Expert Oracle Exadata - Parent Directory

CHAPTER 14 STORAGE LAYOUTTable 14-2 shows the performance effect on the ASM disk groups from the first to the last grid diskcreated when the interleaving allocation policy is used.Table 14-2. I/O Performance – Interleaving PolicyASM Disk GroupI/O Performance RatingDATA_DG 1RECO_DG 1SYSTEM_DG 2Fourth disk group 2Fifth disk group 3As you can see, the main difference between the default policy and the interleaving policy is thatdefault provides finer-grained control over ASM disks. With the default policy you have the ability tochoose which set of grid disks will be given the absolute fastest position on the disk. The interleavingpolicy has the effect of evening out the performance of grid disks. In practice, this gives the first two setsof grid disks (for DATA_DG and RECO_DG) the same performance characteristics. This may be useful if theperformance demands of the first two disk groups are equal. In our experience this is rarely the case.Usually there is a clear winner when it comes to the performance demands of a database environment.Tables, indexes, online redo logs (the DATA_DG disk group) have much higher performance requirementsthan database backups, archived redo logs, and flashback logs, which are usually stored in the RECO diskgroup. Unless there are specific reasons for using interleaving, we recommend using the default policy.Creating Grid DisksBefore we run through a few examples of how to create grid disks, let’s take a quick look at some of theirkey attributes:• Multiple grid disks may be created on a single cell disk, but a grid disk may not spanmultiple cell disks.• Storage for grid disks is allocated in 16M Allocation Units (AUs) and is rounded downif the size requested is not a multiple of the AU size.• Grid disks may be created one at a time or in groups with a common name prefix.• Grid disk names must be unique within a storage cell and should be unique across allstorage cells.• Grid disk names should include the name of the cell disk on which they reside.Once a grid disk is created, its name is visible from ASM in the V$ASM_DISK view. In other words, griddisks = ASM disks. It is very important to name grid disks in such a way that they can easily be associatedwith the physical disk to which they belong in the event of disk failure. To facilitate this, grid disk namesshould include both of the following:477