Oracle Database 11 g - Online Public Access Catalog

CHAPTER 9 ■ STORAGE MANAGEMENT 385■Note ASM best practices recommend only two diskgroups. The general philosophy is that two diskgroups,DATA and FRA (for the flash recovery area), will suit the majority of the technical requirements of most companies.There are site-specific considerations that may need to be considered. For example, if your company utilizesa combination of RAID 1+0 and RAID 5 on the same server, you must create separate diskgroups since youdo not want to mix multiple RAID levels in a single diskgroup. In this case, this particular company had terabytes ofASM storage on RAID 5 for BLOBs and RAID 1+0 for relational OLTP data.Additionally, if you have differently sized disks, you have another candidate for additional diskgroups. Anothergood case for a separate diskgroup is when the disk attributes and speed are different from others.Allocation Unit (AU) SizesPrior to Oracle Database 11g, the AU size could not be specified at diskgroup creation time. AllAUs were 1MB. There is a workaround, but not very many DBAs are aware that you can havemultiple au_sizes in Oracle Database 10g. Specifying au_size in Oracle Database 10g meanssetting an underscore initialization parameter and creating the diskgroup. If you want multipleAU sizes, you must set the initialization parameter and bounce the ASM instance with the new AUsize. The following underscore initialization parameters allow a 16MB AU size and 1MB AUstripe size. This is recommended only for VLDB databases or may be suitable for databases thathave large objects (BLOBs and CLOBs).• _asm_ausize=16777216• _asm_stripesize=1048576In Oracle Database 11g, the au_size attribute can be specified only at diskgroup creationtime. Because it involves storage characteristics, this attribute cannot be modified using thealter diskgroup command.Starting in Oracle Database 11g, you can set the ASM allocation unit (AU) size from 1MBall the way to 64MB in powers of 2. As of Oracle Database 11g, the valid extent sizes are 1, 2, 4,8, 16, 32, or 64MB. The larger AUs can be beneficial for large VLDB databases or data warehousesthat perform large sequential reads. In addition, organizations that store BLOB or SecureFilesinside the database can benefit from larger AUs.Variable-Size ExtentsVariable-size extents provide support for larger ASM files. Moreover, this feature reduces theSGA requirements to manage the extent maps in the RDBMS instance. Setting the AU to ahigher number also reduces the metadata space usage since it reduces the number of extentpointers associated with the metadata. Moreover, variable-size extents can significantly improvedatabase open time and reduce memory utilization in the shared pool. Variable-size extentsallow you to support databases that are hundreds of TB and even several PB in size.