IBM DS6000 - Manual (Page 273)

Chapter 22. Global Mirror overview 249
Figure 22-3 Synchronous data replication, freeze, and restart without recovery required
The sequence of events in Figure 22-3 is the following:
1. Update intent to logging file.
2. Link between A2 and B2 becomes unavailable, or the storage disk subsystem with A2
fails, and a rolling disaster develops.
3. The first attempt to write to the database volume A2 will return a unit check. The host
re-drives the I/O. When only the links fail, the subsystem triggers an
extended long busy
(ELB). This is also called an
automation window
.
4. Within this automation window, or ELB window, a freeze operation removes all related
paths between both sites for the selected LSSs. From now on, no data is replicated any
longer between the selected LSSs.
5. After the ELB expires, or a RUN command is issued to end the freeze period, the I/O is
re-driven, and it may successfully complete when the pair is suspended, or may fail when
CRIT(HEAVY) is specified or the storage disk subsystem 3 is not available any longer.
6. When the I/O fails, then a
recovery required
is written to the database configuration file on
A3. Note volume A3 is no longer replicated due to the previous freeze. Note also that
volume A2 and B2 are still identical and at the very same level of data currency —that of
the moment previous to the error.
7. The primary site eventually becomes unavailable and a switch over to the secondary site
is carried out.
8. Database subsystem restart discovers
no recovery required
indication because all related
data is consistent —neither the
recovery required
indication in step 6 was transmitted to
B3. Then, the database subsystems continues immediately after the restart phase is
finished.
Primary
Primary
A
Primary
Primary
A
2D00
Primary
Primary
A
Primary
B3
3E00
Subsystem
Database
Primary
A
2C00
Primary
Primary
A
Primary
A3
2E00
Primary
Primary
A
Primary
A1
2C00
B1
3C00
Storage Disk
Subsystem 5 Storage Disk
Subsystem 6
Storage Disk
Subsytem 1
Storage Disk
Subsystem 2
1
3
6
Primary
Primary
A
Primary
Primary
A
2D00
Primary
Primary
A
Primary
B2
3D00
Primary
Primary
A
Primary
A2
2D00
Replicate
Synchronous
Storage Disk
Subsystem 3
Storage Disk
Subsystem 4
1
Secondary Primary
RECON'
Log'
DB'
RECON
Log
DB
Subsystem
Database
Recover A2
8
7
4
5
8 8
Replicate
Synchronous
Replicate
Synchronous
4
4
4
2
Restart
Fail over to secondary site
Hold I/O
Redrive
I/O