Used LTO Cartridges. A smart way to stretch your media budget, or a sure way to lose your data?

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1 Used LTO Cartridges A smart way to stretch your media budget, or a sure way to lose your data? As tape markets mature, used media abound. With the price of used tape cartridges falling to a fraction of new ones a question for the budgetconscious data manager should be: is used tape media safe? Could you be risking your company s valuable data (and quite possibly your career) by purchasing used cartridges? The answer depends on which cartridges you are buying. Used cartridges can be the deal of a lifetime, but they can also be your worst nightmare. It can be difficult to get reliable information about the used tape cartridges. Knowing your supplier always helps, but does your supplier really know where his cartridges have been? Happily, for buyers of used LTO media there is a very reliable historical data source. That source is a memory chip inside the cartridge which contains a wealth of information about the past life of the cartridge. This article describes what information is available on the cartridge memory chip (CM), what it means, how to access this chip, and how to make an informed buying decision based on these data Sherman Drive, Longmont, CO (fax)

2 New does not necessarily mean better It would seem obvious that, given enough funds, it would always be best to buy a new cartridge. Surprisingly, however, this is not necessarily true. Up to a point, tape cartridges can actually improve with use. New tapes carry debris generated during manufacturing by the slitting process. The debris can contaminate the tape drive components and cause data errors. Some of the debris is transferred from the tape to the tape drive each time a tape is used. After a while the tape is as clean as it will ever be. Continued use beyond this point, however, does not improve the tape. Eventually it begins to wear out and the error rate jumps up. Figure 1 charts the life of a tape. The bathtub -shaped curve shows that the error rate of brand new tapes is elevated. At P1 the tape has been cleaned because of the transfer of debris to the drive and the error rate drops. If the tape has been used in a controlled environment, the error rate should remain relatively low up to P2. After P2 the error rate increases steeply. The tape has entered the end of its life cycle and should not be used any longer. Essentially then, the best cartridge would be one that has been used a little but not too much in a controlled environment by a well-designed and adjusted tape drive. So if someone has kindly cleaned a tape for you and offers to sell it to you at a discount, that used cartridge may be an excellent deal. Relevant data in the memory chip Not all of the parameters which are stored in the memory chip are relevant to a buyer of a used cartridge. The most important statistics, however, should include the number of errors, the number of bytes read/written, the number of loads, and the age of the cartridge. Percentage of records with errors Error statistics are perhaps the most important indications of the cartridge s health. There are three types of errors: Recovered data errors, unrecovered data errors, and servo errors. Of course, the more data that have been transferred to and from the drive the more errors are to be expected. We therefore consider important all errors (not the absolute number of errors) in the percentage of records that had recovered-write, recovered-read, and fatal-suspended-write errors. During a write process, recovered write errors occur when the readback check of the data shows that the record had too many errors. The error correction circuitry of the tape drive may have been able to correct these errors and the data may have been read back perfectly, but a drive may decide to allow only a few errors in order to have sufficient margins for a later restore process. The drive will then rewrite the same record further down the tape. This is a fairly common operation caused mainly by media Used LTO cartridges page 2

3 defects and is of little concern if the number of recovered write errors is not excessive. Recovered read errors occur when the error correction logic is not able to correct errors during a read process. The drive will retry the operation which involves stopping the tape, moving it back and reading the record again. Sometimes the retry operation will be successful. These read errors are much more serious than recovered write errors so it is best to have very few read-retries. Unrecovered read and write errors are the most undesirable errors since they always cause a complete crash of the backup or restore process. If this happens during a data restore and if you do not have another copy of the data, you will soon be getting those phone calls from very unhappy users. Unrecovered read errors are especially undesirable. The third category of error statistics consists of Servo Errors. LTO tape is manufactured with five servo bands pre-written by the tape manufacturer 1. Tape drives use these bands to locate the data tracks. A Suspended Writes error means that during a write process the information in a servo band could not correctly be decoded, so the drive was unable to write the data at this location. The drive will write the data further down the tape when it regains the ability to read the servo data. The drive will bypass the area of tape where the servo bands were unreadable. The usable length of such a tape becomes slightly shorter, but this is of little concern as long as there are not too many suspended writes. If the drive is unable to read the servo band at any location the write operation is suspended. This is called Fatal Suspended Writes, since the drive is not able to recover from this failure. If the word fatal scares you, it ought to. Because LTO tape drives cannot regenerate servo bands, once the servo information is destroyed the tape is completely unusable. This is a cartridge you do not want under any circumstances. Number of bytes read and written The number of bytes read and written refers to the total amount of data that has been transferred to and from the cartridge 2. A cartridge should have been used a little but not too much. The tape should have been moved several times from the beginning of tape all the way to the end of tape. These full-length passes are ideal for removing debris. In a typical backup operation the desired number of passes is achieved after the tape has been written to approximately 20% of its capacity. After that the tape enters that long, slow descent into its twilight years. Number of cartridge loads If the number of times the cartridge has been loaded is disproportionately high relative to the number of bytes written or read, it means that each time the cartridge was loaded, only a few records were 1 The cartridge memory contains the name of the manufacturer who wrote the servo track as well as the name of the cartridge manufacturer. These two names are not always identical because several LTO media manufacturers sell their cartridges under multiple brand names. Here, then, is where the actual manufacturer of the tape media can be found. 2 To be precise, the memory contains the number of data sets written or read. The number of bytes can be easily calculated from these numbers. Used LTO cartridges page 3

4 accessed. Accessing random records on tape requires movement of the tape to locate the records. If this is the case, the actual amount of tape that moved through the drive s tape path is probably much higher than the figure of number of bytes transferred suggests. It should ideally require only one load to write at least one wrap (i.e., one movement of tape between beginning and end of tape in either direction 3 ). The number of loads should not exceed 50 times the number of bytes transferred divided by the total cartridge capacity. Age of cartridge An old cartridge is not necessarily a bad cartridge. A cartridge should last for a long time without any negative effects on its performance if it has been stored in a controlled environment. However, the older the cartridge is it is natural that more opportunities would arise for its abuse. Given a choice, a newer cartridge is preferable to an old cartridge, so statistically old age is considered as a weak negative. Pointing fingers When a bad tape cartridge is found there are many directions in which to point a finger of blame. The cartridge memory chip is written and read by the tape drive. If a tape drive is malfunctioning the cartridge could appear defective because the drive may falsely increment the error counters. The error condition may exist only temporarily when, for example, debris is deposited on the head. Movement of tape across the head or other conditions can remove the debris and the drive may function correctly thereafter. In this case the damage is limited. A permanently defective drive can increment the error counters of the cartridge so much that a perfectly good cartridge could appear to be damaged. Fortunately there are some indicators that can narrow down the possibilities of blame. In addition to the overall number of errors made during the life of a tape, the cartridge memory also contains details of each of the last four sessions. If a cartridge has a high error count but performed flawlessly the last four times the cartridge was used, we tend to blame the errors on the tape drive. This is because tape cartridges generally cannot repair themselves (bad sections are simply skipped over). However, if most of these errors occurred during the last four sessions, the cartridge is more likely to be faulty. The details of the last four sessions also contain the serial numbers of the tape drives which were used, so if the errors occurred on several different drives (with of course the same tape) it is a very strong indicator that the problem is with the cartridge. How to extract the information There are at least two ways to extract the CM information. It is available through the host interface of LTO tape drives. Special software on the host can analyze the data. 3 This is due to the serpentine nature of tape writing. Used LTO cartridges page 4

5 The CM can also be read externally without having to load the cartridge into a drive. Through a wireless RF interface, an external device can access the CM. The device manufactured by our company that connects to a system through either a USB interface. Application software that runs on most Windows operating systems reads and displays the data. Because the cartridge does not need to be loaded into a drive, the operation is almost instantaneous and allows many cartridges to be evaluated in a very short period of time. Putting the pieces together Looking at all the data to evaluate a cartridge can be time consuming, tedious, and inconsistent. Not everyone gives each parameter the same weight. We found it more helpful to create a single score number that is automatically and consistently calculated from all available data. We call this score number the Tape Evaluation Score (TES) with values ranging from 0 (very bad) to 100 (very good). In addition to the CM statistics discussed in this article, we also display the TES. In general we discourage the purchase of used tape cartridges with a TES score lower than 50. It is fairly safe to purchase cartridges with a TES of 75 and higher. Cartridges with a TES between 50 and 75 are a bit risky, but if you get a really good price on it, and you don t mind an occasional job crash, we recommend: go for it. Conclusion Purchasing used LTO cartridges can be frugal and safe if you know what you are doing. There is no reason to blindly purchase used LTO cartridges. If you don t know where a cartridge has been, you may be gambling with your company s data. So before purchasing any used cartridges, ask your vendor for quality information (the TES or similar) about the cartridge. The cartridge knows its past history. Don t you think you should know it too? Used LTO cartridges page 5