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SUMMARY use none none generating todeploy none in ean 13 generating c# After you m none for none ake all reasonable efforts to avoid physical IO by reducing workload and optimizing memory, it s time to configure the IO subsystem so that it can meet the resulting IO demand. Disk devices provide lower latency when they are only partially filled with data and when they are producing only a fraction of maximum possible throughput. Keeping disks less than 50-percent full and at less than 50 percent to 75 percent of maximum throughput is a possible rule of thumb for optimizing service time.

Throughput is generally achieved by using multiple disk drives and striping data across the devices. Throughput goals can be achieved only if you acquire enough disks to meet the aggregate IO demand. The two most popular ways to spread data across Oracle datafiles are RAID5 and striping (RAID0, RAID10, RAID 0+1).

RAID5 imposes a heavy penalty on write performance and is not recommended even for primarily readonly databases unless there is no temporary segment IO. Striping is the technique of choice on performance grounds. Because temporary segment IO and permanent segment IO have such different IO characteristics and diverse service-level expectations, it can often be a good idea to separate temporary tablespace datafiles on their own disk volumes.

For redo and archive logs, RAID5 is even more undesirable and should generally not be used unless performance is not important. Redo logs do not always benefit from striping in any case: Alternating redo logs across two devices and placing the archive destination on a striped volume is often the high-performance solution. Flashback logs can be stored together with archive logs on a fine-grained striped device, although better performance will often be obtained by allocating the flashback recovery area its own dedicated disk volume.

. RFID 22 . ADVANCED IO TECHNIQUES In this cha pter we build on the IO tuning fundamentals of the last chapter and consider some specific technologies and IO optimizations. Oracle s Automatic Storage Management (ASM) provides a storage management layer tightly integrated with the Oracle software stack. DBAs can use ASM to implement striping, mirroring, and load balancing on top of directly attached vanilla disks or on logical disks exposed by a storage array.

ASM can be used to implement many of the optimizations discussed in the previous chapter, and offers unique monitoring and tuning opportunities. Spinning magnetic disk devices continue to be the basis for the vast majority of database storage. However, Solid State Disk (SSD) is becoming increasingly attractive as a means of overcoming the unavoidable latency that is a consequence of the mechanics of magnetic disk.

Although SSD offers a way to mitigate latency limitations, throughput generated by large disk arrays can overload the channels and processing capabilities of the database server. The Oracle/HP Exadata storage server mitigates these limitations by implementing some database operations directly in the storage unit. Finally, we discuss the possibility of optimizing IO through changes to the Oracle block size (and generally advise against it).

. AUTOMATIC S none none TORAGE MANAGEMENT (ASM). Automatic S torage Management (ASM) was introduced in Oracle 10g to provide cluster-ready storage management facilities tightly integrated with the Oracle software stack. ASM provides a storage virtualization layer that automates many 641. 22 file le none none vel database administration tasks and which provides striping, redundancy, load balancing, and other services to an Oracle database. From Oracle 11g release 2 forward, ASM can provide clustered filesystem capabilities as well: the ASM Cluster File System (ACFS). ASM can provide storage services across a cluster and is ideally suited to providing the shared disk storage subsystem required by Real Application Clusters (RAC); consequently, ASM is particularly popular in conjunction with RAC.

However, the services and benefits of ASM are equally valid for a single instance Oracle database. ASM services can be enabled on top of directly attached disk devices (Just a Bunch Of Disks [JBOD]) or on top of Logical Units (LUNs) exposed by a storage array or volume manager. In either case, the disks are exposed to the Oracle database as a small number of Disk Groups, each of which can contain any number of logical disks.

When using ASM, DDL operations that formally specified file locations need to specify only an ASM Disk Group: ASM determines the optimal location for the files on disk. ASM ARCHITECTURE ASM is implemented by a stripped-down Oracle instance. This instance supports no datafiles or database sessions.

The purpose of the ASM instance is to maintain and determine the mapping between the database objects and the blocks on disk that store the data for that object. ASM determines the physical placement based on the redundancy (for example, mirroring) and striping characteristics of the file. ASM distributes data evenly across the disks in the disk group and rebalances the data should a disk be added or removed or if a rebalance operation is requested.

Figure 22-1 provides a high-level overview of the ASM and RDBMS interactions. From the RDBMS point of view, segments have extents that are stored on datafiles. These datafiles are stored in an ASM Disk Group.

Asides from the Disk Group and logical ASM filename, the RDBMS maintains no information about the physical layout of the data file on disk. Although ASM determines where each data block should be stored on disk, it s the RDBMS that actually does the reading and writing. When a new block is created, ASM advises the RDBMS where on disk to write that block.

When the RDBMS wants to read a logical block, it asks ASM for the physical location. The RDBMS remains responsible for all the physical IOs; ASM simply advises the RDBMS where to direct those IOs. ASM MONITORING All the IO monitoring techniques outlined in the previous chapter are relevant for ASM.

However, ASM can provide additional insight into IO at the ASM disk.
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