A Faster Tape Drive or Media Does Not Necessarily Mean Better Overall Performance

Answer

When comparing performance of one tape drive or media vs another multiple factors must be taken into account including but not limited to the following:

- Compressibility of the data being saved
- Maximum native transfer rate of the tape drives or tape media
- Speed of the interface

A note about tape media: A higher generation tape drive using previous generation media will have a maximum data transfer rate generally equivalent to the maximum native transfer rate of the media. For example using an LTO4 cartridge in an LTO5 tape drive will cause the maximum native transfer rate to be generally equivalent to LTO4 even though the tape drive is LTO5. This has to do with various specifications of the media; for example, the previous generation media has fewer tracks. The newer generation drive may perform slightly better using the previous generation media than the previous generation drive. This is because the newer drive may move the tape at a slightly higher speed than the previous generation drive.

As data flows to the tape drive it first enters a buffer in the drive. The servo in the tape drive moves the tape over the read/write head and data is taken from the buffer and written to the tape media. Depending on the parameters used in the save command the data is compacted as it is written to the tape media. If the buffer in the tape drive is full the servo speed in the tape drive is adjusted higher up to it's maximum speed (speed matching) and depending on the compressibility of the data an actual transfer rate of something higher than the maximum native transfer rate is achieved.

An example of this calculation:

- Native Max Rate of LTO 6: 160 MB/sec
- Data compressibility: 3:1
- Actual Speed data is written to media: 160 MB/sec * 3 = 480 MB/sec

Types of data vary greatly in it's compressibility. For example large physical files may compress at 8:1 whereas images in the IFS may be much closer to 1:1.

When considering buying a tape drive or media with higher maximum native transfer rate capabilities for performance reasons some analysis should be done. If the current data transfer rate being achieved is not exceeding the maximum native transfer rate of the tape drive or media currently in use then a new faster tape drive or media will either have no affect on the overall data transfer rate or in some cases could possibly be slower than the tape drive and/or media currently in use. If this is the case the main reason to buy a newer generation tape drive/media would be to gain additional capacity per tape cartridge.

The maximum capabilities of all of the components in the interface between the tape drive up to and including the IO adapter in IBM i system must be analyzed. The number of devices sharing a common interface must also be taken into account. The distance between the IO adapter and the next component must be considered.

A short example of a SAN fabric:

- Adapter in IBM i is an IBM 5774 which is a 4 Gigabit PCI Express Dual Port Fibre Channel Adapter. The adapter automatically negotiates the highest data rate between the adapter and an attaching device at 1 Gbps, 2 Gbps, or 4 Gbps of which the device or switch is capable. Distances between the adapter and an attaching device or switch can reach up to 500 meters running at 1 Gbps data rate, up to 300 meters running at 2 Gbps data rate, and up to 150 meters running at 4 Gbps data rate.

- 2 Gbps fibre switch

- IBM 3573 (TS3100) with LTO Ultrium 6 full-height tape drive: 8 Gbps Fibre Channel

In the example above the 4 Gbps 5774 adapter is capable of moving about (400 MB/sec * 80%) or 320 MB/sec on each port. The 2 Gbps ports in the fibre switch are capable of about (200 MB/sec * 80%) or about 160 MB/sec. The 8 Gbps port in the LTO 6 tape drive is capable of about (800 MB/sec * 80%) or about 640 MB/sec. The weakest link is the fibre switch ports. In this example 160 MB/sec is all the faster data can be moved over the entire interface. Also, it is possible that more than one drive in the tape library could be sharing parts of that interface further limiting the performance if multiple devices are being used concurrently. This interface will not be able to keep the LTO6 tape drive busy unless the data is not compressible at all.

Below is an example of how an LTO6 may be slower than LTO5:

An LTO5 tape drive/media has a maximum native data transfer rate of 140 MB/s. LTO6 drive/media has a maximum native data transfer rate of 160 MB/s. For a specific type of data the LTO5 may achieve a compression ratio of 7.5:1 and because of the improved compression hardware for L6 media the compression ratio for the same data may be 8.4:1. This means that the data rate across the fibre channel interface needs to be faster for the LTO6 than the LTO5 to keep the buffer full and the servo running fast.

If both devices are running on a 4 Gbps interface data cannot be sent any faster to the LTO6 drive/media. The LTO6 tape drive spends most of it's time moving the tape at its slowest speed setting, but that's not slow enough for the data rate at the media so the drive has to stop, backhitch and start more frequently. The LTO5 drive/media can move the tape slower without stopping so it doesn't have to stop, backhitch and start as frequently. The combination of the interface speed, compressibility of the data, and the drive rates and servo speeds means that the LTO6 drive/media ends up running slightly slower than the LTO5 drive for highly compressible data.

Note: This is not an exhaustive discussion of factors that affect save/restore performance using tape drives. This document does not discuss system workload or resources for example. There are far too many factors to consider and the topic is too detailed to be considered in detail in this document.

Note: The IBM i global support center cannot provide design or sizing functions in this area.