Data Representation Recovery from Disk Crashes – 13.4 Presented By: Deepti Bhardwaj Roll No. 223_103 SJSU ID:

Contents Recovery from Disk Crashes Mirroring as a Redundancy Technique Parity Blocks An Improving: RAID Coping With Multiple Disk Crashers

Recovery from Disk Crashes: Ways to recover data  The most serious mode of failure for disks is “head crash” where data permanently destroyed.  So to reduce the risk of data loss by disk crashes there are number of schemes which are know as RAID (Redundant Arrays of Independent Disks) schemes.

Recovery from Disk Crashes: Ways to recover data Continue : Recovery from Disk Crashes: Ways to recover data  Each of the schemes starts with one or more disks that hold the data and adding one or more disks that hold information that is completely determined by the contents of the data disks called Redundant Disk.

Mirroring as a Redundancy Technique  Mirroring Scheme is referred as RAID level 1 protection against data loss scheme.  In this scheme we mirror each disk.  One of the disk is called as data disk and other redundant disk.  In this case the only way data can be lost is if there is a second disk crash while the first crash is being repaired.

Parity Blocks  RAID level 4 scheme uses only one redundant disk no matter how many data disks there are.  D isadvantages of Mirroring: uses so many redundant disks  In the redundant disk, the ith block consists of the parity checks for the ith blocks of all the data disks.  It means, the jth bits of all the ith blocks of both data disks and redundant disks, must have an even number of 1’s and redundant disk bit is used to make this condition true.

Parity Blocks – Reading disk Reading data disk is same as reading block from any disk. We could read block from each of the other disks and compute the block of the disk we want to read by taking the modulo-2 sum. disk 2: disk 3: disk 4: If we take the modulo-2 sum of the bits in each column, we get disk 1:

Parity Block - Writing When we write a new block of a data disk, we need to change that block of the redundant disk as well. One approach to do this is to read all the disks and compute the module-2 sum and write to the redundant disk. But this approach requires n-1 reads of data, write a data block and write of redundant disk block. Total = n+1 disk I/Os

Continue : Parity Block - Writing Better approach will require only four disk I/Os 1. Read the old value of the data block being changed. 2. Read the corresponding block of the redundant disk. 3. Write the new data block. 4. Recalculate and write the block of the redundant disk.

Parity Blocks – Failure Recovery If any of the data disk crashes then we just have to compute the module-2 sum to recover the disk. Suppose that disk 2 fails. We need to re compute each block of the replacement disk. We are given the corresponding blocks of the first and third data disks and the redundant disk, so the situation looks like: disk 1: disk 2: ???????? disk 3: disk 4: If we take the modulo-2 sum of each column, we deduce that the missing block of disk 2 is :

An Improvement: RAID 5 RAID 4 is effective in preserving data unless there are two simultaneous disk crashes. Shortcoming of RAID level 4: suffers from a bottleneck defect (when updating data disk need to read and write the redundant disk); Whatever scheme we use for updating the disks, we need to read and write the redundant disk's block. If there are n data disks, then the number of disk writes to the redundant disk will be n times the average number of writes to any one data disk. However we do not have to treat one disk as the redundant disk and the others as data disks. Rather, we could treat each disk as the redundant disk for some of the blocks. This improvement is often called RAID level 5.

Continue : An Improvement: RAID 5 For instance, if there are n + 1 disks numbered 0 through n, we could treat the ith cylinder of disk j as redundant if j is the remainder when i is divided by n+1. For example, n = 3 so there are 4 disks. The first disk, numbered 0, is redundant for its cylinders numbered 4, 8, 12, and so on, because these are the numbers that leave remainder 0 when divided by 4. The disk numbered 1 is redundant for blocks numbered 1, 5, 9, and so on; disk 2 is redundant for blocks 2, 6. 10,..., and disk 3 is redundant for 3, 7, 11,....

Coping With Multiple Disk Crashes RAID 6 : Deals with any number of disk crashes if using enough redundant disks By this strategy the two simultaneous crashes are correctable.  The bits of disk 5 are the modulo-2 sum of the corresponding bits of disks 1, 2, and 3.  The bits of disk 6 are the modulo-2 sum of the corresponding bits of disks 1, 2, and 4.  The bits of disk 7 are the module2 sum of the corresponding bits of disks 1, 3, and 4

Coping With Multiple Disk Crashes – Reading/Writing We may read data from any data disk normally. To write a block of some data disk, we compute the modulo-2 sum of the new and old versions of that block. These bits are then added, in a modulo-2 sum, to the corresponding blocks of all those redundant disks that have 1 in a row in which the written disk also has 1.

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