Dependent write operations

To preserve the integrity of data that is being written, ensure that dependent writes are run in the intended sequence of the application.

The following list is a typical sequence of write operations for a database update transaction:
  1. A write operation updates the database log so that it indicates that a database update is about to take place.
  2. A second write operation updates the database.
  3. A third write operation updates the database log so that it indicates that the database update has completed successfully.

The database ensures correct ordering of these writes by waiting for each step to complete before starting the next. The database log is often placed on a different volume than the database. In this case, ensure that FlashCopy operations are performed without changing the order of these write operations. For example, consider the possibility that the database (update 2) is copied slightly earlier than the database log (update 1 and 3), which means the copy on the target volume will contain updates (1) and (3) but not (2). In this case, if the database is restarted from a backup made from the FlashCopy target disks, the database log indicates that the transaction has completed successfully when, in fact, it has not. The transaction is lost and the integrity of the database is compromised.

Protection against incomplete write operations and the extended database recovery that might result is provided by Lenovo Storage V7000.

You can process a FlashCopy operation on multiple volumes as an atomic operation to create a consistent image of user data. To use FlashCopy this way, the Lenovo Storage V7000 supports the concept of a consistency group. A consistency group can contain an arbitrary number of FlashCopy® mappings, up to the maximum number of FlashCopy mappings that are supported by a Lenovo Storage V7000clustered system.You can use the command-line interface (CLI) startfcconsistgrp command to start the point-in-time copy for the entire consistency group. All of the FlashCopy mappings in the consistency group are started at the same time, resulting in a point-in-time copy that is consistent across all of the FlashCopy mappings that are contained in the consistency group.

See the following website for the latest maximum configuration support:

http://support.lenovo.com/us/en/products/servers/lenovo-storage

Parent topic: FlashCopy function