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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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IV Data Storage and Querying
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Note that a parity block cannot store parity for blocks in the same disk, since then a disk failure would result in loss of data as well as of parity, and hence would not be recoverable Level 5 subsumes level 4, since it offers better read write performance at the same cost, so level 4 is not used in practice RAID level 6, the P + Q redundancy scheme, is much like RAID level 5, but stores extra redundant information to guard against multiple disk failures Instead of using parity, level 6 uses error-correcting codes such as the Reed Solomon codes (see the bibliographical notes) In the scheme in Figure 114g, 2 bits of redundant data are stored for every 4 bits of data unlike 1 parity bit in level 5 and the system can tolerate two disk failures Finally, we note that several variations have been proposed to the basic RAID schemes described here Some vendors use their own terminology to describe their RAID implementations2 However, the terminology we have presented is the most widely used
1134 Choice of RAID Level
The factors to be taken into account when choosing a RAID level are Monetary cost of extra disk storage requirements Performance requirements in terms of number of I/O operations Performance when a disk has failed Performance during rebuild (that is, while the data in a failed disk is being rebuilt on a new disk) The time to rebuild the data of a failed disk can be signi cant, and varies with the RAID level that is used Rebuilding is easiest for RAID level 1, since data can be copied from another disk; for the other levels, we need to access all the other disks in the array to rebuild data of a failed disk The rebuild performance of a RAID system may be an important factor if continuous availability of data is required, as it is in high-performance database systems Furthermore, since rebuild time can form a signi cant part of the repair time, rebuild performance also in uences the mean time to data loss
2 For example, some products use RAID level 1 to refer to mirroring without striping, and level 1+0 or level 10 to refer to mirroring with striping Such a distinction is not really necessary since not striping can simply be viewed as a special case of striping, namely striping across 1 disk
Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
IV Data Storage and Querying
11 Storage and File Structure
The McGraw Hill Companies, 2001
RAID
RAID level 0 is used in high-performance applications where data safety is not critical Since RAID levels 2 and 4 are subsumed by RAID levels 3 and 5, the choice of RAID levels is restricted to the remaining levels Bit striping (level 3) is rarely used since block striping (level 5) gives as good data transfer rates for large transfers, while using fewer disks for small transfers For small transfers, the disk access time dominates anyway, so the bene t of parallel reads diminishes In fact, level 3 may perform worse than level 5 for a small transfer, since the transfer completes only when corresponding sectors on all disks have been fetched; the average latency for the disk array thus becomes very close to the worst-case latency for a single disk, negating the bene ts of higher transfer rates Level 6 is not supported currently by many RAID implementations, but it offers better reliability than level 5 and can be used in applications where data safety is very important The choice between RAID level 1 and level 5 is harder to make RAID level 1 is popular for applications such as storage of log les in a database system, since it offers the best write performance RAID level 5 has a lower storage overhead than level 1, but has a higher time overhead for writes For applications where data are read frequently, and written rarely, level 5 is the preferred choice Disk storage capacities have been growing at a rate of over 50 percent per year for many years, and the cost per byte has been falling at the same rate As a result, for many existing database applications with moderate storage requirements, the monetary cost of the extra disk storage needed for mirroring has become relatively small (the extra monetary cost, however, remains a signi cant issue for storage-intensive applications such as video data storage) Access speeds have improved at a much slower rate (around a factor of 3 over 10 years), while the number of I/O operations required per second has increased tremendously, particularly for Web application servers RAID level 5, which increases the number of I/O operations needed to write a single logical block, pays a signi cant time penalty in terms of write performance RAID level 1 is therefore the RAID level of choice for many applications with moderate storage requirements, and high I/O requirements RAID system designers have to make several other decisions as well For example, how many disks should there be in an array How many bits should be protected by each parity bit If there are more disks in an array, data-transfer rates are higher, but the system would be more expensive If there are more bits protected by a parity bit, the space overhead due to parity bits is lower, but there is an increased chance that a second disk will fail before the rst failed disk is repaired, and that will result in data loss
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