
Global
Mirror
processing
is
most
often
associated
with
disaster
recovery
or
preparing
for
disaster
recovery.
However,
it
can
also
be
used
for
everyday
processing
and
data
migration.
The
Global
Mirror
function
is
designed
to
mirror
data
between
volume
pairs
of
a
storage
unit
over
greater
distances
without
affecting
overall
performance.
It
is
also
designed
to
provide
application
consistent
data
at
a
recover
(or
remote)
site
in
case
of
a
disaster
at
the
local
site.
By
creating
a
set
of
remote
volumes
every
few
seconds,
this
function
addresses
the
consistency
problem
that
can
be
created
when
large
databases
and
volumes
span
multiple
storage
units.
With
Global
Mirror,
the
data
at
the
remote
site
is
maintained
to
be
a
point-in-time
consistent
copy
of
the
data
at
the
local
site.
Global
Mirror
is
based
on
existing
Copy
Services
functions:
Global
Copy
and
FlashCopy.
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.
Your
reasons
for
using
Global
Mirror
processing
might
include
the
following:
v
Support
for
virtually
unlimited
distances
between
the
local
and
remote
sites,
with
the
distance
typically
limited
only
by
the
capabilities
of
your
network
and
the
channel
extension
technology.
This
unlimited
distance
enables
you
to
choose
your
remote
site
location
based
on
business
needs
and
enables
site
separation
to
add
protection
from
localized
disasters.
v
A
consistent
and
restartable
copy
of
the
data
at
the
remote
site,
created
with
minimal
impact
to
applications
at
your
local
site.
v
Data
currency,
where
your
remote
site
might
lag
behind
your
local
site
by
3
to
5
seconds,
minimizing
the
amount
of
data
exposure
in
the
event
of
an
unplanned
outage.
The
actual
lag
in
data
currency
that
you
experience
can
depend
upon
a
number
of
factors,
including
specific
workload
characteristics
and
bandwidth
between
the
local
and
remote
sites.
v
Session
support
whereby
data
consistency
at
the
remote
site
is
internally
managed
across
up
to
eight
storage
units
that
are
located
across
the
local
and
remote
sites.
v
Efficient
synchronization
of
the
local
and
remote
sites
with
support
for
failover
and
failback
modes,
helping
to
reduce
the
time
that
is
required
to
switch
back
to
the
local
site
after
a
planned
or
unplanned
outage.
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.
Chapter
8.
Optional
feature
codes
for
the
DS8000
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