<s>
OpenSSI	B-Operating_System
is	O
an	O
open-source	B-Application
single-system	B-Operating_System
image	I-Operating_System
clustering	B-Architecture
system	O
.	O
</s>
<s>
OpenSSI	B-Operating_System
is	O
based	O
on	O
the	O
Linux	B-Application
operating	I-Application
system	I-Application
and	O
was	O
released	O
as	O
an	O
open	O
source	O
project	O
by	O
Compaq	O
in	O
2001	O
.	O
</s>
<s>
It	O
is	O
the	O
final	O
stage	O
of	O
a	O
long	O
process	O
of	O
development	O
,	O
stretching	O
back	O
to	O
LOCUS	B-Operating_System
,	O
developed	O
in	O
the	O
early	O
1980s	O
.	O
</s>
<s>
OpenSSI	B-Operating_System
allows	O
a	O
cluster	O
of	O
individual	O
computers	O
(	O
nodes	O
)	O
to	O
be	O
treated	O
as	O
one	O
large	O
system	O
.	O
</s>
<s>
OpenSSI	B-Operating_System
is	O
designed	O
to	O
be	O
used	O
for	O
both	O
high	B-Architecture
performance	I-Architecture
and	O
high	O
availability	O
clusters	O
.	O
</s>
<s>
It	O
is	O
possible	O
to	O
create	O
an	O
OpenSSI	B-Operating_System
cluster	O
with	O
no	B-Architecture
single	I-Architecture
point	I-Architecture
of	I-Architecture
failure	I-Architecture
,	O
for	O
example	O
the	O
file	O
system	O
can	O
be	O
mirrored	O
between	O
two	O
nodes	O
,	O
so	O
if	O
one	O
node	O
crashes	O
the	O
process	O
accessing	O
the	O
file	O
will	O
fail	O
over	O
to	O
the	O
other	O
node	O
.	O
</s>
<s>
OpenSSI	B-Operating_System
provides	O
a	O
single	O
process	O
space	O
–	O
every	O
process	O
is	O
visible	O
from	O
every	O
node	O
,	O
and	O
can	O
be	O
managed	O
from	O
any	O
node	O
using	O
the	O
normal	O
Linux	B-Application
commands	O
(	O
ps	O
,	O
kill	O
,	O
renice	O
and	O
so	O
on	O
)	O
.	O
</s>
<s>
The	O
Linux	B-Application
/proc	O
virtual	O
filesystem	O
shows	O
all	O
running	O
processes	O
on	O
all	O
nodes	O
.	O
</s>
<s>
The	O
implementation	O
of	O
the	O
single	O
process	O
space	O
is	O
accomplished	O
using	O
the	O
VPROC	O
abstraction	O
invented	O
by	O
Locus	B-Operating_System
for	O
the	O
OSF/1	O
AD	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
OpenSSI	B-Operating_System
allows	O
migration	O
of	O
running	O
processes	O
between	O
nodes	O
.	O
</s>
<s>
Processes	O
can	O
be	O
manually	O
migrated	O
,	O
either	O
by	O
the	O
process	O
calling	O
the	O
special	O
OpenSSI	B-Operating_System
migrate(2 )	O
system	O
call	O
,	O
or	O
by	O
writing	O
a	O
node	O
number	O
to	O
a	O
special	O
file	O
in	O
the	O
processes	O
/proc	O
directory	O
.	O
</s>
<s>
OpenSSI	B-Operating_System
uses	O
an	O
algorithm	O
developed	O
by	O
the	O
MOSIX	B-Operating_System
project	O
for	O
determining	O
the	O
load	O
on	O
each	O
node	O
.	O
</s>
<s>
OpenSSI	B-Operating_System
provides	O
a	O
single	O
root	O
for	O
the	O
cluster	O
-	O
from	O
any	O
node	O
the	O
same	O
files	O
and	O
directories	O
are	O
available	O
.	O
</s>
<s>
OpenSSI	B-Operating_System
uses	O
several	O
mechanisms	O
to	O
provide	O
the	O
single	O
root	O
–	O
CFS	O
(	O
the	O
OpenSSI	B-Operating_System
Cluster	B-Application
File	I-Application
System	I-Application
)	O
,	O
SAN	O
cluster	O
filesystems	O
and	O
parallel	O
mounts	O
of	O
network	B-Protocol
file	I-Protocol
systems	I-Protocol
.	O
</s>
<s>
OpenSSI	B-Operating_System
uses	O
the	O
context	O
dependent	O
symbolic	O
link	O
(	O
CDSL	O
)	O
feature	O
,	O
inspired	O
by	O
HP	O
's	O
TruCluster	B-General_Concept
system	O
,	O
to	O
allow	O
access	O
to	O
node-specific	O
files	O
in	O
a	O
manner	O
transparent	O
to	O
non	O
cluster-aware	B-Operating_System
applications	I-Operating_System
.	O
</s>
<s>
CFS	O
,	O
the	O
OpenSSI	B-Operating_System
Cluster	B-Application
File	I-Application
System	I-Application
provides	O
transparent	O
inter-node	O
access	O
to	O
an	O
underlying	O
real	O
file	O
system	O
on	O
one	O
node	O
.	O
</s>
<s>
The	O
OpenSSI	B-Operating_System
CFS	O
implementation	O
is	O
remarkably	O
similar	O
to	O
that	O
used	O
by	O
HP	O
TruCluster	B-General_Concept
.	O
</s>
<s>
CFS	O
is	O
also	O
used	O
to	O
co-ordinate	O
access	O
to	O
shared	B-Operating_System
memory	I-Operating_System
segments	O
.	O
</s>
<s>
CFS	O
can	O
be	O
used	O
in	O
a	O
fault	O
tolerant	O
system	O
by	O
using	O
shared	O
disk	O
subsystems	O
(	O
dual	O
ported	O
SCSI	B-Architecture
or	O
SAN	O
)	O
,	O
or	O
by	O
using	O
DRBD	B-Application
.	O
</s>
<s>
OpenSSI	B-Operating_System
can	O
use	O
SAN	O
based	O
clustered	B-Application
file	I-Application
systems	I-Application
for	O
its	O
root	O
provided	O
they	O
provide	O
a	O
POSIX	O
compatible	O
file	O
system	O
interface	O
.	O
</s>
<s>
Currently	O
Lustre	B-Application
and	O
GFS	B-Application
have	O
been	O
tested	O
.	O
</s>
<s>
With	O
a	O
clustered	B-Application
file	I-Application
system	I-Application
,	O
each	O
node	O
mounts	O
the	O
file	O
system	O
in	O
parallel	O
and	O
access	O
to	O
the	O
files	O
goes	O
directly	O
from	O
the	O
node	O
to	O
the	O
file	O
system	O
.	O
</s>
<s>
OpenSSI	B-Operating_System
mounts	O
NFS	B-Protocol
files	O
systems	O
in	O
parallel	O
on	O
each	O
node	O
.	O
</s>
<s>
Every	O
node	O
accesses	O
the	O
NFS	B-Protocol
server	O
directly	O
.	O
</s>
<s>
OpenSSI	B-Operating_System
provides	O
cluster-wide	O
access	O
to	O
all	O
I/O	O
devices	O
on	O
the	O
system	O
,	O
with	O
some	O
limitations	O
-	O
it	O
is	O
not	O
possible	O
for	O
a	O
node	O
to	O
mount	O
a	O
block	O
device	O
from	O
another	O
node	O
.	O
</s>
<s>
The	O
udev	B-Application
device	O
manager	O
is	O
used	O
to	O
manage	O
the	O
/dev	O
directory	O
.	O
</s>
<s>
Each	O
node	O
runs	O
its	O
own	O
copy	O
of	O
udev	B-Application
to	O
create	O
the	O
appropriate	O
device	O
nodes	O
in	O
a	O
subdirectory	O
of	O
/dev	O
,	O
/dev/1	O
for	O
node	O
1	O
,	O
/dev/2	O
for	O
node	O
2	O
and	O
so	O
on	O
.	O
</s>
<s>
OpenSSI	B-Operating_System
provides	O
internode	O
access	O
to	O
all	O
the	O
standard	O
Linux	B-Application
inter-process	O
communication	O
mechanisms	O
,	O
shared	B-Operating_System
memory	I-Operating_System
,	O
semaphores	B-Operating_System
,	O
SYSV	B-Operating_System
message	O
queues	O
,	O
pipes	O
and	O
Unix	B-Protocol
domain	I-Protocol
sockets	I-Protocol
.	O
</s>
<s>
In	O
order	O
to	O
implement	O
cluster	O
wide	O
shared	B-Operating_System
memory	I-Operating_System
–	O
distributed	B-Operating_System
shared	I-Operating_System
memory	I-Operating_System
–	O
OpenSSI	B-Operating_System
uses	O
the	O
CFS	O
token	O
system	O
.	O
</s>
<s>
OpenSSI	B-Operating_System
uses	O
LVS	B-Operating_System
to	O
provide	O
fault-tolerant	O
load	O
balanced	O
IP	B-Protocol
services	O
.	O
</s>
<s>
The	O
OpenSSI	B-Operating_System
software	O
is	O
available	O
for	O
various	O
Linux	B-Application
distributions	I-Application
.	O
</s>
<s>
The	O
OpenSSI	B-Operating_System
kernel	B-Operating_System
is	O
distribution	O
independent	O
but	O
various	O
distribution	O
specific	O
Linux	B-Application
user	O
level	O
systems	O
need	O
to	O
be	O
modified	O
,	O
for	O
example	O
the	O
init	B-Operating_System
process	O
and	O
the	O
system	B-Device
startup	I-Device
scripts	O
.	O
</s>
<s>
In	O
2008	O
,	O
work	O
was	O
in	O
progress	O
to	O
port	O
OpenSSI	B-Operating_System
to	O
Debian	O
Etch	O
and	O
Lenny	O
.	O
</s>
<s>
The	O
origins	O
of	O
OpenSSI	B-Operating_System
date	O
back	O
to	O
the	O
early	O
1980s	O
,	O
when	O
the	O
LOCUS	B-Operating_System
distributed	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
was	O
developed	O
at	O
UCLA	O
.	O
</s>
<s>
The	O
team	O
that	O
developed	O
LOCUS	B-Operating_System
went	O
on	O
to	O
form	O
the	O
Locus	B-Operating_System
Computing	O
Corporation	O
and	O
produced	O
various	O
versions	O
of	O
the	O
LOCUS	B-Operating_System
technology	O
under	O
several	O
names	O
,	O
culminating	O
in	O
the	O
development	O
of	O
the	O
UnixWare	B-Operating_System
NonStop	I-Operating_System
Clusters	I-Operating_System
product	O
at	O
Tandem	O
Computers	O
,	O
which	O
had	O
by	O
that	O
time	O
acquired	O
the	O
LOCUS	B-Operating_System
team	O
and	O
rights	O
to	O
the	O
technology	O
.	O
</s>
<s>
NonStop	B-Operating_System
Clusters	I-Operating_System
for	O
Unixware	O
was	O
commercialized	O
by	O
SCO	O
as	O
an	O
add-on	O
for	O
UnixWare	O
.	O
</s>
<s>
When	O
SCO	O
stopped	O
selling	O
NonStop	B-Operating_System
Clusters	I-Operating_System
,	O
the	O
former	O
Locus	B-Operating_System
team	O
,	O
now	O
working	O
for	O
Compaq	O
(	O
which	O
had	O
acquired	O
Tandem	O
in	O
the	O
interim	O
)	O
,	O
ported	O
the	O
NonStop	B-Operating_System
Clusters	I-Operating_System
code	O
to	O
Linux	B-Application
and	O
released	O
it	O
as	O
open	O
source	O
.	O
</s>
<s>
The	O
team	O
at	O
Compaq	O
continued	O
to	O
develop	O
the	O
system	O
,	O
now	O
called	O
OpenSSI	B-Operating_System
,	O
for	O
some	O
time	O
after	O
HP	O
acquired	O
Compaq	O
.	O
</s>
<s>
OpenSSI	B-Operating_System
is	O
currently	O
developed	O
by	O
an	O
independent	O
team	O
.	O
</s>
