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RAID TECHNOLOGY

RAID TECHNOLOGY. MAGNETIC DISK STORAGE. Before we can fully understand RAID, we must first understand the inner workings of a magnetic hard disk, and the problems often associated with a disk. A “hard” disk is different from the also well known

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RAID TECHNOLOGY

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  1. RAID TECHNOLOGY

  2. MAGNETIC DISK STORAGE • Before we can fully understand RAID, we must first understand the inner workings of a magnetic hard disk, and the problems often associated with a disk. • A “hard” disk is different from the also well known “floppy” disk because it has a fixed,rigid platter that is coated in a magnetic material, which stores the actual information. • This rigid platter has a motor that causes it to spin at speeds varying from 5400 rotations per minute (RPM), up to 15,000 RPM.

  3. There are several obvious points of failure in a hard disk: moving mechanical parts can deteriorate and break, or there can be manufacturing defects, such as surface imperfections, or dust contamination. • RAID DEFINED “RAID” was originally coined by David A.Patterson, Garth Gibson, and Randy H. Katz of theUniversityof California at Berkeley, in their 1988 paper, “A Case for Redundant Arrays of Inexpensive Disks” (David A Patterson 1988).

  4. RAID is a method of combining several hard disk drives into one logical unit, called a LUN, or a "logical unit number”, appearing as a single device to the host system. Disk protection is achieved any number of ways based on the particular implementation of RAID. “RAID”technology was developed to address the Fault tolerance and performance limitations of conventional disk storage” (Technick.net).

  5. HISTORY • By the time “A Case for Redundant Arrays of Inexpensive Disks” had been written in 1988, magnetic disks had become very large, dense, and costly pieces of equipment.Often requiring a strictly controlled environment, use of these disks was generally restricted to only the largest computer systems. • With the number of personal computers on the rise and an increasing reliability on electronic data storage, disk designers began to design smaller and cheaper disks, as an alternative to these “single large expensive disks”(SLEDs).

  6. When compared to the larger expensive discs, it was found that these smaller“inexpensive” disks were at least equal to, if not better than, the larger expensive disks, in terms of both I/Os per second per actuator and price per megabyte. Most importantly, these smaller disks had the same basic functionality as the larger disks, including an embedded SCSI chip controller, which could be used as a Direct Memory Access device at the end of the SCSI bus.

  7. Based on the fact that the smaller disks had the same functionality as the larger disks,storage systems could be built as arrays of these in expensive disks. The idea was to not only overcome the problems associated with the single large disks, but to also achieve better performance and increased reliability for both small and large data storage systems.

  8. RELIABILITY • RAID addresses the issue of disk reliability by making use of “redundancy”. This means thatdata is redundantly distributed across all (or some) of the disks for the purposes of fault tolerance and data protection. If one disk fails or if a piece of data becomes corrupted, it can be recovered from one or more of the other disks. • In dealing with redundancy, two things need to be considered: • 1. Calculating the redundant information in the event of an error.

  9. 2. The method of distributing the information across the disks . These two points are considered and addressed based on the type of RAID. Performance Disk performance is enhanced based on the fact that there are multiple disks working in parallel. Depending on the distribution of data, different pieces of information can be read from different disks at the same time .

  10. RAID TECHNOLOGY RAID LEVEL O : In a RAID 0 system data are split up in blocks that get written across all the drives in the array. By using multiple disks (at least 2) at the same time, this offers superior I/O performance. This performance can be enhanced further by using multiple controllers, ideally one controller per disk.

  11. Advantages RAID 0 offers great performance, both in read and write operations. There is no overhead caused by parity controls. All storage capacity is used, there is no disk overhead. The technology is easy to implement.

  12. Disadvantages: RAID 0 is not fault-tolerant. If one disk fails, all data in the RAID 0 array are lost. It should not be used on mission-critical systems. Ideal use: RAID 0 is ideal for non-critical storage of data that have to be read/written at a high speed, such as on a Photoshop image retouching station. RAID level 1 – Mirroring: Data are stored twice by writing them to both the data disk (or set of data disks) and a mirror disk (or set of disks) . If a disk fails, the controller useseither the data drive or the mirror drive for data recovery and continues operation. You need at least 2 disks

  13. for a RAID 1 array. RAID 1 systems are often combined with RAID 0 to improve performance. Such a system is sometimes referred to by the combined number: a RAID 10 system.

  14. Advantages: RAID 1 offers excellent read speed and a write-speed that is comparable to that of a single disk. In case a disk fails, data do not have to be rebuild, they just have to be copied to the replacement disk. RAID 1 is a very simple technology. Disadvantages: The main disadvantage is that the effective storage capacity is only half of the total disk capacity because all data get written twice. Software RAID 1 solutions do not always allow a hot swap of a failed disk (meaning it cannot be replaced while the server keeps running). Ideally a hardware controller is used.

  15. Ideal use: RAID-1 is ideal for mission critical storage, for instance for accounting systems. It is also suitable for small servers in which only two disks will be used. RAID level 3: On RAID 3 systems, data blocks are subdivided (striped) and written in parallel on two or more drives. An additional drive stores parity information. You need at least 3 disks for a RAID 3 array.

  16. Since parity is used, a RAID 3 stripe set can withstand a single disk failure without losing data or access to data. Advantages: RAID-3 provides high throughput (both read and write) for large data transfers. Disk failures do not significantly slow down throughput. Disadvantages: This technology is fairly complex and too resource intensive to be done in software. Performance is slower for random, small I/O operations.

  17. Ideal use: RAID 3 is not that common in prepress. RAID level 5: RAID 5 is the most common secure RAID level. It is similar to RAID-3 except that data are transferred to disks by independent read and write operations (not in parallel). The data chunks that are written are also larger. Instead of a dedicated parity disk, parity information is spread across all the drives. You need at least 3 disks for a RAID 5 array.

  18. A RAID 5 array can withstand a single disk failure without losing data or access to data. Although RAID 5 can be achieved in software, a hardware controller is recommended. Often extra cache memory is used on these controllers to improve the write performance.

  19. Advantages: Read data transactions are very fast while write data transaction are somewhat slower (due to the parity that has to be calculated). Disadvantages: Disk failures have an effect on throughput, although this is still acceptable. Like RAID 3, this is complex technology. Ideal use: RAID 5 is a good all-round system that combines efficient storage with excellent security and decent performance. It is ideal for file and application servers.

  20. RAID level 10 – Combining RAID 0 & RAID 1 RAID 10 combines the advantages (and disadvantages) of RAID 0 and RAID 1 in one single system. It provides security by mirroring all data on a secondary set of disks (disk 3 and 4 in the drawing below) while using striping across each set of disks to speed up data transfers.

  21. Future RAID: RAID DP: Conventional RAID schemes which use a single parity drive (RAID 3 for example) can easily recover one failed disk. But what if a second disk fails before the previous has been rebuilt? This is very common considering the amount of time it can take to rebuild an entire drive (Sunstar Company 2007). RAID DP uses a second parity disk which can rebuild a second drive in a double disk failure situation. Parity for the second drive is calculated differently than the first.

  22. RAID Z: The future may not be too far away. Sun introduced the ZFS file system with the release of their Solaris 10 operating system. The “Z” stands for Zettabyte, which to put in perspective, the IDC estimates that by 2010, there will be 988 Exabyte, just under a Zett a byte, in all computer storage worldwide [29]. ZFS is a 128bit File system, which gives its user 16 billion times the capacity of 32 or 64 bit systems (Microsystems 2007). RAID Controllers: A RAID controller acts as an interface between the host system and the RAID system. It

  23. serves to manage the physical disks and deliver the data to the host system. Interfacing with RAID: A storage system can be connected to the host system in a number of ways. One way uses “Serial Advanced Technology Attachment”, or SATA, which communicates over a serial link (Planet 2003). Another way is “Small Computer System Interface”, or SCSI,which communicates using a series of commands and is typically faster, but more expensive (Planet 2004; Adaptec 2007). iSCSI and Fibre Channel are other technologies typically used (Freeman 2004).

  24. Software Based RAID: A type of “Direct Attached Storage”, or DAS, a software based RAID system functions through the operating system. It is typically lower in performance than hardware based RAID, “due to the lack of dedicated hardware” (STATS 2004). It relies on the host system CPU for all operations. Software RAID is very easy to set up and is typically very flexible (STATS 2004). Hardware Based RAID: Another kind of DAS, hardware based RAID, functions similarly to Software Based RAID except that it uses an additional piece of hardware for interfacing with the external disks.

  25. Conclusion Information is being generated at an These controller cards offload processor intensive RAID operations to enhance performance. They may use SATA, SCSI, iSCSI, or Fiber Channel technology. Conclusion: Information is being generated at an alarmingly fast rate. The world relies heavily on data storage. RAID provides a reliable and safe way to store and manage data, but there are many options. Depending on implementation, RAID can be configured for enhanced performance, increased reliability, or some combination of both.

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