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HP-UX System Administrator's Guide: Routine Management Tasks: HP-UX 11i Version 3 > Appendix A Using High Availability Strategies
Disk Arrays Using RAID Data Protection Strategies |
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RAID stands for Redundant Arrays of Independent Disks. Various configurations or RAID levels are available. We will mention several. In a RAID 1 configuration, all data is duplicated on two or more disks. In hardware mirroring, each disk has a “twin,” a backup disk containing an exact copy of its data. Some RAID 1 implementations duplicate not only the disks but the array controller and the power supply as well. In the case of software mirroring (discussed in “Using Software Mirroring as a Disk Protection Strategy”), the original data and its copied data may be spread over more than one disk as a result of using LVM or VxVM software to manage your disk storage. This configuration interleaves data in blocks across multiple disks. RAID 0 offers increased performance because several I/O transfers can be done at the same time. However, it does not provide data redundancy in the event of disk failure. This type of array uses a separate data protection disk to store encoded data. RAID 3 is designed to provide a high transfer rate. RAID 3 organizes data by segmenting a user data record into either bit- or byte-sized chunks and evenly spreading the data across N drives in parallel. One of the drives acts as a parity drive. In this manner, every record that is accessed is delivered at the full media rate of the N drives that comprise the stripe group. The drawback is that every record I/O stripe accesses every drive in the group. You may not write to a RAID 3 array, except in full data stripe logical blocks. This limits application design flexibility and also the user’s ability to have different arrays run at different RAID levels on the same system. RAID 3 is not well suited for multiple process I/O (long or short) and is especially not suited for any application that requires a high I/O per second rate with any degree of randomness. On the other hand, RAID 3 will deliver excellent performance for single process/single stream long sequential I/O requests. RAID 3 provides consistently lower I/O performance when compared to standalone disks except when the I/O size is less than or equal to 64 KB. RAID 3 architecture should only be chosen in a case where the user is virtually guaranteed that there will be only a single, long process accessing sequential data. A video server and a graphics server would be good examples of proper RAID 3 applications. RAID 3 is so limited that it becomes a poor choice in most other cases. With this RAID level, both data and encoded data protection information are spread across all the drives in the array. Level 5 is designed to provide a high transfer rate (a one-way transmission of data) and a moderate I/O rate (a two-way transmission of data). In RAID 5 technology, the hardware reads and writes parity information to each module in the array. If a module fails, the system processor can reconstruct all user data from the user data and parity information on the other disk modules. When a failed disk module is replaced, the system processor automatically rebuilds the disk array using the information stored on the remaining modules. The rebuilt disk array contains an exact replica of the information it would have contained had the original disk module never failed. RAID 5 requires fewer drives than RAID 1 or RAID 1/0 which is a combination of RAID 1 and RAID 0. Disk striping is used and parity data is distributed for optimum performance. In RAID 5, three to sixteen drives can be configured per group. Five drives to a group are typical. The data are distributed across multiple drives preventing the I/O slowdown caused by constant hits on a single drive. RAID 5 is not quite as robust as RAID 1/0 and can only sustain the loss of one disk per group. RAID 5 is the most versatile RAID level for most applications. RAID 5 is a good choice where multitasking applications require a large history database with a high read rate, or a database that uses a normal or less-than-normal percentage of write operations, where writes are 33% or less of all I/O operations. RAID 5 provides consistently high performance for large input/output operations, greater or equal to 64 KB, but poor for smaller I/O sizes. |
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