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Friday, February 12, 2010

Performance Issues of Tertiary storage

There are three aspects of tertiary-storage performance :
- Speed : There are two aspects of speed in tertiary storage are bandwidth and
Bandwidth is measured in bytes per second.
* Sustained bandwidth – average data rate during a large transfer; # of bytes/transfer time.Data rate when the data stream is actually flowing.
* Effective bandwidth – average over the entire I/O time, including seek or locate, and cartridge switching. It drive’s overall data rate.

Access latency is the amount of time needed to locate data.
* Access time for a disk – It moves the arm to the selected cylinder and wait for the rotational latency; < 35 milliseconds.
* Access on tape requires winding the tape reels until the selected block reaches the tape head; tens or hundreds of seconds.
* Generally say that random access within a tape cartridge is about a thousand times slower than random access on disk.
The low cost of tertiary storage is a result of having many cheap cartridges share a few expensive drives. A removable library is best devoted to the storage of infrequently used data, because the library can only satisfy a relatively small number of I/O requests per hour.

- Reliability : A fixed disk drive is likely to be more reliable than a removable
disk or tape drive. An optical cartridge is likely to be more reliable than a
magnetic disk or tape. A head crash in a fixed hard disk generally destroys the data, whereas the failure of a tape drive or optical disk drive often leaves the data cartridge unharmed.

- Cost : The main memory is much more expensive than disk storage. The cost per megabyte of hard disk storage is competitive with magnetic tape if only one tape is used per drive. The cheapest tape drives and the cheapest disk drives have
had about the same storage capacity over the years. Tertiary storage gives a cost savings only when the number of cartridges is considerably larger than the number of drives.

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