SIOS DataKeeper Intent Log

SIOS DataKeeper uses an intent log (also referred to as a bitmap file) to track changes made to the source, or to target volume during times that the target is unlocked. This log is a persistent record of write requests which have not yet been committed to both servers.

The intent log gives SIOS DataKeeper the ability to survive a source or target system failure or reboot without requiring a full mirror resync after the recovery of the system.

There is a performance overhead associated with the intent log, since each write to the volume must also be reflected in the intent log file. To minimize this impact, it is recommended that the intent logs be stored on a physical disk that is not involved in heavy read or write activity. See Relocation of Intent Log for more information.

DataKeeper Bitmap File Size Calculation

• One bit per block (each block is 64KB)

• One bit represents 64KB of volume space/

Example: For a 640GB volume the bitmap is 1,310,720 bytes

640 × 1073741824 / 65536 / 8

That is 1,310,720 bytes for the bitmap.

Non-Shared Volumes

By default, this intent log feature is enabled, and the intent log files are stored in a subdirectory called “Bitmaps” under the directory where SIOS DataKeeper was installed.

To create the intent log file in a directory other than the default location, set the BitmapBaseDir registry entry to a directory where SIOS DataKeeper will create the file. See Relocation of Intent Log for more information.

To disable the intent log feature, clear the BitmapBaseDir registry entry (set it to an empty string) on all current and potential mirror endpoint servers. Disabling the intent log requires a reboot on each of these systems in order for this setting to take effect. Keep in mind that if this feature is disabled, a full resync will be performed in the event of a source system failure.

Shared Volumes

When replicating shared volumes, the intent log files are stored in a subdirectory called “ReplicationBitmaps” on the replicated volume itself. This is necessary to allow switchover to the other shared source servers without resulting in a full resync of the data.

SIOS does not recommend relocating intent logs from their default locations.

Configuration Issue

When configuring a BitmapBaseDir registry entry, make sure that the folder and drive letter specified exist. If configured with a drive letter that does not exist, the following message will be received upon system boot up:

Global bitmap volume {drive letter}: has not been detected yet. Mirror source threads may hang if this volume does not exist. Check to make sure that the BitmapBaseDir registry entry specifies a valid volume for storage of bitmaps.