Metro
Mirror
copying
supports
a
maximum
distance
of
300
km
(186
mi).
Delays
in
response
times
for
Metro
Mirror
are
proportional
to
the
distance
between
the
volumes.
However,
100%
of
the
source
data
is
available
at
the
recovery
when
the
copy
operation
ends.
The
following
procedure
describes
how
data
is
written
for
a
Remote
Mirror
and
Copy
operation.
A
copy
to
the
target
storage
unit
is
synchronous
with
the
source
volume‘s
I/O
operation.
1.
An
application
requests
a
write
I/O
to
the
source
storage
unit.
The
write
I/O
is
written
into
cache
and
nonvolatile
storage
(NVS).
2.
Metro
Mirror
sends
the
write
I/O
to
the
target
storage
unit
cache
and
NVS.
3.
The
storage
unit
at
the
recovery
site
signals
that
the
write
operation
has
completed
when
the
updated
data
is
in
its
cache
and
NVS.
4.
When
the
storage
unit
at
the
production
site
receives
notification
from
the
target
storage
unit
that
the
write
operation
has
completed,
it
returns
the
I/O
completed
status
to
your
application.
Consistency
groups
Consistency
groups
are
used
for
controlling
error
situations
and
for
maintaining
data
consistency
at
a
recovery
site.
You
can
create
paths
from
a
source
LSS
to
a
target
LSS
have
the
consistency
group
option
enabled.
All
volume
pairs
in
remote
mirror
and
copy
relationships
from
this
source
to
target
LSS
(sharing
the
same
paths)
belong
to
the
consistency
group.
Other
volumes
pairs
are
not
affected.
With
the
consistency
group
option
enabled,
when
an
error
occurs
on
any
volume
pairs
or
on
the
links
that
are
associated
with
these
LSS
pairs,
an
alert
is
issued
and
I/O
to
all
duplex
remote
mirror
and
copy
volumes
on
LSS
pairs
will
be
queued
either
until
a
consistency
group
created
operation
is
run
or
the
consistency
group
timeout
time
expires,
allowing
external
automation
to
use
the
consistency
group
created
operation
to
create
a
dependent
write
consistent
set
of
target
volumes
over
any
number
of
LSS
and
disk
storage
units.
External
automation
or
command-line
interface
commands
can
issue
a
″
freeze
″
to
all
LSS
pairs
that
relate
to
the
application,
affecting
the
set
of
source
volumes
in
the
consistency
group.
The
freeze
operation
causes
the
source
volumes
to
suspend
and
their
associated
paths
to
end.
Therefore,
all
associated
updates
at
the
recovery
site
are
halted,
resulting
in
a
consistent
data
checkpoint.
The
automation
program
is
not
a
part
of
the
software
provided
with
the
storage
unit;
it
must
be
supplied
by
the
user.
However,
IBM
has
offerings
to
assist
with
this
automation.
For
more
information,
contact
your
IBM
storage
representative.
A
consistency
grouping
timeout
function
gives
automation
software
the
time
to
issue
the
automated
requests
to
ensure
consistency.
The
consistency
grouping
timeout
is
a
value
that
specifies
the
length
of
time
that
the
volume
remains
in
long-busy
(unavailable)
following
an
error.
You
can
change
this
value
by
accessing
Copy
Services
functions
from
the
Web
interface
(DS
Storage
Manager
)
and
navigating
to
the
Paths
page
and
then
to
the
LSS
options
page.
The
default
value
for
this
parameter
is
set
to
two
minutes.
Note:
The
consistency
group
option
is
not
required
for
Global
Mirror
or
for
Global
Copy.
Global
Copy
always
keeps
a
inconsistent
copy
of
the
volumes
at
the
recovery
site.
The
volume
pairs
become
consistent
only
when
all
updates
122
DS8000
User’s
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