
Global
Copy
is
a
nonsynchronous
mirroring
function
and
is
an
alternative
mirroring
approach
to
Metro
Mirror
operations.
Host
updates
to
the
source
volume
are
not
delayed
by
waiting
for
the
update
to
be
confirmed
by
a
storage
unit
at
your
recovery
site.
The
source
volume
sends
a
periodic,
incremental
copy
of
updated
tracks
to
the
target
volume
instead
of
a
constant
stream
of
updates.
There
is
no
guarantee
that
dependent
write
operations
are
transferred
in
the
same
sequence
that
they
have
been
applied
to
the
source
volume.
This
nonsynchronous
operation
results
in
a
“fuzzy
copy”
at
the
recovery
site;
however,
through
operational
procedures,
you
can
create
a
point-in-time
consistent
copy
at
your
recovery
site
that
is
suitable
for
data
migration,
backup,
and
disaster
recovery
purposes.
To
ensure
that
a
consistent
copy
of
the
data
is
created,
you
can
periodically
switch
from
Global
Copy
to
Metro
Mirror
mode.
Then,
either
stop
application
I/O
or
freeze
write
applications
to
source
volumes
and
wait
for
all
pending
updates
to
be
copied
to
the
recovery
site.
At
this
point,
you
can
create
a
FlashCopy
operation
at
your
recovery
site
to
obtain
a
consistent
data.
The
Global
Copy
function
can
operate
at
very
long
distances—well
beyond
the
300
km
distance
that
is
supported
for
Metro
Mirror—and
with
minimal
impact
to
applications,
with
the
distance
limited
only
by
the
network
and
the
channel
extended
technology.
During
a
disaster,
data
can
be
restored
only
to
the
last
known
consistent
increment
that
was
created.
This
means
that
data
that
is
written
to
the
production
site
but
is
waiting
to
be
transferred
to
the
recovery
site
is
lost
whenever
the
two
storage
units
can
no
longer
communicate.
Be
aware
that
the
use
of
Global
Copy
functions
do
not
guarantee
against
data
loss.
Global
Mirror
functions,
on
the
other
hand,
provide
recoverable
copies
of
production
data
at
a
long
distance
recovery
site
by
continually
forming
consistent
sets
of
data
with
no
significant
performance
impact.
This
allows
a
quick
restart
at
your
recovery
site
in
case
of
a
disaster
at
the
production
site.
The
following
describes
the
Global
Copy
write
sequence:
1.
During
a
Global
Copy
operation,
the
storage
unit
at
your
production
site
captures
information
about
updates
to
the
source
and
periodically
sends
those
updates
to
the
target
volume
at
the
recovery
site.
2.
After
the
initial
copy
of
tracks,
the
storage
unit
series
periodically
starts
a
synchronization
cycle
where
all
updated
tracks,
in
ascending
order
from
the
lowest
numbered
track,
are
copied
from
the
source
volume
to
the
target
volume.
The
storage
unit
updates
the
target
tracks
with
the
current
information
for
each
track,
regardless
of
the
number
of
updates
between
the
time
that
each
track
was
last
copied,
the
current
time,
and
the
order
in
which
the
updates
occurred.
3.
When
this
process
completes,
the
cycle
is
repeated.
There
is
little
response
time
degradation
on
application
write
operations
in
extended
distance
mode.
4.
Write
updates
to
the
source
volume
receive
an
immediate
completion
because
the
synchronization
cycle
is
independent
of
the
updates
to
the
source
volume.
Global
Mirror
Global
Mirror
processing
provides
a
long-distance
remote
copy
solution
across
two
sites
for
open
systems
or
System
z
(or
both)
data
using
asynchronous
technology.
This
processing
is
accomplished
using
the
DS
Storage
Manager
or
the
DS
command-line
interface
(CLI).
114
DS8000
User’s
Guide
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