Slide 1 What Happens Before A Disk Fails? Randi Thomas, Nisha Talagala

Slide 2 Motivation ISTORE: –Proposes to take advantage of predicted failures to improve system robustness –Uses a switched network design to connect intelligent devices to each other to improve system performance. »Therefore ISTORE devices do not share electrical connections »Is this another ISTORE advantage? This talk examines: –The potential to predict failures for disk devices –If and how the failure of a device sharing electrical connections with other devices affects those other devices

Slide 3 Just Before a Disk Fails... Can we predict the disk failure? To answer we will investigate: –What kind of log messages does the system generate? –When do these messages get generated? –How do we distinguish a failing disk from a non-failing disk? Are the other connected devices in the system affected in any way? To answer we will investigate: –Are there correlations between the logged messages?

Slide 4 * Which Logs on What System? –The Error Logs Generated by Berkeley’s Tertiary Disk System –Log Dates: January to November, 1998 * The Tertiary Disk Application –A WEB Accessible Image Collection –Available 24 hours/day, 7 days/week

Slide 5 Outline * Tertiary Disk Architecture Example of a log Message What Kind of Messages are generated? Can we predict the disk failure? Are the other connected devices in the system affected in any way? Summary and Conclusion

Slide 6 The Tertiary Disk Architecture 20 PCs (m0-m19): –200 MHz Pentium Pros –96 MB of RAM –Running FreeBSD version 2.2 –Connected through a switched Ethernet network –Hosts a set of disks using fast-wide SCSI 2 in the single ended mode »Using twin channel SCSI controllers Total of 368 Disks –8 GB each –State of the Art in 1996

Slide 7 The Tertiary Disk Architecture 4 PCs (m0 - m3) have 28 or more disks each: –2-3 SCSI Chains per PC –9-15 Disks per SCSI chain 16 PCs (m4 - m19) have 16 disks each: –2 SCSI Chains per PC –8 Disks per SCSI chain SCSI bus made up of: –SCSI cable: Connects the controller and enclosure –Backplane of the enclosure

Slide 8 The Tertiary Disk Architecture To Ethernet Switch SCSI Cable SCSI Backplane Disk Enclosure SCSI Controller Ethernet Terminator

Slide 9 Outline Tertiary Disk Architecture * Example of a log Message What Kind of Messages are generated? Can we predict the disk failure? Are the other connected devices in the system affected in any way? Summary and Conclusion

Slide 10 Example of A Log Message Oct 22 14:53:50 m6 /kernel: (da1:ahc0:0:1:0): WRITE(06). CDB: a c b1 bf 80 0 Oct 22 14:53:50 m6 /kernel: (da1:ahc0:0:1:0): HARDWARE FAILURE info:cb1bf asc:44,0 Oct 22 14:53:50 m6 /kernel: (da1:ahc0:0:1:0): Internal target failure field replaceable unit: 1 sks:80,3 Month Day Time --> Oct 22 14:53:50 Machine name --> m6 Source of message --> kernel reporting message Error Device --> disk = da1, SCSI bus = ahc0 Description of Error --> Write request had a write fault and caused a HW Failure More information --> Driver & SCSI Controller Codes

Slide 11 Outline Tertiary Disk Architecture Example of a log Message * What Kind of Messages are generated? Can we predict the disk failure? Are the other connected devices in the system affected in any way? Summary and Conclusion

Slide 12 What kind of messages are generated? Data Disk Error Messages: –Hardware Error: The command unsuccessfully terminated due to a non-recoverable hardware failure. (Type is given in the message) –Medium Error: The operation was unsuccessful due to a flaw in the medium --> usually recommends reassigning sectors –Recoverable Error: The last command completed with the help of some error recovery at the target --> e.g. if the drive dynamically reassigned a bad sector to available spare sector –Not Ready: The drive cannot be accessed at all SCSI Error Messages: –Time Outs: Can happen in any of the SCSI bus phases, i.e. message, data, idle. Response: a BUS RESET command –Parity: Cause of an aborted request

Slide 13 Outline Tertiary Disk Architecture Example of a log Message What Kind of Messages are generated? * Can we predict the disk failure? Are the other connected devices in the system affected in any way? Summary and Conclusion

Slide 14 m0: SCSI Time Outs+Recovered Errors SCSI Bus 0

Slide 15 m0: SCSI Time Outs+Recovered Errors SCSI Bus 4

Slide 16 m0: SCSI Time Outs+Recovered Errors SCSI Bus 0

Slide 17 m0: SCSI Time Outs+Recovered Errors SCSI Bus 0

Slide 18 Can we predict a disk failure? Yes, we can look for Recovered Error messages --> on : –There were 433 Recovered Error Messages –These messages lasted for slightly over an hour between: »12:43 and 14:10 On : Disk 5 on m0 was “fired”, i.e. it was about to fail so it was swapped Another example...

Slide 19 m11: SCSI Time Outs SCSI Bus 0

Slide 20 m11: SCSI Time Outs + Hardware Failures SCSI Bus 0

Slide 21 Can we predict a disk failure? Yes, we can also look for Hardware Failure messages --> –These messages lasted for 8 days between: » and –On disk 9 there were: »1763 Hardware Failure Messages, and »297 Timed Out Messages Disk 9 on SCSI Bus 0 of m11 was “fired”, i.e. it was about to fail so it was swapped on

Slide 22 Outline Tertiary Disk Architecture Example of a log Message What Kind of Messages are generated? Can we predict the disk failure? * Are the other connected devices in the system affected in any way? Summary and Conclusion

Slide 23 Are the other connected devices in the system affected in any way? Yes, observe the Time Out message traffic on other disks on the same SCSI bus for --> –The same 8 day period: » and What about predicting other kinds of failures besides just disk failures? --> –Distinguishing between failing and non-failing disks...

Slide 24 m2: SCSI Bus 2 Parity Errors

Slide 25 m2: SCSI Bus 2 Parity Errors

Slide 26 Can We Predict Other Kinds of Failures? Yes, the flurry of parity errors on m2 occurred between: – and , as well as – and On –m2 had a bad enclosure --> cables or connections defective –The enclosure was then swapped Note: The activity logs are not available for the earlier time period.

Slide 27 Can We Distinguish a Failing Disk From a Non-Failing Disk? Yes... SCSI Error Messages alone --> No impending disk failure –As in the m2 Parity example Disk Error Messages alone or accompanied by SCSI Error Messages --> High Probability of an impending disk failure e.g. –ALONE: m0 had only Recovered Error Messages: »Disk 5 was about to fail and therefore was “fired” –BOTH: m11 had both Hardware Failure Disk Messages and Time Out SCSI Messages: »Disk 9 was about to fail and therefore was “fired”

Slide 28 Outline Tertiary Disk Architecture Example of a log Message What Kind of Messages are generated? Can we predict the disk failure? Are the other connected devices in the system affected in any way? * Summary and Conclusion

Slide 29 Total Disk & SCSI Errors Per Machine

Slide 30 Summary and Conclusion Disks don’t fail very often –In the 10 months of logs, only two disks failed –We have only 2 data points for these conclusions! We can predict disk failures and other kinds of failures with enough time to do something about it There are correlations between the logged messages: –Hardware Failure Messages on one disk device propagates as Time Out Messages on: »not only the failing disk, »but also other disks on the same SCSI bus

Slide 31 Back Up Slides

Slide 32 m0: SCSI Time Outs SCSI Bus 2