中国科学院：CERN专题计算学校《T-CSC数据存储》课程教学资源（讲义）Preserving data-pres

Preserving data 4 risks co ncy safety c/e Preserving data Sebastien Ponce sebastien.ponce@cern.ch CERN Thematic CERN School of Computing 2018 1/43 S.Ponce-CERN

Preserving data 1 / 43 S. Ponce - CERN risks consistency safety c/c Preserving data S´ebastien Ponce sebastien.ponce@cern.ch CERN Thematic CERN School of Computing 2018

Preserving data 花5 In the previous episodes... o We've found out how to store data efficiently o And how to distribute it o And even how to distribute the computation 2/43 S.Ponce-CERN

Preserving data 2 / 43 S. Ponce - CERN risks consistency safety c/c In the previous episodes... We’ve found out how to store data efficiently And how to distribute it And even how to distribute the computation Today Let’s make sure we do not lose or corrupt our nice data !

Preserving data In the previous episodes... We've found out how to store data efficiently o And how to distribute it o And even how to distribute the computation Today Let's make sure we do not lose or corrupt our nice data 2/43 S.Ponce-CERN

Preserving data 2 / 43 S. Ponce - CERN risks consistency safety c/c In the previous episodes... We’ve found out how to store data efficiently And how to distribute it And even how to distribute the computation Today Let’s make sure we do not lose or corrupt our nice data !

Preserving data risls conaistency safety c/c Outline Risks of data loss and corruption ② Data consistency ●Checksums ●Block checksums ③ Data safety ●Redundancy ●Parity ●Erasure coding Conclusion 3/43 S.Ponce-CERN

Preserving data 3 / 43 S. Ponce - CERN risks consistency safety c/c Outline 1 Risks of data loss and corruption 2 Data consistency Checksums Block checksums 3 Data safety Redundancy Parity Erasure coding 4 Conclusion

Preserving data risks consistency safety c/e Risks of data loss and corruption ① Risks of data loss and corruption 2 Data consistency Data safety Conclusion 4/43 S.Ponce-CERN

Preserving data 4 / 43 S. Ponce - CERN risks consistency safety c/c Risks of data loss and corruption 1 Risks of data loss and corruption 2 Data consistency 3 Data safety 4 Conclusion

Preserving data risks consistency Risks for my data -Hardware some numbers for disks probability of losing a disk per year:few %up to 10% with 60K disks,it's around 10 per day 。and all files are lost o one unrecoverable bit error in 1014 bits read/written for 1GB files,that's one file corrupted per 10K files written some numbers for tapes probability of losing a tape per year:10-4 and you recover most of the data on it o net result is 10-7 file loss per year one unrecoverable bit error in 1019 bits read/written for 1GB files,that's one file corrupted per 1G files written 5/43 S.Ponce-CERN

Preserving data 5 / 43 S. Ponce - CERN risks consistency safety c/c Risks for my data - Hardware some numbers for disks probability of losing a disk per year : few %, up to 10% with 60K disks, it’s around 10 per day and all files are lost one unrecoverable bit error in 1014 bits read/written for 1GB files, that’s one file corrupted per 10K files written some numbers for tapes probability of losing a tape per year : 10−4 and you recover most of the data on it net result is 10−7 file loss per year one unrecoverable bit error in 1019 bits read/written for 1GB files, that’s one file corrupted per 1G files written

Preserving data risks consistency Risks for my data -Software BUGS o in your software oe.g.scheduling twice a transfer,not receiving data on the second run and overwriting the correct file with an empty one o in your dependencies .e.g.the transfer protocol used does not support checksum and data may be corrupted by TCP(checksum is only 16 bit,one corrupted packet in 65536 will go through) o in the OS or common libraries e.g.libc locks not being atomic o in the hardware-that is in the micro code running inside e.g.RAID controllers o in your admin tools o e.g.recycling a tape that was not empty 6/43 S.Ponce-CERN

Preserving data 6 / 43 S. Ponce - CERN risks consistency safety c/c Risks for my data - Software BUGS ! in your software e.g. scheduling twice a transfer, not receiving data on the second run and overwriting the correct file with an empty one in your dependencies e.g. the transfer protocol used does not support checksum and data may be corrupted by TCP (checksum is only 16 bit, one corrupted packet in 65536 will go through) in the OS or common libraries e.g. libc locks not being atomic in the hardware - that is in the micro code running inside e.g. RAID controllers in your admin tools e.g. recycling a tape that was not empty

Preserving data Risks for my data-Human factor Real life cases that went wrong reinstall (and wipe)old machine p23425a4752 Oh no,I actually meant p42532a8779...bad cut and paste rm-rf/top/data/alltimes /2015/04/crap one space too much and all data are gone.... activate garbage collection on pool XYZ,it's full wasn't it tape backed up no oups.... 7/43 S.Ponce-CERN

Preserving data 7 / 43 S. Ponce - CERN risks consistency safety c/c Risks for my data - Human factor Real life cases that went wrong reinstall (and wipe) old machine p23425a4752 Oh no, I actually meant p42532a8779... bad cut and paste rm -rf /top/data/alltimes /2015/04/crap one space too much and all data are gone.... activate garbage collection on pool XYZ, it’s full wasn’t it tape backed up ? no ? oups

Preserving data risks consistency safety c/e Risks for my data -conclusion 8/43 S.Ponce-CERN

Preserving data 8 / 43 S. Ponce - CERN risks consistency safety c/c Risks for my data - conclusion You will lose/corrupt data ! better to be able to know when and what even better if you can repair

Preserving data risks conaistency Risks for my data -conclusion You will lose/corrupt data 8/43 S.Ponce-CERN

Preserving data 8 / 43 S. Ponce - CERN risks consistency safety c/c Risks for my data - conclusion You will lose/corrupt data ! better to be able to know when and what even better if you can repair