Best Practice guidelines for using persistent binding in NetBackup Shared Storage Option
Details:
Without persistent binding, the following problems with tape drives may occur:
Media not ejecting, the wrong media mounting in drives, and tape drives down.
Solution:
This is a narrative description of NetBackup (tm) operation on a Windows 2000 operating system.
The Fibre Host Bus Adaptor (HBA) featured in this TechNote is the Emulex LP8000.
Ensuring proper tape drive configuration in a Storage Area Network (SAN) environment is critical to the proper operation of NetBackup Shared Storage Option (SSO). Often, Storage Area Network (SAN) hardware is configured to use automatic device discovery. This practice works well with disk storage devices, but is not recommended for tape storage sub-systems where persistent hardware device configuration or identification is necessary.
The Windows operating system detects the SAN hardware SCSI devices via a Fibre HBA. This detection phase then stores or maps SCSI drive information in the Windows registry.
The Windows operating system must properly detect and map all SCSI devices in the registry before NetBackup device configuration. In NetBackup device configuration, SCSI device information is gathered from the operating system and placed into the NetBackup device database. The database stores the exact SCSI location of every device detected by the HBA.
When the SAN hardware is configured for automatic detection, any change in the SAN system will trigger the operating system to re-scan the HBA and remap the devices in the registry. In such an event, the new tape device mapping can present a conflict within the NetBackup device database. As a result, NetBackup will no longer communicate with the proper tape device and device errors will occur.
Example: NetBackup may attempt to mount media into a tape device from the previous configuration. NetBackup is configured to mount media into drive 1, but after a hardware re-scan the device is mapped to drive 2. This new configuration, although detected by the operating system, is not automatically updated in NetBackup. This will cause the backup to fail.
A second example is when NetBackup queries the SCSI device and cannot sense the device on the bus. This condition will result in a missed drive signal stating that the "device is not ready" or that it is "in use" which can be interpreted as a hardware problem, and will down the drive.
A third example in this situation is that a media eject may not occur and is presented to the operator as a hardware problem. This condition may interrupt NetBackup processes and ultimately NetBackup will refuse to release the drive for further NetBackup drive operations.
To counteract the effect of SAN Hardware automatic device detection, it is important to ensure that consistent SCSI device path and Logical Unit Number (LUN) mapping information remains consistent even after a reboot.
Note: NetBackup 4.5 also counteracts the effect of SAN Hardware automatic device detection.
Reference:
NetBackup 4.5 Release Notes for UNIX and Windows
page 1: NetBackup 4.5 improves the product's robustness in SAN environments
page 29: Windows NT and Windows 2000 path re-mapping for tape drives
During device manager startup, serialized drives are verified, and if necessary, the paths are corrected in the Media Manager device database.
Below are some examples of HBA configurations when using Emulex LP8000 series cards.
Clearing the Automatically Map SCSI Devices check box will prevent the SCSI device path information from changing. Changes in the SCSI device path information could conflict with other SAN equipment and result in tape devices going down or cause interruption to backup or restore operations.
Clearing the Automatic LUN Mapping check box will allow proper configuration of your mapped SCSI IDs to the appropriate World Wide Name (WWN) of the HBA. Differences in LUN mapping information can also cause problems similar to those caused by inconsistencies in the SCSI device paths listed above.
LP9000 series HBAs share a similar configuration utility. The rules of persistent binding, LUN mapping, and SCSI device path consistencies vary between HBA manufacturers, but the need for consistent hardware mapping information is essential for proper communication.
Additional setup and configuration information of LP8000 HBAs including screen shots can be found at:
Below is an excerpt from Emulex which illustrates the advantages of persistent binding and usefulness in backups directed to fibre attached tape devices:
"A Fibre Channel target is assigned its WWNN at loop initialization time; the SCSI target ID for that target is assigned by the device driver when the device is first discovered. It is possible for the WWNN to change between one loop initialization and the next. Every time a system boots or a target is added to or removed from the Fibre Channel, the loop will be re-initialized. After a system has booted, it will maintain a constant view of the same target ID because the driver software remaps the SCSI target ID to the new WWNN on the fly.
In an environment that utilizes persistent binding, once a system has booted, it will maintain a constant view of the same target ID because the driver software remaps the SCSI target ID to the new WWNN on the fly.
A second system may use a different SCSI target ID for that target. Thus, an administrator seeking to work with the same target across multiple hosts must be prepared to encounter the situation where the same Fibre Channel target is known by different SCSI target IDs. Use persistent binding to maintain a consistent target ID on all systems. A common scenario for this type of situation would be a network administrator who needs to consistently map to a tape drive for backup purposes. "
Bob Stump
VERITAS
"Ain't it the truth? Ain't it the truth?" Bert Lahr, Wizard of Oz 1939
