Global
Mirror
provides
a
two-site
extended
distance
remote
copy
option
for
disaster
recovery.
With
Global
Mirror,
the
data
that
the
host
writes
to
the
storage
unit
at
the
production
(local)
site
is
asynchronously
shadowed
to
the
storage
unit
at
the
recovery
site.
The
recovery
site
can
be
located
well
beyond
the
300
km
limit
that
is
associated
with
Metro
Mirror
with
minimal
delays
in
response
times.
Separating
volumes
by
larger
distances
provides
greater
protection
against
regional
disasters.
Only
the
capabilities
of
the
network
and
channel
extension
technologies
limit
distance.
Global
Mirror
is
based
on
existing
Copy
Services
functions:
Global
Copy
and
FlashCopy.
With
Global
Mirror,
a
consistent
copy
of
the
data
is
automatically
maintained
on
the
storage
unit
at
the
recovery
site.
This
consistency
is
ensured
if
the
order
of
dependent
write
operations
to
disks
or
disk
groups
is
maintained.
With
Global
Copy
operations,
data
consistency
is
guaranteed
at
the
recovery
site
by
creating
″
freeze
and
run
″
tasks
and
then
using
FlashCopy
to
create
point-in-time
copies
at
the
recovery
site.
Global
Mirror
operations
periodically
invoke
a
point-in-time
FlashCopy
at
the
recovery
site,
at
regular
intervals,
without
disrupting
the
I/O
to
the
source
volume,
thus
giving
a
continuously
updating,
nearly
up-to-date
data
backup.
Then,
by
grouping
many
volumes
into
a
session,
which
is
managed
by
the
master
storage
unit,
you
can
copy
multiple
volumes
to
the
recovery
site
simultaneously
while
maintaining
point-in-time
consistency
across
those
volumes.
To
better
understand
how
Global
Mirror
works,
you
must
be
familiar
with
the
following
terms:
Master
The
master
storage
unit
controls
the
creation
of
consistency
groups
in
the
Global
Mirror
session.
The
master
storage
unit
sends
commands
to
subordinate
storage
units.
A
storage
unit
is
allowed
to
be
a
master
for
only
one
Global
Mirror
session.
Subordinate
The
subordinate
storage
unit
receives
commands
from
a
master
storage
unit.
The
subordinate
storage
unit
is
identified
when
a
Global
Mirror
session
is
started.
The
subordinate
storage
unit
forms
consistency
groups
and
performs
other
Global
Mirror
processing.
A
subordinate
storage
unit
can
only
be
controlled
by
one
storage
unit
master.
Session
A
session
is
a
collection
of
volumes
across
multiple
storage
units
that
are
managed
together
to
create
consistent
copies
of
data.
The
session
is
identified
with
an
ID
that
is
unique
across
the
enterprise.
The
ID
identifies
volumes
that
will
participate
in
the
Global
Mirror
consistency
group.
A
session
is
opened
on
each
LSS
in
the
enterprise
that
has
or
might
have
volumes
that
will
participate
in
the
Global
Mirror
consistency
group
that
is
associated
with
the
specific
session
ID.
Control
path
The
control
path
is
established
from
the
master
storage
unit
to
the
subordinate
storage
unit
when
more
than
one
storage
unit
participates
in
the
Global
Mirror
session.
If
there
is
only
one
storage
unit
that
is
involved,
you
do
not
have
to
create
a
control
path.
The
master
storage
unit
communicates
directly
with
its
subordinate
storage
unit.
Fast
restore
operations:
12
Introduction
and
Planning
Guide
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