Transparency to the backup application

Because there is a significant performance increase when writing data to disk, using virtual tape libraries reduces the amount of time that it takes to backup the data.

Virtual tape libraries

Like their physical counterparts, virtual tape libraries have cartridge slots, one or more tape drives, an import/export slot, and even a robotic arm. These attributes (number of slots, capacity of tape drives, and so on) are set when the virtual library is created. The NearStore VTL comes with a number of predefined library and virtual tape drive types. You can also create new library types to fit your needs.

Virtual tape drives

The NearStore VTL includes a number of virtual tape drive types. The type specifies such properties as vendor information, the capacity of the drive, and tape optimization algorithm.

Virtual tapes

Virtual tapes are the NearStore VTL equivalent of physical tapes. Like physical tapes, virtual tapes have a specified capacity and are labeled by barcodes. Backup applications use these barcodes to track virtual tapes in the same way that they track physical tapes. In fact, there is a one-to-one correspondence between virtual and physical barcodes.

Control of physical library entry/exit port (EEP)

The correspondence between virtual tape movement and physical tape movement even extends to the movement of tapes into and out of libraries. When you move a virtual tape, that has a corresponding physical tape, to the virtual library entry/exit port, the NearStore VTL can move the physical tape to the physical library EEP. (This is controlled by the Move Physical Tapes to EEP After Virtual Export option.)

Control of ACSLS managed libraries

The NearStore VTL appliance can use ACSLS-controlled libraries. The ACSLS server and the VTL appliance communicate over a LAN, while the ACS tape drives communicate with the NearStore VTL appliance over the SAN. The NearStore VTL appliance does not communicate directly with the ACS robot arms.

In addition to networking and SAN requirements, before using an ACSLS with a NearStore VTL appliance, you must set the Advanced Option Use ACSLS to control physical libraries.

Import/Export

When you import a physical tape, the NearStore VTL creates a virtual tape that is the exact copy of that physical tape, including barcode and file mark information. Likewise, when you export a virtual tape, the NearStore VTL sends the data from the virtual tape to an attached physical tape device, resulting in a physical copy of the virtual tape.

You can also use the cloning feature of the backup application as a slower alternative. The tapes that the NearStore VTL creates are in the native format of the backup application.

Shadow tape pool and shadow tapes

When you export a virtual tape, in parallel with the creation of the physical tape, the NearStore VTL can create a shadow tape stored in the shadow tape pool. The backup application continues to manage the physical tape, with the shadow tape invisible to the backup application (and invisible to the library on the NearStore VTL). If you later import the physical tape, the NearStore VTL immediately moves the shadow tape to the library, which makes it available for reading while the physical tape is still being read. You can also manually import a shadow tape back to a virtual library.

Shadow tapes are assigned a retention period. When a backup application is writing to the NearStore VTL, if the NearStore VTL needs additional space for an existing virtual tape, the VTL reclaims shadow tape space. Space is reclaimed first from expired shadow tapes, but if after reclaiming expired tapes, more space is still needed, space is reclaimed from unexpired shadow tapes.

Smart sizing

All data is compressible at different rates. This means that each time you back up data to physical tape, the actual amount of data that the physical tape drive writes to tape depends on the compressibility of the data. Because the NearStore VTL must be able to export virtual tapes to physical tape, it is crucial that the amount of data on the virtual tape can fit on the physical tape, without going over its capacity. If the data from a virtual tape cannot fit on a physical tape, you cannot export or clone it.

Most VTL vendors solve this issue by matching the capacity of virtual tapes to the native (uncompressed) capacity of the corresponding physical tapes. This is not an efficient solution, because it does not take compression into account. For example, if the physical tape drive can compress the data at a ratio of 2:1, over half of the physical tape would remain unused when you exported the virtual tape.

With smart sizing, the data stream is monitored to determine the compressibility of the data. This allows the VTL to determine the appropriate size to make the virtual tape so that it correctly fits on its corresponding physical tape. There is no performance impact when using smart sizing. Note that smart sizing is completely different from disk compression. Smart sizing optimizes the utilization of physical tape media, while disk compression minimizes the space used on disk by the VTL. With hardware compression this is an extremely effective method of saving space on disk. Smart sizing and disk compression can be used at the same time on the same NearStore VTL appliance.

Disk compression

While smart sizing adjusts the size of a virtual tape to match the capacity of a physical tape by taking in account the physical tape drives own data compression algorithm. VTL data compression reduces the size of the data stored on a virtual tape, which in turn saves disk capacity. Some models of NearStore VTL appliances support hardware-based disk compression. Hardware-based compression is high-speed, and recommended when your appliance supports it. Models without hardware-based compression can use software-based data compression, but this can significantly reduce data throughput. Data compression can be separately enabled or disabled for each virtual tape (the default, data compression on or off, can be set separately for each virtual library when the library is created). When compression is enabled, the NearStore VTL appliance selects hardware compression if available, or software compression, if hardware compression is not available. For both hardware- and software- based compression, the compression ratio depends on the actual data compressed, but is typically about 2:1.

Smart cloning and tape exporting

When a tape is cloned or exported, and then later the (virtual) tape is updated, NearStore VTL keeps track of the location of change—the point starting from which the physical tape and the virtual tape differ. When the tape is again cloned or exported, the NearStore VTL need only write data to the physical tape beginning at the location of change. Smart cloning is the default behavior.

Direct Tape Creation

The NearStore VTL can export data to physical tapes directly by copying virtual tapes to a physical tape library connected to its backend. This feature is also referred to as “direct tape creation”. The physical tapes are written in native backup application format, so when virtual tapes are exported to physical tape there is a 100% match between the two. This means that any virtual tape stored on the NearStore VTL and then written to physical tape can be restored from any standard tape drive that can is supported by the backup server. It is therefore possible to back up data to disk while still keeping the data offsite on physical tapes that are stored in the native format of the backup application. Each virtual tape has an export status that suggests whether there is a need to copy this tape to a physical one or not. The export status is therefore one of the two:

Tape exports are triggered when:

After the tape export is done, the virtual tape is deleted (unless Shadow Tape is enabled).

Ease of use

Because the NearStore VTL virtual tape libraries function in much the same way as physical tape libraries (including the way they are displayed to backup applications), you do not have to alter the existing backup and restore process. You can back the up data to disk using the existing infrastructure and work procedures.