<s>
Device	B-Application
Mapper	I-Application
Multipath	O
Input	B-General_Concept
Output	I-General_Concept
often	O
shortened	O
to	O
DM-Multipathing	O
and	O
abbreviated	O
as	O
DM-MPIO	B-Application
provides	O
input-output	B-General_Concept
(	O
I/O	B-General_Concept
)	O
fail-over	O
and	O
load-balancing	B-Application
by	O
using	O
multipath	B-General_Concept
I/O	I-General_Concept
within	O
Linux	B-Application
for	O
block	O
devices	O
.	O
</s>
<s>
By	O
utilizing	O
device-mapper	B-Application
,	O
the	O
multipathd	O
daemon	O
provides	O
the	O
host-side	O
logic	O
to	O
use	O
multiple	O
paths	O
of	O
a	O
redundant	O
network	B-Architecture
to	O
provide	O
continuous	O
availability	O
and	O
higher-bandwidth	O
connectivity	O
between	O
the	O
host	O
server	O
and	O
the	O
block-level	O
device	O
.	O
</s>
<s>
DM-MPIO	B-Application
handles	O
the	O
rerouting	O
of	O
block	O
I/O	B-General_Concept
to	O
an	O
alternate	O
path	O
in	O
the	O
event	O
of	O
a	O
path	O
failure	O
.	O
</s>
<s>
DM-MPIO	B-Application
can	O
also	O
balance	O
the	O
I/O	B-General_Concept
load	O
across	O
all	O
of	O
the	O
available	O
paths	O
that	O
are	O
typically	O
utilized	O
in	O
Fibre	B-Architecture
Channel	I-Architecture
(	O
FC	O
)	O
and	O
iSCSI	B-Protocol
SAN	O
environments	O
.	O
</s>
<s>
DM-MPIO	B-Application
is	O
based	O
on	O
the	O
device	B-Application
mapper	I-Application
,	O
which	O
provides	O
the	O
basic	O
framework	O
that	O
maps	O
one	O
block	O
device	O
onto	O
another	O
.	O
</s>
<s>
When	O
utilizing	O
Linux	O
DM-MPIO	O
in	O
a	O
datacenter	B-Operating_System
that	O
has	O
other	O
operating	B-General_Concept
systems	I-General_Concept
and	O
multipath	O
solutions	O
,	O
key	O
components	O
of	O
path	O
management	O
must	O
be	O
considered	O
.	O
</s>
<s>
Load	B-Application
balancing	I-Application
The	O
workload	O
is	O
distributed	O
across	O
the	O
available	O
hardware	O
components	O
.	O
</s>
<s>
Path	O
failover	O
and	O
recovery	O
Utilizes	O
redundant	O
I/O	B-General_Concept
channels	O
to	O
redirect	O
application	O
reads	O
and	O
writes	O
when	O
one	O
or	O
more	O
paths	O
are	O
no	O
longer	O
available	O
.	O
</s>
<s>
DM-MPIO	B-Application
started	O
as	O
a	O
patch	O
set	O
created	O
by	O
Joe	O
Thornber	O
,	O
and	O
was	O
later	O
maintained	O
by	O
Alasdair	O
G	O
Kergon	O
at	O
Red	O
Hat	O
.	O
</s>
<s>
It	O
was	O
included	O
in	O
mainline	O
Linux	B-Application
with	O
kernel	B-Operating_System
version	O
2.6.12	B-Application
,	O
which	O
was	O
released	O
on	O
June	O
17	O
,	O
2005	O
.	O
</s>
<s>
DM-MPIO	B-Application
in	O
Linux	B-Application
consists	O
of	O
kernel	B-Operating_System
components	O
and	O
user-space	O
components	O
.	O
</s>
<s>
Kernel	B-Operating_System
–	O
device-mapper	B-Application
–	O
block	O
subsystem	O
that	O
provides	O
layering	O
mechanism	O
for	O
block	O
devices	O
.	O
</s>
<s>
dm-multipath	O
–	O
kernel	B-Operating_System
module	O
implementing	O
the	O
multipath	O
device-mapper	O
target	O
.	O
</s>
<s>
User-space	O
–	O
multipath-tools	O
–	O
provides	O
the	O
tools	O
to	O
manage	O
multipathed	O
devices	O
by	O
instructing	O
the	O
device-mapper	B-Application
multipath	O
module	O
what	O
to	O
do	O
.	O
</s>
<s>
Multipath	O
:	O
scans	O
the	O
system	O
for	O
multipathed	O
devices	O
,	O
assembles	O
them	O
,	O
updates	O
the	O
device-mapper	B-Application
'	O
s	O
map	O
.	O
</s>
<s>
The	O
configuration	O
file	O
/etc/multipath.conf	O
makes	O
many	O
of	O
the	O
DM-MPIO	B-Application
features	O
user-configurable	O
.	O
</s>
<s>
The	O
multipath	O
command	O
and	O
the	O
kernel	B-Operating_System
daemon	O
multipathd	O
use	O
information	O
found	O
in	O
this	O
file	O
.	O
</s>
<s>
The	O
file	O
is	O
only	O
consulted	O
during	O
the	O
configuration	O
of	O
the	O
multipath	B-General_Concept
devices	I-General_Concept
.	O
</s>
<s>
Blacklisted	O
devices	O
(	O
blacklist	O
)	O
:	O
User	O
specifies	O
the	O
list	O
of	O
devices	O
that	O
is	O
not	O
to	O
be	O
under	O
the	O
control	O
of	O
DM-MPIO	B-Application
.	O
</s>
<s>
Blacklist	O
exceptions	O
(	O
blacklist_exceptions	O
)	O
:	O
Specific	O
devices	O
to	O
be	O
treated	O
as	O
multipath	B-General_Concept
devices	I-General_Concept
even	O
if	O
listed	O
in	O
the	O
blacklist	O
.	O
</s>
<s>
HBA	O
:	O
Host	B-Architecture
bus	I-Architecture
adapters	I-Architecture
provide	O
the	O
physical	O
interface	O
between	O
the	O
input/output	B-General_Concept
(	O
I/O	B-General_Concept
)	O
host	O
bus	O
of	O
Fibre	B-Architecture
Channel	I-Architecture
devices	O
and	O
the	O
underlying	O
Fibre	B-Architecture
Channel	I-Architecture
network	B-Architecture
.	O
</s>
<s>
DM	O
Path	O
States	O
:	O
The	O
device	B-Application
mapper	I-Application
's	O
view	O
of	O
the	O
path	O
condition	O
.	O
</s>
<s>
Active	O
:	O
The	O
last	O
I/O	B-General_Concept
operation	I-General_Concept
sent	O
through	O
this	O
path	O
successfully	O
completed	O
.	O
</s>
<s>
Failed	O
:	O
The	O
last	O
I/O	B-General_Concept
operation	I-General_Concept
sent	O
through	O
this	O
path	O
did	O
not	O
successfully	O
complete	O
.	O
</s>
<s>
Number	O
of	O
seconds	O
:	O
Wait	O
for	O
a	O
specified	O
number	O
of	O
seconds	O
to	O
allow	O
the	O
I/O	B-General_Concept
to	O
stabilize	O
,	O
then	O
failback	O
to	O
the	O
highest	O
priority	O
path	O
.	O
</s>
<s>
Active/Active	O
:	O
In	O
a	O
system	O
that	O
has	O
two	O
storage	O
controllers	O
,	O
each	O
controller	O
can	O
process	O
I/O	B-General_Concept
.	O
</s>
<s>
Active/Passive	O
:	O
In	O
a	O
system	O
that	O
has	O
two	O
storage	O
controllers	O
,	O
only	O
one	O
controller	O
at	O
a	O
time	O
is	O
able	O
to	O
process	O
I/O	B-General_Concept
,	O
the	O
other	O
(	O
passive	O
)	O
is	O
in	O
a	O
standby	O
mode	O
.	O
</s>
<s>
WWID	O
:	O
Worldwide	O
Identifier	O
is	O
an	O
identifier	O
for	O
the	O
multipath	B-General_Concept
device	I-General_Concept
that	O
is	O
guaranteed	O
to	O
be	O
globally	O
unique	O
and	O
unchanging	O
.	O
</s>
