Disk Scrubbing in Large Archival Storage Systems. Thomas Schwarz, S.J. 1,2 Qin Xin 1,3 , Ethan Miller 1 , Darrell Long 1 , Andy Hospodor 1,2 , Spencer Ng 3 1 Storage Systems Resource Center, U. of California, Santa Cruz 2 Santa Clara University, Santa Clara, CA
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Disk Scrubbing in Large Archival Storage Systems
Thomas Schwarz, S.J.1,2 Qin Xin1,3, Ethan Miller1, Darrell Long1, Andy Hospodor1,2, Spencer Ng3
1 Storage Systems Resource Center, U. of California, Santa Cruz
2 Santa Clara University, Santa Clara, CA
3 Hitachi Global Storage Technologies, San Jose Research Center,
This is one back of the envelope calculation based on numbers by one anonymous disk manufacturer. The results seem to be accepted by many.
Random Scrubbing: Scrub an S-block at random. (Exponential distribution).
Deterministic Scrubbing: Scrub an S-block at regular intervals.
Try to scrub when you access the disk anyway.
“Piggyback scrubs on disk accesses”
Efficiency depends on the frequency of accesses.
MTBA: Mean Time Between Accesses (103 hours).
Average scrub interval 104 hours.
Block MTBF 105 hours.
N = 250 disks.
Device failure rate: 5·105 hours
Block failure rate: 10-5
Time to read disk: 4 hours.
Deterministic: without considering power-up effects.
Deterministic with cycling: considering power-up effects.
Opportunistic does not pay power-on penalty, but runs disk longer.
Random does not pay power-on penalty. Random with cycling would be below the deterministic with cycling graph.
Mirrored reliability block
Scrub frequently: You never know what you might find.
Mirrored disks using opportunistic scrubbing (no power-on penalty).
Assumes a high disk access rate.
RAID 5 redundancy scheme
Opportunistic scrubbing with ~ three disk accesses per year.
Observe that additional scrubbing leads to more power-on cycles that slightly increase occurrence of data losses.