Data Domain: Virtual Tape Library Best Practices Guide

VTL Best Practices Guide

PURPOSE

This document provides Best Practice guidelines to help ensure optimal performance and of the Data Domain Virtual Tape Library (VTL) in backup environments and also to ensure ease of supporting and maintaining the product.

APPLIES TO

• All Data Domain systems
• All Software Releases supporting VTL
• VTL Protocol
• Third-Party Backup Application ("BA") such as NetWorker, TeraSAM, and so forth

SOLUTION

1. Basic Guidelines to avoid poor performance:
   1. It is critical that you ensure that a VTL qualifier has been completed for your installation, and verified for proper operation. Use of unsupported HBAs, drivers, and so forth, are a common source of problems.
   2. Attempt to keep the Data Domain system less than 85% full. Filesystem Cleaning and other operations are faster and more efficient when the system has enough available disk to perform these important tasks.
   3. Try to schedule Filesystem Clean (also known as Garbage Collection or file system cleaning) to run during times when active backups are not running.
   4. The default Filesystem Clean schedule is sufficient in most environments. Consult the document "Scheduling Cleaning on a Data Domain System: Best Practices 12089" to better understand this process. If you still feel there is a reason to alter the default schedule to have Filesystem Clean run more often, contact Data Domain Support to discuss.
   5. Do not schedule Replication to overlap with your active VTL backup window. Both processes require substantial resources and complete faster if run separately rather than concurrently.
   6. Never use encryption, multiplexing, or pre-compression or client-side deduplication from the client BA (that is, NetWorker, TeraSAM), as they greatly reduce the compression factor obtained on the Data Domain system. Perform these activities on the Data Domain system only. Some backup applications turn on these features by default (that is, HP Data Protector defaults to multiplexing), so check that all these are turned OFF for your application.
   7. Although the Data Domain system may offer higher limits in configuration options (number of streams, throttling, replication, and so forth), use of more moderate configurations often offers the best overall performance. 
   8. Be certain that you read and understand all alerts from the Data Domain system. If you do not understand an alert, call support for clarification.
   9. Do not use the default pool for everything. Create at least one other pool and create all tapes in the pools you created. If you use replication (or may in the future), it is important to create and use between 5 and 10 replication contexts (that is, a VTL pool) for improved performance.
   10. Ensure that your specific components are compatible with VTL.

2. VTL Components

   1. Initiators:
      1. It is required that the FC initiator port must be dedicated to only Data Domain VTL devices.
      2. Only initiators that must communicate with a particular set of VTL target ports on a Data Domain system should be zoned with that Data Domain system.
      3. Create a useful alias for every initiator you zone and connect to the Data Domain system, preferably including the hostname and port in the alias name. 
      4. Use only one-to-one zoning; create zones on your Fibre Channel switch composed of only one initiator and one target per zone. 
   2. Slots: 
   • The number of slots or drives a library should have is dictated by how many simultaneous backup and restore streams are running. Drive counts are also constrained by the configuration and overall performance limits of your particular Data Domain system. Slot counts are typically based on how many tapes are used over a retention policy cycle.

   3. Cartridge Access Points:

      • See the Data Domain Integration documentation for your specific Backup Application to determine if Cartridge Access Points (CAPs) must be emulated for your particular environment.

   4. Changer:

      • There can be only one changer per VTL.
      • Often, the changer model you should select depends on your specific configuration:
        • Use the RESTORER-L180 library emulation when using Symantec Backup software
        • Use the TS3500 library emulation when using IBM System i platform
        • You can also use the TS3500 library emulation when using TeraSAM 6.2 on AIX 6.1 and AIX 5.3 platforms.
        • Most other installations should use the L180 library emulation (non-Symantec, non-IBM system i)

   5. Tape Drive

      1. auto-offline:  If a tape is loaded, the drive is online. In this state, the changer is unable to move a tape from the drive without first unloading the tape. However, if Auto-Offline is enabled, there is an implicit drive unload and so the tape can be moved from the drive even if an Unload command has not been issued by the application. This setting can be useful for certain applications, and is global across the VTL service (one setting for all drives).
      2. auto-eject:  If a tape is moved from a drive or slot to a CAP, it goes directly to the vault. This setting can be useful for those applications that check to see that tapes have been removed from the CAP. They will fail their library "eject" operation if tapes are still in the CAP after a time delay. Auto-eject makes these applications happy because tapes disappear immediately from the CAPs. This setting is also global across the VTL service (one setting for all drives)
      3. It is best to use only one type of tape drive per library.

   6. Target HBAs

      1. Consider spreading the backup load across multiple FC ports on the Data Domain system in order to avoid bottlenecks on a single port. 
      2. Verify the speed of each FC port on the switch to confirm that the port is configured for the wanted rate.
      3. Set secondary ports to "none" unless explicitly necessary for your particular configuration.
      4. Configure the host operating system driver for LUN persistent binding. Doing so avoids situations where, because of target changes, backup software or the operating system must be reconfigured. 

• VTL Operation

  1. Slots: Create sufficient slots to contain the number of tapes you have created. Creating a few extra slots is not a problem, if it is not an excessive quantity.
  2. CAPs
  3. Tapes
     1. Create only as many tapes as required to satisfy your backup requirements. The starting tape count is less than twice the disk space available on the Restorer. Creating too many virtual tapes can create a scenario where the Data Domain system can fill up prematurely and cause an unexpected system outage. As the global compression statistics become available, additional tapes can be added incrementally. 
     2. If the system does reach 100% full you must delete any empty tapes that may exist on the system, then expire enough data to get the system below 80% capacity. In order to avoid this time-consuming task, in order to prevent a system-full event from occurring.
     3. On a replication destination system, never read from a tape that is being replicated.
     4. Always use unique tape barcodes, even across different pools.
     5. Always use the same tape suffix (size) across all pools. If for some reason you must use a different suffix, at a minimum you should keep the same suffix within a pool.
     6. Optimal size of tapes depends on multiple factors, including the specific BA being used, and the characteristics of the data being backed-up. In general, it is better to use a larger number of smaller tapes than a smaller number of big tapes, in order to control disk usage and prevent system full conditions.
     7. For TeraSAM, it is recommended to use smaller tapes (that is, 30-50G) in order to help reclaim space more quickly.

  4. Back-up Applications

     1. Ensure that you are using the largest optimal block size for your BA for maximum performance operating with the Data Domain system. The optimal number depends on many factors, such as disk speed, OS caching, and your specific backup software. See your vendor's recommendations, and the Integration Guides.
     2. In general, a tape block size that is a multiple of 64K offers better performance, but be sure to check the Best Practices or Integration guides for your specific software (see below for links). If accessing the Data Domain device with multiple backup servers, use the largest block size accessible by all servers in the environment (especially in a heterogeneous OS environment).

• Access Groups

  1. Numbering of devices within each discrete VTL access group should begin with LUN 0.
  2. It is best to avoid changing VTL access group configuration while the Data Domain system is under heavy load.
  3. It is recommended that you have exactly one initiator per access group.

• Statistics

  1. When using the VTL filemark cache statistics, the reset of the statistics should be done prior to tapes being loaded into the drives. If the reset of the statistics is performed after the tapes are loaded and accessed in the tape drives, the vtl show detailed-stats command may be misleading. The report may display the "free" counts to be greater than the "alloc" counts, which is unexpected but harmless in this case. This is caused by resetting the statistics of the drives while they are in use; the reset of the statistics is not an atomic operation.
  2. As a general best practice, the statistics should be reset prior to tapes being loaded into the drives.