
between
two
storage
units
that
can
be
located
up
to
300
km
from
each
other.
With
Metro
Mirror
copying,
the
source
and
target
volumes
can
be
on
the
same
storage
unit
or
on
separate
storage
units.
You
can
locate
the
storage
unit
at
another
site,
some
distance
away.
Metro
Mirror
is
a
synchronous
copy
feature
where
write
operations
are
completed
on
both
copies
(local
and
remote
site)
before
they
are
considered
to
be
complete.
Synchronous
mirroring
means
that
a
storage
server
constantly
updates
a
secondary
copy
of
a
volume
to
match
changes
made
to
a
source
volume.
The
advantage
of
synchronous
mirroring
is
that
there
is
minimal
host
impact
for
performing
the
copy.
The
disadvantage
is
that
since
the
copy
operation
is
synchronous,
there
can
be
an
impact
to
application
performance
because
the
application
I/O
operation
is
not
acknowledged
as
complete
until
the
write
to
the
target
volume
is
also
complete.
The
longer
the
distance
between
primary
and
secondary
storage
units,
the
greater
this
impact
to
application
I/O,
and
therefore,
application
performance.
Asynchronous
mirroring
(Global
Copy)
Copies
data
nonsynchronously
and
over
longer
distances
than
is
possible
with
the
Metro
Mirror
feature.
When
operating
in
Global
Copy
mode,
the
source
volume
sends
a
periodic,
incremental
copy
of
updated
tracks
to
the
target
volume
instead
of
a
constant
stream
of
updates.
This
causes
less
impact
to
application
writes
for
source
volumes
and
less
demand
for
bandwidth
resources,
while
allowing
a
more
flexible
use
of
the
available
bandwidth.
The
updates
are
tracked
and
periodically
copied
to
the
target
volumes.
As
a
consequence,
there
is
no
guarantee
that
data
is
transferred
in
the
same
sequence
that
was
applied
to
the
source
volume.
To
get
a
consistent
copy
of
your
data
at
your
remote
site,
you
must
periodically
switch
from
Global
Copy
to
Metro
Mirror
mode,
then
either
stop
the
application
I/O
or
freeze
data
to
the
source
volumes
using
a
manual
process
with
freeze
and
run
commands.
The
freeze
and
run
functions
can
be
used
with
external
automation
software
such
as
Geographically
Dispersed
Parallel
Sysplex
™
(GDPS
®
),
which
is
available
for
System
z
environments,
to
ensure
data
consistency
to
multiple
Metro
Mirror
volume
pairs
in
a
specified
logical
subsystem.
Common
options
for
Metro
Mirror
and
Global
Copy
include
the
following
modes:
Suspend
and
resume
If
you
schedule
a
planned
outage
to
perform
maintenance
at
your
remote
site,
you
can
suspend
Metro
Mirror
or
Global
Copy
processing
on
specific
volume
pairs
during
the
duration
of
the
outage.
During
this
time,
data
is
no
longer
copied
to
the
target
volumes.
Because
the
primary
storage
unit
keeps
track
of
all
changed
data
on
the
source
volume,
you
can
resume
operations
at
a
later
time
to
synchronize
the
data
between
the
volumes.
Copy
out-of-synchronous
data
You
can
specify
that
only
data
that
was
updated
on
the
source
volume
while
the
volume
pair
was
suspended
be
copied
to
its
associated
target
volume.
Copy
an
entire
volume
or
not
copy
the
volume
You
can
copy
an
entire
source
volume
to
its
associated
target
volume
to
guarantee
that
the
source
and
target
volume
contain
the
Chapter
1.
Introduction
to
IBM
System
Storage
DS8000
series
27
Содержание DS8000 - ADDITIONAL INFORMATION
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