1. 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 3 Hitachi Global Storage Technologies, San Jose Research Center,

2. Introduction Large archival storage systems: Protect data more proactively Keep disks powered off for long periods of time Have low rate of data access Protect data by storing it redundantly.

3. Introduction Failures can happen At the block level. At the device level. Failures may remain undetected for long periods of time. A failure may unmask one or more additional failures. Reconstruction procedure accesses data on other devices. Those devices can have suffered previous failures.

4. Introduction We investigate the efficacy of disk scrubbing. Disk Scrubbing accesses a disk to see whether the data can still be read. Reading a single block shows that the device still works. Reading all blocks shows that we can read all the data on the block.

5. Contents 1. Disk Failure Taxonomy 2. System Overview 3. Disk Scrubbing Modeling 4. Power Cycles and Reliability 5. Optimal Scrubbing Interval 6. Simulation Results

6. Disk Failure Taxonomy Disk Blocks 512B sector uses error control coding Read to a block successfully either corrects all errors, or retries and then: flags block as unreadable, or misreads block. Disk Failure Rates Depend highly on Environment: Temperature, Vibrations, Air quality Age. Vintage.

7. Disk Failure Taxonomy Block Failure Rate estimate: Since: 1/3 of all field returns for server drives are due to hard errors. RAID users (90%) do not return drives with hard errors. 10% of all disks sold account for 1/3 of all errors. Hence: Mean Time between Block Failures is 3/10 MTBF of all disk failures. Mean time to disk failure is 3/2 of MTBF. 1 million hour rated drive has 3*10 5 mean time between block failure. 1.5*10 6 mean time between disk failure. This is one back of the envelope calculation based on numbers by one anonymous disk manufacturer. The results seem to be accepted by many.

8. System Overview Disks are powered down when not in use. Use m+k redundancy scheme: Store data in large blocks. m blocks grouped into an r-group. Add k parity data blocks to r-group. Small blocks lead to fast reconstruction and good reconstruction load distribution. Large blocks have slightly better reliability.

9. System Overview Disk Scrubbing Scrub an S - block Can read one block  device not failed. Can read all blocks  can access all data. Can read and verify all blocks  data can be read correctly. Use “algebraic signatures” for that. Can even verify that parity data accurately reflects client data.

10. System Overview If a bad block is detected, we usually can reconstruct its contents with parity / mirrored data. Scrubbing finds the error before it can hurt you.

11. Modeling Scrubbing Random Scrubbing: Scrub an S-block at random. (Exponential distribution). Deterministic Scrubbing: Scrub an S-block at regular intervals.

12. Modeling Scrubbing Opportunistic Scrubbing: 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 (10 3 hours). Average scrub interval 10 4 hours. Block MTBF 10 5 hours.

13. Power Cycling and Reliability Turning a disk on or off has a significant impact. Even if disks move actuators away from surface (laptop disks). No direct data to measure impact of Power On Hours (POH). Extrapolate from Seagate data: One on / off cycle is roughly equivalent to running a disk for eight hours.

14. Determining Scrubbing Intervals Interval too short: Too much traffic  Disks busy  Increased error rate  Lower system MTBF. Interval too long: A failure more likely to unmask other failures.  More failures catastrophic.  Lower system MTBF.

15. Determining Scrubbing Intervals Mirrored reliability block N = 250 disks. Device failure rate: 5·10 5 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.

16. Determining Scrubbing Intervals Scrub frequently: You never know what you might find. Mirrored disks using opportunistic scrubbing (no power-on penalty). Assumes a high disk access rate.

17. Simulation Results 1PB archival data store. Disks have MTBF of 10 5 hours. 10,000 disk drives 10GB reliability blocks. ~1TB/day traffic

18. Simulation Results Two-way Mirroring

19. Simulation Results RAID 5 redundancy scheme

20. Simulation Results Mirroring. 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.

21. Conclusions We have shown that disk scrubbing is a necessity for very large scale storage systems. Our simulations show the impact of power-on / power-off on reliability. We also note that lack of numbers on disk drive reliability prevents public research.