Chapter 12 4 Mass Storage Systems Chapter 12 3 Mass Storage Systems Chapter 12 1 Chapter 12 2 Disk Attachment Disk Scheduling Chapter 12 3 Disk Management Swap Space Management Overview of Mass Storage Structure RAID Structure Chapter 12 4 RAID Structure Stable Storage Implementation Tertiary Storage Devices Operating System Issues Performance Issues Operating System Concepts 12 2 Silberschatz Galvin and Gagne 2005 RAID Continued Operating System Concepts 12 3 Silberschatz Galvin and Gagne 2005 RAID different source modified here and there RAID is a technology that employs the simultaneous use of two or more hard disk drives to achieve greater levels of performance reliability and or larger data volume sizes The phrase RAID is an umbrella term for computer data storage schemes that can divide and replicate data among multiple hard disk drives RAID s various designs all involve two key design goals increased data reliability and increased input output performance When several physical disks are set up to use RAID technology they are said to be in a RAID array This array distributes data across several disks but the array is seen by the computer user and operating system as one single disk Operating System Concepts 12 4 Silberschatz Galvin and Gagne 2005 RAID continued Features Principle Some arrays are redundant in a way such that write operations result in extra data derived from original data captured across an array of disks organized so that the failure of one sometimes more disks in the array will not result in loss of data a bad disk is replaced by a new one and the data on it can be reconstructed from the remaining data and the extra data Other RAID arrays are arranged so that they are faster to write to and read from than a single disk It is important to note that a redundant array allows less data to be stored For instance a 2 disk RAID 1 array loses half of its capacity because data is stored twice and a RAID 5 array with several disks loses the capacity of one full disk There are various combinations of these approaches giving different trade offs of protection against data loss capacity and speed RAID levels 0 1 and 5 are the most commonly found and cover most requirements Operating System Concepts 12 5 Silberschatz Galvin and Gagne 2005 RAID continued Important Levels RAID 0 striped disks distributes data across several disks in a way which gives improved speed and full capacity but all data on all disks will be lost if any one disk fails figures ahead RAID 1 mirrored disks could be described as a backup solution uses two possibly more disks which each store the same data hence the term mirrored so that data is not lost as long as one disk survives Total capacity of the array is just the capacity of a single disk The failure of one drive in the event of a hardware or software malfunction does not increase the chance of a failure nor decrease the reliability of the remaining drives second third etc RAID 5 striped disks with parity combines three or more disks in a way that protects data against loss of any one disk Here capacity of the array is reduced by one disk RAID 6 less common can recover from the loss of two disks Operating System Concepts 12 6 Silberschatz Galvin and Gagne 2005 RAID Levels 0 1 and 2 In RAID Level 0 we ve got no redundancy Our array of disks has striping at the block level In RAID Level 1 we have disk mirroring For RAID Level 2 I will not discuss because it is rarely used Operating System Concepts 12 7 Silberschatz Galvin and Gagne 2005 RAID continued Computations Involved RAID involves significant computation when reading and writing information With true RAID hardware the disk controller does all of this computation work In other cases the operating system or simpler and often less expensive controllers require the host computer s processor to do the computing which reduces the computer s performance on processorintensive tasks Simpler RAID controllers may provide only levels 0 and 1 which require less processing Operating System Concepts 12 8 Silberschatz Galvin and Gagne 2005 RAID continued Rebuilding Systems A very nice feature is that RAID systems with redundancy continue working without interruption when one or sometimes more disks of the array fail although they are vulnerable to further failures When the bad disk is replaced by a new one the array is rebuilt while the system continues to operate normally Some systems have to be shut down when removing or adding a drive others support this kind of hot swapping allowing drives to be replaced without powering down RAID with hot swap drives is often used in high availability systems where it is essential that the system keeps running as much of the time as possible Operating System Concepts 12 9 Silberschatz Galvin and Gagne 2005 RAID continued Backup RAID is not in general a good alternative to backing up data In general data may become damaged or destroyed without harm to the drive s on which it is stored For example Part of the data may be overwritten by a system malfunction A file may be damaged or deleted by user error or malice and not noticed for days or weeks Of course the entire array is at risk of catastrophes such as theft flood and fire Presented ahead are a few more visuals from the text and some additional information Please also note that there are additional configurations of RAIDs that we will not discuss Operating System Concepts 12 10 Silberschatz Galvin and Gagne 2005 RAID Level3 RAID 3 is a bit interleaved parity organization Unlike memory systems disk controllers can detect whether a sector has been read correctly so a single parity bit can be used for error correction as well as for detection If one of the sectors is damaged we know exactly which sector it is and the controller can figure out whether any bit in the sector is a 1 or a 0 by computing the parity of the corresponding bits from the sectors in the other disks If the parity of the remaining bits is equal to the stored parity the missing bit is 0 otherwise it is 1 RAID level 3 is good as level 2 but is less expensive in the number of extra disks required it has only onedisk overhead so level 2 is not used in practice Operating System Concepts 12 11 Silberschatz Galvin and Gagne 2005 RAID Levels 4 RAID 4 is block interleaving Operating System Concepts 12 12 Silberschatz Galvin and Gagne 2005 More RAID Levels RAID6 more Text elaborates in more detail on RAID6 and cites that additional redundant information is