One definition of duty cycle specific to disk drives is this excerpt from a patent description (designed to enforce certain duty cycles):
The disk drive duty cycle can be expressed as a ratio=Ta/t for a given time period t where Ta is the amount of time the disk drive is actively processing read/write commands during the time period. For example, if during a time period t=60 seconds, the disk drive was actively processing read/write commands for a collective total of Ta=15 seconds, then the average duty cycle for that time period would be 25%.
Some SATA drives out there say they have a duty cycle of 100% (24×7 usage) but others are less than 100%. What happens if you exceed the duty cycle? Drives fail more often than the stated MTBF/MTTF. But at what rate?
How can you tell, though, to what extent you are using your drives in terms of a duty cycle? For example, what if you have a general purpose HPC cluster. That cluster accesses data through a clustered file system across multiple arrays of hundreds of disks. I/O patterns will vary. Some weeks you may pull 2 GB/s from your disk array while other weeks you may only pull 750 MB/s. On the other hand, some weeks you may be pulling 40,000 IOPS from your disk arrays while other weeks you are at 20,000 IOPS.
So to determine your actual utilization, would you add up the total IOPS of the SATA drives in your disk array and then track the IOPS actually performed by that array. Take the duty cycle ratio to equal the IOPS performed divided by max possible IOPS and see if that duty cycle ratio is within the threshold of the type of SATA drives you have in your array? Perhaps you take a monthly average? Or would you need to consider the amount of time spent reading/writing–i.e., would a single IO reading/writing more data sequentially count more against the duty cycle than a smaller single IO?
On the other hand, some sources have indicated that drive reliability (ala FC vs SATA) is not so much due to duty cycles related to drive mechanics but to the possibility of bit errors during rebuilds of RAID sets. Some FC drives have more measures taken to correct errors compared to SATA drives resulting in bit errors being perhaps 100 times more likely on SATA drives vs. FC drives (1 in 10^14 bits vs. 1 in 10^16 bits).
Here is some other info I’m taking a look at in regard to drive reliability issues:
- Are Disks the Dominant Contributor for Storage Failures?
- SATA Bit Error Rate Column Puts Some RAID-Based Storage Systems Vendors on the Defensive
- Disk failures in the real world: What does an MTTF of 1,000,000 hours mean to you?
- Panasas Sets New Standard for Storage System Reliability with Tiered-Parity™ Architecture