are
transmitted
and
copied
to
the
recovery
site
while
the
application
write
operations
are
quiesced.
Global
Mirror
uses
a
function
called
a
session
to
control
the
formation
of
consistency
groups.
(A
session
is
a
collection
of
volumes
across
multiple
storage
units
that
are
managed
together
to
create
consistent
copies
of
data.)
The
formation
of
these
consistency
groups
is
coordinated
by
the
″
master
″
storage
unit,
which
sends
commands
over
remote
mirror
and
copy
links
to
its
″
subordinate
″
storage
units.
The
following
describes
the
behavior
of
these
volume
pairs
when
an
error
occurs
that
affects
a
volume
in
a
consistency
group:
v
Without
the
consistency
group
option
enabled,
the
storage
unit
causes
the
volume
where
the
error
is
detected
to
enter
a
suspended
state,
but
updates
to
that
volume
are
still
allowed.
v
With
the
consistency
group
option
enabled,
the
volume
becomes
suspended
and
additionally
enters
a
long-busy
state.
Updates
are
not
possible
to
volumes
from
the
source
LSS
to
target
LSS
that
are
sharing
the
same
path.
A
critical
mode
determines
the
behavior
of
the
remote
mirror
and
copy
pairs
or
consistency
groups
after
a
failure
in
communication
between
the
source
and
target
storage
units
when
all
the
paths
between
a
pair
are
lost.
You
can
enable
this
mode
also
by
accessing
Copy
Services
functions
from
the
DS
Storage
Manager
and
by
navigating
to
the
Paths
page
and
then
to
the
LSS
options
page.
Failover
and
failback
operations
The
failover
operation
is
the
process
of
switching
production
to
a
backup
facility
(normally
your
recovery
site).
A
failback
operation
is
the
process
of
returning
production
to
its
original
location
after
a
disaster
or
a
scheduled
maintenance
period.
There
are
times,
both
planned
and
unplanned,
when
it
is
necessary
to
suspend
disk
mirroring
and
to
make
use
of
the
secondary
storage
unit
in
your
configuration.
As
a
manual
process,
this
can
be
complex.
However,
failover
and
failback
recovery
operations
are
available
to
simplify
this
process
and
reduce
the
risk
of
error
and
the
time
it
takes
to
switch
sites
and
restart
I/O
operations.
Failover
is
the
process
of
temporarily
switching
production
to
a
backup
facility
(normally
your
recovery
site)
following
a
scheduled
maintenance
period
or
a
disaster
at
your
production
(or
local)
site.
A
failover
operation
is
always
followed
by
a
failback
operation,
which
is
the
process
of
returning
production
to
its
original
location.
These
operations
use
remote
mirror
and
copy
functions
to
reduce
the
time
that
is
required
to
synchronize
volumes
after
switching
sites
during
planned
or
unplanned
outages.
The
failover
and
failback
operations
allow
change
recording
to
be
enabled
on
the
target
volumes
without
having
to
communicate
between
the
target
and
source
storage
units.
This
method
eliminates
the
need
to
perform
a
full
volume
copy
from
your
recovery
site
to
the
production
site,
which
can
reduce
the
time
that
is
required
to
resume
operations
at
your
production
site.
In
a
typical
remote
mirror
and
copy
environment,
processing
will
temporarily
failover
to
the
storage
unit
at
your
recovery
site
if
an
outage
occurs
at
the
production
site.
Through
use
of
failover
operations,
the
state
of
a
storage
unit
in
your
target
configuration
changes.
As
a
result,
the
storage
unit
is
recognized
as
the
source
storage
unit
in
the
pair.
Because
the
failover
process
puts
the
volumes
into
a
suspended
state,
changes
are
tracked
within
a
bitmap.
Assuming
that
change
Chapter
8.
Optional
feature
codes
for
the
DS8000
123
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