Expert Oracle Exadata - Parent Directory

CHAPTER 14 STORAGE LAYOUTLinux O/SCell SystemAreaCell SystemAreaPhysicalDiskLUNCell DiskGrid Disk 1Grid Disk 2Grid Disk 3Grid Disk nFigure 14-1. The relationship between physical disks and grid disksCell DiskWith the introduction of ASM, Oracle provided a way to combine many physical disks into a singlestorage volume called a disk group. Disk groups are the ASM replacement for traditional file systems andare used to implement Oracle’s SAME (Stripe and Mirror Everything) methodology for optimizing diskperformance. As the name implies, the goal of SAME is to spread I/O evenly across all physical disks inthe storage array. Virtualizing storage in this way allows multiple databases to share the same physicaldisks. It also allows physical disks to be added or removed without interrupting database operations. If adisk must be removed, ASM automatically migrates its data to the other disks in the disk group before itis dropped. When a disk is added to a disk group, ASM automatically rebalances data from other disksonto the new disk. In a very basic ASM configuration, LUNs are presented to ASM as ASM disks. ASMdisks are then used to create disk groups, which in turn are used to store database files such as datafiles,control files, and online redo logs. The Linux operating system presents LUNs to ASM as native blockdevices such as /dev/sda1. Exadata virtualizes physical storage through the use of grid disks and ASMdisk groups. Grid disks are used for carving up cell disks similar to the way partitions are used to carve upphysical disk drives. Figure 14-2 shows the relationship between cell disks, grid disks, and ASM diskgroups.ASM Disk GroupGrid Disk 1Grid Disk 2Grid Disk 3Grid Disk nCell Disk 1Grid Disks 1-nFigure 14-2. ASM disk group with its underlying grid disks, and cell disks468