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
The
fast
reverse
option
allows
you
to
reverse
a
FlashCopy
relationship
without
waiting
for
the
background
copy
of
a
previous
FlashCopy
to
finish.
In
disaster
recovery
situations,
when
you
use
Global
Mirror,
you
must
use
volumes
with
consistent
data
at
the
target
site.
Part
of
the
setup
requires
that
you
enable
the
fast
restore
option,
a
copy
option
that
is
available
for
FlashCopy
operations.
(This
option
applies
to
a
Global
Mirror
operation,
which
is
based
on
existing
Global
Copy
and
FlashCopy
operations
at
the
target
site.)
Use
fast
restore
option
to
reverse
a
FlashCopy
target
volume
and
allow
consistent
data
to
be
copied
back
to
its
associated
source
volume,
without
having
to
wait
for
the
background
copy
from
the
original
source
to
the
original
target
to
complete.
You
can
then
vary
the
volumes
online
and
start
your
applications.
You
can
enable
the
fast
reverse
option
by
accessing
Copy
Services
functions
from
the
Web
interface
called
the
IBM
TotalStorage
DS
Storage
Manager
and
navigating
to
the
FlashCopy
page
and
then
to
the
Select
copy
options
page.
Chapter
8.
Optional
feature
codes
for
the
DS8000
119
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