Back to the Future with Tape Drives Page 2Understanding your application(s) environment is critical to developing a good architecture. Tape drives and the associated libraries have different characteristics for tape load, tape ready, position and rewind time. In some cases this is not important in applications, such as backup, where in most cases all you are doing is loading the tape and writing large amounts of data sequentially.
On the other hand, with hierarchical storage management, applications tape load, position and rewind time becomes a critical issue, especially for reading data back. HSM applications are become more popular given the length of time required for backup with increasing storage densities. In fact, StorageTek developed the T9840A and B drives specifically for HSM application with small files, as it has a 4 second load time and average 8 seconds to first data byte. Typical other products are 6 to 15 times the T9840A and B time to first byte.
But if the files are large, load and position time become insignificant compared with transfer time. If you have a 20-gigabyte file and with compression the transfer rate is 30 megabytes/sec, the transfer time is 682 seconds. With a 50-second load and position time this is only about 7.5 percent of the total time. A good rule of thumb when setting up a system is to try and keep load and position to 10 percent of the time to write the data.
For HSM applications, of course, reading is a different story, as most applications can consolidate the files ensure large amounts of data is written. Reading for HSM applications becomes an issue for using tape for HSM at it requires an understanding of the recall rate of the files, the size of the files recalled, and -- most important -- the speed requirement for recall. A credit-card company that stores information to provide approval codes is far different than a research site doing genetic research recalling a gene for comparison.
Given all this: is tape dead? A number of the large storage vendors pronounced tape dead three years ago, two years ago, last year, and likely next year. Tape has some significant advantages over disk storage, so it will be some time before tape is dead. Here are some reasons why:
- Tape does not require power. Most modern disk drives required power to be powered on for reliability; the Seagate 120GB ATA drive, for example, uses 13 watts. That can get really expensive if you have 400 terabytes of secondary storage.
- Lower error rates. Bit error rates for ATA drives (FC and SCSI drives are an order of magnitude better) are 10 to the 14th while bit error rates for enterprise tape are 10 to the 18th, and other tapes (AIT and DLT) are around 10 to 17th. Tapes are between two and four orders of magnitude more reliable than both ATA and SCSI disk drives.
- Tapes can handle higher shock than disks and still survive. We all have either personally dropped a tape or surely have seen someone drop a tape.
What to Do
For at least the next few years tapes and tape drives will continue to be a critical part of the storage infrastructure. This will continue because tape is far cheaper than disk storage in total cost of ownership, given the issues with power requirements for rotating storage and compression support with tape drives. Almost all of the tapes in the market claim 30 years of shelf life even lower-end tapes. (Of course, having a tape for 30 years might be possible, but how are you doing to be able to read it?) Tapes, as with any storage medium are dependent on outside influences like:
- What is the interface? Try finding an SCSI-1 interface from the early 1990s, much less IPI-3 interface 20 years from now.
- Will the tape drive be available to read the tape? A little over 30 years ago 7-track tapes were state of the art, but not finding one to read a tape today will be next to impossible.
- What is the data format of the tape? Some vendors write in tar format, for example, but will tar or an application like Veritas Netbackup be available in 2033? Tar probably will be, at least.
- What is the data and will any program be able to read it? PDF is a popular format today, and applications can read it today, but what about reading a MS Word 2.0 document from just 10 years ago from tape and try reading it into MS Word 2002?
All in all, it's clear that a migration strategy as part of the initial decision process is essential. Nothing last forever -- especially your data.