Wednesday, May 24, 2006

VTL & Tape: A Symbiotic Relationship

A lot has been written over the past year about advantages and disadvantages of tape. One thing for sure though is that Tape's not going anywhere anytime soon for various reasons some of which are included below:

  1. Tape is deeply entrenched in the Enterprise
  2. Tape's a cost effective long term storage medium
  3. Backup applications understand Tape and perform their best when streaming to a Tape drive rather than a filesystem.
  4. Tape can be easily moved offsite for vaulting purposes.

But Tape has some distinct disadvantages some of which include:

  1. Tapes are unreliable and susceptible to environmental conditions (i.e heat/humidity etc).
  2. You won't know of a bad tape until you attempt to recover from it.
  3. Sharing Tape drives requires additional software and adds cost and complexity.
  4. Streaming to a tape drive is not simple, especially with incremental backups. And while it can be done, via multiplexing, the latter has a significant effect on recovery since all interleaved streams must be read by the backup server.
  5. In order to share Tape libraries between servers additional software must be purchased, adding cost as well as complexity.

One approach that customers have been using to address the above issues is to backup to a conventional disk array using D2D backup. However, what they find is that this approach adds additional configuration steps, in that they would still have to provision storage to the backup application using the disk vendors provisioning tools, still have to create RAID Groups, still have to create LUNs, still have to make decision regarding cache allocations and finally they still have to manage it.

Then, reality sets in...Disk is not easily shared between servers and Operating systems without a Shared SAN filesystem or by carving and managing multiple LUNs to multiple servers/apps. All this means additional cost, complexity and management overhead. Addressing a challenge by making it more challenging is not what people are looking for. This is where the VTL comes into play.

An integrated appliance with single or dual controllers and disk behind, that looks like, feels like tape but it's...Disk. Disk that Emulates Tape Libraries, with Tape drives, slots, Entry/Exit ports and Tape cartridges. Backup SW, since their inception were designed with Tape in mind, not disk. They know Tape, they perform very well with tape. They know little about disk and in some cases do not integrated at all with disk, nor do they perform optimally with disk.

The VTL on the other hand appears to the Backup SW as one or more Tape Libraries of different type and characteristics (drive type, slots #, capacities). They also eliminate the need to stream to disk regardless of the backup you are taking (full/incremental) since inherently disk is faster than tape. This also means that you don't have to multiplex thus making your recovery fast.

You could also easily share a single VTL among multiple servers providing each server with its own dedicated Tape library, drives, slots, robot. Essentially, what you end up is with a centrally located and manage Virtual Library that looks, feels and behaves as a dedicated physical library to each of your servers.

Another benefit of the VTL is that is easily integrated with a real Physical Tape library. In fact, the majority of the implementations require it by positioning the VTL in front of a Physical Tape library. The VTL will then emulate the specific tape library with its associated characteristics such as, number of drives, slots, barcodes, robot etc. After a backup has completed you then have 2 choices with regards to Physical Tape creation.

Traditional Physical Tape Creation Approach

Using this approach, the backup server is responsible for direct physical tape creation. In other words, the backup server controls the copy process as well as providing reporting capabilities incorporated into the backup sw. However, the backup server must process every tape twice which can increase the time required to create offsite tapes. Since the data path goes thru the backup server, this process will require specific windows that do not coincide with a regular backup windows. This method allows for the independent tracking of physical and virtual tapes but the process is slower from a performance perspective. Every VTL vendor supports this method.

VTL Direct Tape Creation Approach

Under this scenario, after the backup to the Virtual Tape is complete, the backup application will issue an eject to the virtual tape based on an aging policy. At this point, the Virtual Tape contents are copied to the Physical tape, in the background, using the same barcodes. Upon completion, the virtual tape is deleted from the virtual library. The benefit of this approach is that the backup server is not involved in the process. The requirement with this approach is that the VTL must be 100% compatible with the Backup application media management and be able to write the backup in the backup application's native format. Netapp's Nearstore VTL offers this approach as well as the Traditional Method while others offer one or the other.

There are many more useful features a VTL provides. One that I find extremely useful is the ability to create Shadow Tapes. What is a Shadow Tape?

When you export a Virtual Tape, in parallel with the creation of the Physical Tape, the VTL creates a shadow tape that is stored in a shadow vault. The backup application continues to manage the Physical tape while the shadow tape is invisible. If you later import the Physical Tape, the shadow Tape is moved form the vault into the library, which makes it available for reading immediately. The VTL manages the retention and expiration of shadow tapes.

VTLs are packed with many more features, some of which I'll be addressing in the next couple of days as a follow up to this writeup as well as give an overview of Netapp's Nearstore VTL story.

1 comment:

bhoja said...

Nice article on VTL's. Please add more information for this article. Very lucid and intersting ..keep up ..nick.