Administrator's Guide


Mirroring the Database and Recovery Log

Mirroring can be crucial in the recovery process. Consider the following scenario: Because of a sudden power outage, a partial page write occurs. The recovery log is 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.

This section explains how to:


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

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, allocate them as shown in Table 30.

Table 30. 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

TSM 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 75 shows a mirrored database in which VOL1-VOL5 are mirrored by VOLA-VOLE.

Figure 75. Mirrored Volumes

Mirrored Volumes


Use the DSMFMT command to format the space. 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, TSM 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 Administrator's Reference for details.

Specifying Mirroring and Database Page Shadowing Server Options

TSM provides four server options for database and recovery log mirroring so you can specify your preferred level of protection, recoverability and performance.

  1. MIRRORREAD specifies how mirrored volumes are accessed when the server reads the recovery log or a database page during normal processing. You may specify MIRRORREAD LOG for reading recovery log pages, or MIRRORREAD DB for reading database pages. MIRRORREAD LOG (or DB) NORMAL specifies that only one mirrored volume is read to obtain the desired page. MIRRORREAD LOG (or DB) VERIFY specifies that all mirrored volumes for a page be read, compared, and re-synchronized if necessary. MIRRORREAD LOG (or DB) VERIFY can decrease server performance as each mirrored volume for the page is accessed on every read.

  2. MIRRORWRITE specifies how mirrored volumes are written to. You may issue MIRRORWRITE LOG or DB, and then specify that write operations for the database and the recovery log be specified as SEQUENTIAL or PARALLEL:

  3. DBPAGESHADOW=YES mirrors the latest batch of pages written to a database so that the server can recover those pages that have been partially written to should an outage occur that affects both mirrored volumes.

  4. DBPAGESHADOWFILE specifies the name of the database page shadowing file.

    DBPAGESHADOW and DBPAGESHADOWFILE coordinate with the MIRRORWRITE server option and its specifications of DB and SEQUENTIAL or PARALLEL like this:

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

The following type of information is displayed:


+--------------------------------------------------------------------------------+
|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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