O
NSC	O
was	O
available	O
on	O
commodity	O
PC	O
hardware	O
,	O
although	O
SCO	O
recommended	O
that	O
systems	O
with	O
more	O
than	O
two	O
nodes	O
used	O
the	O
ServerNet	B-General_Concept
interconnect	O
.	O
</s>
<s>
but	O
this	O
was	O
not	O
to	O
be	O
,	O
Caldera	O
Systems	O
ceased	O
distribution	O
of	O
NSC	O
,	O
replacing	O
it	O
by	O
the	O
Reliant	O
HA	O
clustering	O
solution	O
and	O
in	O
May	O
2001	O
Compaq	O
announced	O
that	O
it	O
would	O
release	O
a	O
GPLed	O
version	O
of	O
the	O
NSC	O
code	O
,	O
which	O
eventually	O
became	O
OpenSSI	B-Operating_System
.	O
</s>
