Administrator's Guide


Mirroring the Database and Recovery Log

This section explains how to:


Task Required Privilege Class
Define database and recovery log volumes System or unrestricted storage
Query mirrored volumes Any administrator

The following scenario shows the importance of mirroring in the recovery process: As the result of a sudden power outage, a partial page write occurs. The recovery log is now corrupted and not completely readable. Without mirroring, transaction recovery operations cannot complete when the server is restarted. However, if the recovery log is mirrored and a partial write is detected, a mirror volume can be used to construct valid images of the missing pages.

Allocating Volume Copies to Separate Physical Disks

By separating volume copies on different physical devices, you protect the server against media failure and increase the availability of the database and recovery log. If you cannot assign each volume copy to its own physical disk, then allocate them as shown in Table 20.

Table 20. Separating Volume Copies
Physical Disk Volume Volume
Physical Disk 1 Database volume copy 1 Recovery log volume copy 3
Physical Disk 2 Recovery log volume copy 1 Database volume copy 2
Physical Disk 3 Database volume copy 3 Recovery log volume copy 2

ADSM mirrored volumes must have at least the same capacity as the original volumes.

Defining Database or Recovery Log Mirrored Volumes

To mirror the database or recovery log, define a volume copy for each volume in the database or recovery log.

For example, the database consists of five volumes named VOL1, VOL2, VOL3, VOL4, and VOL5. To mirror the database, you must have five volumes that match the original volumes in size. Figure 61 shows a mirrored database in which VOL1-VOL5 are mirrored by VOLA-VOLE.

Figure 61. Mirrored Volumes

Mirrored Volumes


Format the space by using the DSMFMT command. For example, to format VOLA, a 25MB database volume, enter:

./dsmfmt -m -db vola 25

Then define the group of mirrored volumes. For example, you might enter the following commands:

define dbcopy vol1 vola
define dbcopy vol2 volb
define dbcopy vol3 volc
define dbcopy vol4 vold
define dbcopy vol5 vole

After a volume copy is defined, ADSM synchronizes the volume copy with the original volume. This process can range from minutes to hours, depending on the size of the volumes and performance of your system. After synchronization is complete, the volume copies are mirror images of each other.
Note:The mirror read and mirror write server options specify modes for reading and writing database and recovery log pages. See ADSM Administrator's Reference for details.

Requesting Information about Mirrored Volumes

You can request information about mirrored database or recovery log volumes by using the QUERY DBVOLUME and QUERY LOGVOLUME commands. For example:

query dbvolume

This command results in the following display:


+--------------------------------------------------------------------------------+
|Volume Name    Copy    Volume Name    Copy    Volume Name    Copy               |
|(Copy 1)       Status  (Copy 2)       Status  (Copy 3)       Status             |
|-----------    ------  -------------  ------  -------------  ------             |
|VOL1           Sync'd  VOLA           Sync'd                 Undef-             |
|VOL2           Sync'd  VOLB           Sync'd                 ined               |
|VOL3           Sync'd  VOLC           Sync'd                                    |
|VOL4           Sync'd  VOLD           Sync'd                                    |
|                                                                                |
|VOL5           Sync'd  VOLE           Sync'd                                    |
|                                                                                |
+--------------------------------------------------------------------------------+


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