
The
process
of
forming
consistency
groups
using
Global
Mirror
involves
the
following
steps:
1.
The
consistency
group
is
created.
2.
Data
in
the
consistency
group
is
sent
to
the
remote
site.
3.
Data
in
the
consistency
group
is
saved
as
a
group.
In
a
Metro/Global
Mirror
configuration,
Metro
Mirror
and
Global
Copy
volume
pairs,
combined
with
consistency
groups
using
Global
Mirror
and
FlashCopy,
help
maintain
consistent
and
current
(usually
within
seconds)
data
at
the
remote
site
to
provide
a
true
backup
at
the
remote
site.
Metro/Global
Mirror
introduces
Global
Mirror
and
FlashCopy
change
recording
and
out-of-sync
bitmap
features
that
allow
faster
subsequent
resynchronization
of
the
volumes
at
the
local
site.
During
a
disaster
while
the
intermediate
site
is
in
production
mode,
the
change
recording
feature
continues
to
be
used
to
copy
changed
data
from
the
B
volumes
at
the
intermediate
site
to
the
C
volumes
at
the
remote
site
as
consecutive
sets
of
consistency
groups.
When
the
original
local
site
is
ready
to
resume
production
mode,
the
direction
of
the
Metro
Mirror
configuration
must
be
reversed.
The
out-of-sync
bitmaps
are
used
to
resynchronize
the
volumes
at
the
local
site.
During
normal
operations,
consistency
groups
are
continually
being
created
from
the
volumes
at
the
intermediate
site.
While
consistency
groups
are
being
formed,
host
write
I/Os
are
momentarily
delayed
while
a
consistent
data
point
is
attained
(because
Metro
Mirror
from
the
local
to
intermediate
site
is
synchronous).
Bitmaps
that
track
updates
for
the
consistency
group
are
created
and
data
is
transferred,
or
drained,
to
the
volumes
at
the
remote
site
using
Global
Copy.
Application
writes
for
the
next
consistency
group
are
recorded
in
the
change
recording
bitmap.
The
master
storage
unit
coordinates
the
consistency
group
formation
for
all
volumes
that
belong
to
a
Global
Mirror
session
over
FCP
links
that
are
based
on
user-specified
parameters.
It
also
coordinates
I/O
with
optional
subordinates
to
all
primary
volumes
to
form
consistency
groups.
The
frequency
that
consistency
groups
are
formed
is
based
on
the
number
of
seconds
that
is
specified
for
the
Consistency
Group
Interval
option
and
the
amount
of
time
that
it
takes
to
form
consistency
groups.
Other
Global
Mirror
options
are
described
in
Note:
Be
aware
that
an
insufficient
bandwidth
(caused
by
a
link
failure
or
insufficient
link
configuration)
can
also
increase
the
amount
of
time
that
it
takes
to
drain
a
consistency
group.
In
a
Metro/Global
Mirror
configuration,
if
a
Metro
Mirror
volume
pair
between
the
local
and
intermediate
sites
is
either
duplex
pending
or
becomes
suspended,
the
Global
Mirror
session
will
fail
consistency
group
formation.
This
is
to
ensure
that
the
consistency
group
at
the
remote
site
is,
in
fact,
truly
consistent
with
the
local
site.
In
addition,
there
might
be
cases
where
the
secondary
volume
in
a
Metro
Mirror
relationship
at
the
intermediate
site
is
not
aware
that
its
status
as
a
secondary
is
suspended
and
the
consistency
group
formation
would
continue.
The
Global
Mirror
session
fails
the
consistency
group
formation
as
a
way
to
ensure
that
the
consistency
group
that
is
formed
at
the
remote
site
is
consistent
with
the
local
site.
You
can
query
information
about
the
intermediate
volume’s
status
in
the
Global
Mirror
session
using
the
showgmir
-metrics
command.
Chapter
8.
Optional
feature
codes
for
the
DS8000
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