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Note:
In
a
remote
mirror
and
copy
environment,
when
two
storage
units
are
set
up
in
two
geographically
distinct
locations,
we
refer
to
the
sites
as
Site
A
(the
production
site)
and
Site
B
(the
recovery
site).
Failover
and
failback
operations
A
failover
is
the
process
of
temporarily
switching
production
to
a
backup
facility
(normally
your
recovery
site)
following
a
planned
outage,
such
as
a
scheduled
maintenance
period
or
an
unplanned
outage,
such
as
a
disaster.
A
failback
operation
is
the
process
of
returning
production
to
its
original
location.
These
operations
use
remote
mirror
and
copy
functions
to
help
reduce
the
time
that
is
required
to
synchronize
volumes
after
sites
are
switched
during
planned
or
unplanned
outages.
Global
Mirror
The
Global
Mirror
function
provides
a
two-site
extended
distance
remote
copy
option
for
disaster
recovery
and
backup
solution
for
the
zSeries
and
open
systems
environments.
This
solution
is
based
on
existing
Global
Copy
and
FlashCopy
functions.
With
Global
Mirror,
the
data
that
the
host
writes
to
the
storage
unit
at
the
production
site
is
asynchronously
shadowed
to
the
storage
unit
at
the
recovery
site.
Global
Mirror
operates
over
high-speed,
Fibre
Channel
communication
links
and
is
designed
to
maintain
a
consistent
and
restartable
copy
of
data
at
a
recovery
site
that
can
be
located
at
virtually
unlimited
distance
from
the
production
site.
Comparison
of
licensed
functions
A
key
decision
that
you
must
make
in
planning
for
a
disaster
is
deciding
which
licensed
functions
to
use
to
best
suit
your
environment.
Table
2
provides
a
brief
summary
of
the
characteristics
of
the
Copy
Services
features
that
are
available
for
the
storage
unit.
Table
2.
Comparison
of
licensed
functions
Licensed
function
Description
Advantages
Considerations
Metro
Mirror
Synchronous
data
copy
at
a
distance
No
data
loss,
rapid
recovery
time
for
distances
up
to
300
km.
Slight
performance
impact.
Global
Copy
Continuous
copy
without
data
consistency
Nearly
unlimited
distance,
suitable
for
data
migration,
only
limited
by
network
and
channel
extenders
capabilities.
Copy
is
normally
fuzzy
but
can
be
made
consistent
through
synchronization.
Global
Mirror
Asynchronous
copy
Nearly
unlimited
distance,
scalable,
and
low
recovery
point
objective
(RPO).
The
RPO
is
the
time
needed
to
recover
from
a
disaster;
that
is,
the
total
system
downtime.
RPO
might
grow
when
link
bandwidth
capability
is
exceeded.
20
DS8000
User’s
Guide
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