SAN-Attached Libraries - Best Practices

Overview

The aim of this page is to provide background intelligence which can be leveraged when planning and designing data management- and backup solutions in SAN environments. the following sections provide implementation guidelines for deploying this software in the SAN Fabric. This includes environments with failover and load balancing capabilities for Disk Access, which is usually accomplished with Dual attached HBA configurations. The following information is a collection of best practices based on field experiences.

The recommendations are consistent with recommendations and configurations supported by other industry/SAN experts. Also these recommendations are equally applicable to different vendor's equipments.

MediaAgent HBA Configuration

To configure the environment with HBA fail over and load balancing, you must zone the library/drives to only one of the HBA’s. This means that there will be no fail-over/load balancing for those tape/robotic devices. The primary culprit for tape device problems in this environment is load balancing. The high I/O volume of backups can cause load balancing to ping-pong the device between the HBA's. This inevitably leads to mount problems and data corruption.

Fail-over is less of a problem. If a fail-over occurs, the system will go into a pending state and automatically restart in accordance with its restart rules. However, the fail-over event could change SCSI ID's of the tape devices and cause confusion in a multi-job/multi-device configuration.

Therefore, the best scenario would be one where each MediaAgents has three HBA’s configured, where two HBA’s are configured in load balancing and failover configuration, thereby managing Disk I/O only. A third HBA may be configured to handle Tape I/O exclusively, whereby no load balancing and failover configuration is active on that HBA.

The recommendation of using a third HBA for this purpose is consistent with other industry/SAN experts.

Additionally, it is recommended that backup and normal traffic do not travel concurrently on the same fabric for obvious performance reasons. This is another reason to have a separate HBA for tape backup. This is another recommendation that is consistent with other industry/SAN experts.

For those who persist with running Tape I/O and Disk I/O production/backup traffic on a load-balanced dual fabric, the queue level trigger can be adjusted to minimize the ping-pong of devices between HBA’s or the round-robin option can be used. In either case, the probability of ping-pong and subsequent risk of data corruption cannot be fully eliminated. A third HBA is the only way to avoid the potential problems.

If MediaAgents are equipped with two HBA’s and no separation exists, neither logical channel or physical (HBA), between Tape I/O and Disk I/O to the devices, it is strongly recommended to disable load balancing on the HBA’s altogether as although Disk I/O may benefit from this configuration, Tape I/O may be affected resulting in unpredictable and intermittent issues with Tape Access Paths changing, causing backup issues. Failover can still be enabled on the HBA’s.

Zoning and HBA Configuration

While configuring SAN zoning it is recommended to allow only the SAN MediaAgents to see the library and for each SAN MediaAgent to be in its own zone for the library. A few examples on how to configure zones are provided below.

Example 1:  A dedicated HBA for Disk and a dedicated HBA for Tape Access (Recommended)

The above example shows a simple setup with dedicated HBA for Disk and Tape Access. Multi-path software is not needed in such a setup as there is no failover capability.

Example 2: Dual-attached HBA for Disk Access and a dedicated HBA for Tape Access (Recommended)

The above example shows a setup with two dedicated HBA’s for Disk and a dedicated HBA for tape access. Multi-path software can be used for the HBA’s that are used for Accessing Disk Storage. This can be configured in either the load balancing or the failover mode.

Example 3: Dual attached HBA for Disk Access and a Shared HBA for Tape Access (Not Recommended)*

The above example shows a redundant Setup with 2 HBA’s for disk and a shared HBA for Tape Access. Multi-pathing software should be used only for fail over purposes - this means NO load balancing between both HBA’s.

Also consider that HBA2 should be configured as Standby HBA for Disk Access. This means that HBA1 is the Active HBA for Disk Access and HBA2 is used for Tape Access. When HBA1 is not available Disk and Tape Access will go over HBA2. Although this Setup will work it is generally recommended to use dedicated HBA’s for Tape Access, whenever possible.

*This is also a best practice recommended by most hardware vendors.

Example 4: Failover Configuration for Dual HBA FC Tape Drives (Additional Option)

The above example shows a redundant Setup with 2 HBA for disk access and 2 HBA for tape access. Multi-pathing software should only be used for Disk Access. This can be configured in either load balancing or failover mode. (See Configuring Dual Host Bus Adaptors (HBA) for more information. on setting up the HBA cards.)