Coding and Algorithms for Memories Lecture 13 1

Large Scale Storage Systems 2 Big Data Players: Facebook, Amazon, Google, Yahoo,… Cluster of machines running Hadoop at Yahoo! (Source: Yahoo!) Failures are the norm

Node failures at Facebook 3 Date XORing Elephants: Novel Erasure Codes for Big Data M. Sathiamoorthy, M. Asteris, D. Papailiopoulos, A. G. Dimakis, R. Vadali, S. Chen, and D. Borthakur, VLDB 2013

Problem Setup Disks are stored together in a group (rack) Disk failures should be supported Requirements: – Support as many disk failures as possible – And yet… Optimal and fast recovery Low complexity 4

Reed Solomon Codes 5

Advantages: – Support the maximum number of disk failures – Are very comment in practice and have relatively efficient encoding/decoding schemes Disadvantages – Require to work over large fields Solution: EvenOdd Codes – Need to read all the disks in order to recover even a single disk failure – not efficient rebuild Solution: ZigZag Codes 6

The Repair Problem P1 P3 P4 P2 A disk is lost – Repair job starts Access, read, and transmit data of disks! Overuse of system resources during single repair Goal: Reduce repair cost in a single disk repair Facebook’s storage Scheme: – 10 data blocks – 4 parity blocks – Can tolerate any four disk failures RS code

ZigZag Codes Designed by Itzhak Tamo, Zhiying Wang, and Jehoshua Bruck The goal: construct codes correcting the max number of erasures and yet allow efficient reconstruction if only a single drive fails 8

ZigZag Codes Lower bound: The min amount of data required to be read to recover a single drive failure – (n,k) code: n drives, k information, and n-k redundancy – M- size of a single drive in bits For (n,n-2) code it is required to read at least 1/2 from the remaining drives, that is at least (1/2)(n-1)M bits – The last example is optimal In general, for (n,n-r) code it required to read at least 1/r from the remaining drives (1/r)(n-1)M 9

ZigZag Codes Example 10 info 1info 2info 3 Row parity ZigZag parity

Network Coding for Distributed Storage Goal – show the following: In general, for (n,n-r) code it required to read at least 1/r from the remaining drives (1/r)(n-1)M Network Coding for Distributed Storage Dimakis, Godfrey, Wu, Wainwright, Ramchandran File of size M is partitioned into k pieces of size M/k The k pieces are encoded into n encoded pieces using an (n,k) MDS code 11

Network Coding for Distributed Storage File of size M is partitioned into k pieces of size M/k The k pieces are encoded into n encoded pieces using an (n,k) MDS code 12 y1y1 y1y1 y2y2 y2y2 x1x1 x1x1 x2x2 x2x2 x3x3 x3x3 x4x4 x4x4

Network Coding for Distributed Storage File of size M is partitioned into k pieces of size M/k The k pieces are encoded into n encoded pieces using an (n,k) MDS code 13 y1y1 y1y1 y2y2 y2y2 x1x1 x1x1 x2x2 x2x2 x3x3 x3x3 x4x4 x4x4 x5x5 x5x5 β=? β β

Network Coding for Distributed Storage File of size M is partitioned into k pieces of size M/k The k pieces are encoded into n encoded pieces using an (n,k) MDS code 14 S S x 1 ou t x 2 ou t x 3 ou t x 4 ou t x 5 in β=? β β x 1 in x 2 in x 3 in x 4 in ∞ ∞ ∞ ∞ α=1 DC x 5 ou t ∞ ∞

ZigZag Codes Example 15 aba+ba+2d cdc+dc+b