Performance Measures (Cont.) 加 Mean time to failure(MTTF)-the average time the disk is expected to run continuously without any failure. ·Typically3to5 years Probability of failure of new disks is quite low,corresponding to a "theoretical MTTF"of 500,000 to 1,200,000 hours for a new disk E.g.,an MTTF of 1,200,000 hours for a new disk means that given 1000 relatively new disks,on an average one will fail every 1200 hours MTTF decreases as disk ages Database System Concepts-7th Edition 12.11 ©Silberscha乜，Korth and SudarshanDatabase System Concepts - 7 12.11 ©Silberschatz, Korth and Sudarshan th Edition Performance Measures (Cont.) ▪ Mean time to failure (MTTF) – the average time the disk is expected to run continuously without any failure. • Typically 3 to 5 years • Probability of failure of new disks is quite low, corresponding to a “theoretical MTTF” of 500,000 to 1,200,000 hours for a new disk ▪ E.g., an MTTF of 1,200,000 hours for a new disk means that given 1000 relatively new disks, on an average one will fail every 1200 hours • MTTF decreases as disk ages