<s>
Hierarchical	B-Application
storage	I-Application
management	I-Application
(	O
HSM	B-Application
)	O
,	O
also	O
known	O
as	O
Tiered	B-General_Concept
storage	I-General_Concept
,	O
is	O
a	O
data	B-General_Concept
storage	I-General_Concept
and	O
Data	B-General_Concept
management	I-General_Concept
technique	O
that	O
automatically	O
moves	O
data	O
between	O
high-cost	O
and	O
low-cost	O
storage	B-General_Concept
media	I-General_Concept
.	O
</s>
<s>
HSM	B-Application
systems	O
exist	O
because	O
high-speed	O
storage	O
devices	O
,	O
such	O
as	O
solid	B-Device
state	I-Device
drive	I-Device
arrays	O
,	O
are	O
more	O
expensive	O
(	O
per	O
byte	B-Application
stored	O
)	O
than	O
slower	O
devices	O
,	O
such	O
as	O
hard	B-Device
disk	I-Device
drives	I-Device
,	O
optical	B-Device
discs	I-Device
and	O
magnetic	B-Architecture
tape	I-Architecture
drives	I-Architecture
.	O
</s>
<s>
Instead	O
,	O
HSM	B-Application
systems	O
store	O
the	O
bulk	O
of	O
the	O
enterprise	O
's	O
data	O
on	O
slower	O
devices	O
,	O
and	O
then	O
copy	O
data	O
to	O
faster	O
disk	B-Device
drives	I-Device
when	O
needed	O
.	O
</s>
<s>
The	O
HSM	B-Application
system	O
monitors	O
the	O
way	O
data	O
is	O
used	O
and	O
makes	O
best	O
guesses	O
as	O
to	O
which	O
data	O
can	O
safely	O
be	O
moved	O
to	O
slower	O
devices	O
and	O
which	O
data	O
should	O
stay	O
on	O
the	O
fast	O
devices	O
.	O
</s>
<s>
HSM	B-Application
may	O
also	O
be	O
used	O
where	O
more	O
robust	O
storage	O
is	O
available	O
for	O
long-term	O
archiving	O
,	O
but	O
this	O
is	O
slow	O
to	O
access	O
.	O
</s>
<s>
This	O
may	O
be	O
as	O
simple	O
as	O
an	O
off-site	O
backup	B-Protocol
,	O
for	O
protection	O
against	O
a	O
building	O
fire	O
.	O
</s>
<s>
HSM	B-Application
is	O
a	O
long-established	O
concept	O
,	O
dating	O
back	O
to	O
the	O
beginnings	O
of	O
commercial	O
data	O
processing	O
.	O
</s>
<s>
In	O
a	O
typical	O
HSM	B-Application
scenario	O
,	O
data	O
which	O
is	O
frequently	O
used	O
are	O
stored	O
on	O
warm	O
storage	O
device	O
,	O
such	O
as	O
solid	B-Device
state	I-Device
disk	I-Device
(	O
SSD	B-Device
)	O
.	O
</s>
<s>
The	O
advantage	O
is	O
that	O
the	O
total	O
amount	O
of	O
stored	B-General_Concept
data	I-General_Concept
can	O
be	O
much	O
larger	O
than	O
the	O
capacity	O
of	O
the	O
warm	O
storage	O
device	O
,	O
but	O
since	O
only	O
rarely	O
used	O
files	O
are	O
on	O
cold	O
storage	O
,	O
most	O
users	O
will	O
usually	O
not	O
notice	O
any	O
slowdown	O
.	O
</s>
<s>
Conceptually	O
,	O
HSM	B-Application
is	O
analogous	O
to	O
the	O
cache	B-General_Concept
found	O
in	O
most	O
computer	O
CPUs	O
,	O
where	O
small	O
amounts	O
of	O
expensive	O
SRAM	B-Architecture
memory	O
running	O
at	O
very	O
high	O
speeds	O
is	O
used	O
to	O
store	O
frequently	O
used	O
data	O
,	O
but	O
the	O
least	B-General_Concept
recently	I-General_Concept
used	I-General_Concept
data	O
is	O
evicted	O
to	O
the	O
slower	O
but	O
much	O
larger	O
main	O
DRAM	O
memory	O
when	O
new	O
data	O
has	O
to	O
be	O
loaded	O
.	O
</s>
<s>
In	O
practice	O
,	O
HSM	B-Application
is	O
typically	O
performed	O
by	O
dedicated	O
software	O
,	O
such	O
as	O
IBM	B-General_Concept
Tivoli	I-General_Concept
Storage	I-General_Concept
Manager	I-General_Concept
,	O
or	O
Oracle	B-Application
's	I-Application
SAM-QFS	B-Application
.	O
</s>
<s>
magnetic	O
disk	B-Device
)	O
after	O
they	O
have	O
been	O
moved	O
to	O
a	O
lower	O
level	O
(	O
e.g.	O
</s>
<s>
optical	B-Device
media	I-Device
)	O
is	O
sometimes	O
called	O
file	B-Application
grooming	I-Application
.	O
</s>
<s>
Hierarchical	B-Application
Storage	I-Application
Manager	I-Application
(	O
HSM	B-Application
,	O
then	O
DFHSM	O
and	O
finally	O
DFSMShsm	O
)	O
was	O
first	O
implemented	O
by	O
IBM	O
on	O
March	O
31	O
,	O
1978	O
for	O
MVS	B-Application
to	O
reduce	O
the	O
cost	O
of	O
data	B-General_Concept
storage	I-General_Concept
,	O
and	O
to	O
simplify	O
the	O
retrieval	O
of	O
data	O
from	O
slower	O
media	O
.	O
</s>
<s>
HSM	B-Application
could	O
originally	O
migrate	O
datasets	O
only	O
to	O
disk	B-Device
volumes	O
and	O
virtual	O
volumes	O
on	O
a	O
IBM	B-Device
3850	I-Device
Mass	O
Storage	O
Facility	O
,	O
but	O
a	O
latter	O
release	O
supported	O
magnetic	O
tape	O
volumes	O
for	O
migration	O
level	O
2	O
(	O
ML2	O
)	O
.	O
</s>
<s>
Later	O
,	O
IBM	O
ported	O
HSM	B-Application
to	O
its	O
AIX	B-Application
operating	I-Application
system	I-Application
,	O
and	O
then	O
to	O
other	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
such	O
as	O
Solaris	B-Application
,	O
HP-UX	B-Application
and	O
Linux	B-Application
.	O
</s>
<s>
CSIRO	O
Australia	O
's	O
Division	O
of	O
Computing	O
Research	O
implemented	O
an	O
HSM	B-Application
in	O
its	O
DAD	O
(	O
Drums	O
and	O
Display	O
)	O
operating	O
system	O
with	O
its	O
Document	O
Region	O
in	O
the	O
1960s	O
,	O
with	O
copies	O
of	O
documents	O
being	O
written	O
to	O
7-track	O
tape	O
and	O
automatic	O
retrieval	O
upon	O
access	O
to	O
the	O
documents	O
.	O
</s>
<s>
HSM	B-Application
was	O
also	O
implemented	O
on	O
the	O
DEC	O
VAX/VMS	B-Operating_System
systems	O
and	O
the	O
Alpha/VMS	O
systems	O
.	O
</s>
<s>
The	O
first	O
implementation	O
date	O
should	O
be	O
readily	O
determined	O
from	O
the	O
VMS	B-Operating_System
System	O
Implementation	O
Manuals	O
or	O
the	O
VMS	B-Operating_System
Product	O
Description	O
Brochures	O
.	O
</s>
<s>
More	O
recently	O
,	O
the	O
development	O
of	O
Serial	O
ATA	O
(	O
SATA	O
)	O
disks	B-Device
has	O
created	O
a	O
significant	O
market	O
for	O
three-stage	O
HSM	B-Application
:	O
files	O
are	O
migrated	O
from	O
high-performance	O
Fibre	B-Architecture
Channel	I-Architecture
storage	O
area	O
network	O
devices	O
to	O
somewhat	O
slower	O
but	O
much	O
cheaper	O
SATA	O
disk	B-Device
arrays	O
totaling	O
several	O
terabytes	O
or	O
more	O
,	O
and	O
then	O
eventually	O
from	O
the	O
SATA	O
disks	B-Device
to	O
tape	O
.	O
</s>
<s>
HSM	B-Application
is	O
often	O
used	O
for	O
deep	O
archival	O
storage	O
of	O
data	O
to	O
be	O
held	O
long	O
term	O
at	O
low	O
cost	O
.	O
</s>
<s>
Some	O
HSM	B-Application
software	O
products	O
allow	O
the	O
user	O
to	O
place	O
portions	O
of	O
data	O
files	O
on	O
high-speed	O
disk	B-Device
cache	B-General_Concept
and	O
the	O
rest	O
on	O
tape	O
.	O
</s>
<s>
This	O
is	O
used	O
in	O
applications	O
that	O
stream	O
video	O
over	O
the	O
internet	O
—	O
the	O
initial	O
portion	O
of	O
a	O
video	O
is	O
delivered	O
immediately	O
from	O
disk	B-Device
while	O
a	O
robot	O
finds	O
,	O
mounts	O
and	O
streams	O
the	O
rest	O
of	O
the	O
file	O
to	O
the	O
end	O
user	O
.	O
</s>
<s>
Such	O
a	O
system	O
greatly	O
reduces	O
disk	B-Device
cost	O
for	O
large	O
content	O
provision	O
systems	O
.	O
</s>
<s>
HSM	B-Application
software	O
is	O
today	O
used	O
also	O
for	O
tiering	O
between	O
hard	B-Device
disk	I-Device
drives	I-Device
and	O
flash	B-Device
memory	I-Device
,	O
with	O
flash	B-Device
memory	I-Device
being	O
over	O
30	O
times	O
faster	O
than	O
magnetic	O
disks	B-Device
,	O
but	O
disks	B-Device
being	O
considerably	O
cheaper	O
.	O
</s>
<s>
The	O
key	O
factor	O
behind	O
HSM	B-Application
is	O
a	O
Data	O
migration	O
policy	O
that	O
controls	O
the	O
file	O
transfers	O
in	O
the	O
system	O
.	O
</s>
<s>
More	O
precisely	O
,	O
the	O
policy	O
decides	O
which	O
tier	O
a	O
file	O
should	O
be	O
stored	O
in	O
,	O
so	O
that	O
the	O
entire	O
storage	B-General_Concept
system	I-General_Concept
can	O
be	O
well-organized	O
and	O
have	O
a	O
shortest	O
response	O
time	O
to	O
requests	O
.	O
</s>
<s>
There	O
are	O
several	O
algorithms	O
realizing	O
this	O
process	O
,	O
such	O
as	O
Least	B-General_Concept
Recently	I-General_Concept
Used	I-General_Concept
replacement(LRU )	O
,	O
Size-Temperature	O
Replacement(STP )	O
,	O
Heuristic	O
Threshold(STEP )	O
etc	O
.	O
</s>
<s>
Every	O
time	O
a	O
storage	O
read	O
occurs	O
,	O
the	O
caching	O
software	O
look	O
to	O
see	O
if	O
a	O
copy	O
of	O
this	O
data	O
already	O
exists	O
on	O
the	O
cache	B-General_Concept
and	O
uses	O
that	O
copy	O
,	O
if	O
available	O
.	O
</s>
<s>
Rather	O
than	O
making	O
a	O
copy	O
of	O
frequently	O
accessed	O
data	O
into	O
fast	O
storage	O
,	O
tiering	O
moves	O
data	O
across	O
tiers	O
,	O
for	O
example	O
,	O
by	O
relocating	O
cold	B-General_Concept
data	I-General_Concept
to	O
low	O
cost	O
,	O
high	O
capacity	O
nearline	B-General_Concept
storage	I-General_Concept
devices	O
.	O
</s>
<s>
The	O
basic	O
idea	O
is	O
,	O
mission-critical	O
and	O
highly	O
accesses	O
or	O
"	O
hot	O
"	O
data	O
is	O
stored	O
in	O
expensive	O
medium	O
such	O
as	O
SSD	B-Device
to	O
take	O
advantage	O
of	O
high	O
I/O	O
performance	O
,	O
while	O
nearline	B-General_Concept
or	O
rarely	O
accessed	O
or	O
"	O
cold	O
"	O
data	O
is	O
stored	O
in	O
nearline	B-General_Concept
storage	I-General_Concept
medium	I-General_Concept
such	O
as	O
HHD	O
and	O
tapes	O
which	O
are	O
inexpensive	O
.	O
</s>
<s>
Thus	O
,	O
the	O
"	O
data	O
temperature	O
"	O
or	O
activity	O
levels	O
determines	O
the	O
primary	O
storage	B-General_Concept
hierarchy	I-General_Concept
.	O
</s>
<s>
Microsoft	O
Storage	O
Spaces	O
since	O
version	O
shipped	O
with	O
Windows	B-Device
Server	I-Device
2012	O
R2	O
.	O
</s>
<s>
An	O
older	O
Microsoft	O
product	O
was	O
Remote	O
Storage	O
,	O
included	O
with	O
Windows	B-Application
2000	I-Application
and	O
Windows	B-Application
2003	I-Application
.	O
</s>
