Tape Library

I am often asked this question.  

On initial review of the LTO tape drive specifications it appears that the Full High and Half High tape drives have nearly the same operational specifications. This is especially true if maximum data rate is not a focus requirement. Table 1 are the published specifications for the IBM LTO8 Full High (FH) and Half High (HH).

Table 1

Mean Time Between Failure (MTBF) and Mean Swap Between Failure (MSBF) are values that are determined for any single homogeneous system.  These values are based on other operational specifications that are specific to the individual drives.  Raw performance values such as capacity and through put are based on best possible values in ideal conditions.

So why would I want a Full high drive instead of a half high drive? Besides the loader life indicated in the specification all of the values are the same.  These values do not tell the full story.  In order to create a smaller packaging, the Half high drives require certain trade-offs.

Let’s take a look at the motors that move the tape in and out of the cartridge.  The Full high has a larger and more powerful motor than the half high. This allows the motor on the full high to accelerate at 10m/s/s while the half high can only accelerate at 5m/s/s.  This is very impacting when operating in environments where stops and starts are common.  Stop and starts, referred to as back-hitching, is common when a host is busy and cannot send or receive data as fast as the slower speeds of the tape drive. It is also seen when the application writing to tape is writing small files to tape with synchs between each file. When millions of files can be written to a single tape the result of using an LTO-8 Full High vs a Half High can be writing the files in as little as half the time. This is because a single back hitch takes up to 1.9 seconds longer to complete on a Half High drive than on a Full High drive.

Back hitching impacts are further impacted by the size of the data buffer.  The LTO8 Half High data buffer is considerably smaller than the full high when shared memory usage is taken into consideration.  This means that the greater the delta in input/output between the host and the tape drives, the greater the drive must compensate.  Since the half high tape drive has a smaller data buffer the probability of the tape drive needing to back hitch is increased.  The HH tape drive has less buffer to compensate for the lack of streaming capability from a host.

The motor capability also impacts the maximum locate/search and rewind speeds for the Half High drive.  The difference between the FH and HH maximum locate and rewind speeds is only 1 meter per second and for small and mid-range applications it will not impact the overall environmental performance.  When examining the high locate and seek environments used in enterprise applications the 1 meter per second can impact the time to data in the large solutions.  The amount of impact is dependent on the size of the solution.  For example, if a mount requires locating to end of tape 1 time, reading a small file and rewinding for unload the delta between the Full High and Half High will be approximately 18 seconds to complete the operations.

The loader mechanism in the Full high drive is designed for a higher number of cycles, it is also designed to be more durable for high speed automation environments. In terms of enterprise performance in Tape Automation the best way to improve the time to data is to move the media from a slot to a drive and vice-versa in a very fast manner.  This requires a more robust loader mechanism in the drive to ensure the loader life under the higher duty cycles. In an enterprise automation environment where time to data is critical and many swaps of media are expected the Full High provides the best durability.

Table 2

Looking at Table 2, a full specification of the underlying specs for the drives, we can make the assertion that if I have “enterprise” class of data I should be using a Full High tape drive.  Furthermore, I will have more flexibility in my operations when choosing the LTO Full High tape drive.

As a reader you are most likely aware that the LTO consortium partners (as the TPC) released the specification for LTO-9. The biggest part of this announcement is always the support statement for capacity.  LTO-9 with be released at 18TB per cartridge capacity, up to 400 MBpS native transfer rate, and downward compatible write and read for LTO-8 format.  We will have more on coming attractions for LTO-9 as we get closer to the availability date.

Check out this video blog focused on the capacity point.