Expert Oracle Exadata - Parent Directory

CHAPTER 8 CONFIGURING EXADATAHigh For All: This configuration can significantly reduce available spacein your disk groups. According to Oracle, it is best used when the backupmethod chosen is Backups External to Oracle.High For DATA: In this configuration, ASM redundancy is set to high forDATA, while RECO is set to normal redundancy. This is best suited forsystems that back up the databases to external storage, such as tape,VTL, NFS, and so on.High For RECO: In this configuration, high redundancy is used for RECOand normal redundancy is used for the DATA disk group. Thisconfiguration is best suited for databases that back up to the FRA.Normal For ALL: This configuration uses normal redundancy for all diskgroups. While it provides quite a bit more usable space in the diskgroups, keep in mind that the simultaneous loss of one disk drive fromtwo storage cells will cause ASM to dismount your disk groups. If thathappens, your databases using those disk groups will also be offline.As you consider which protection scheme is right for you, think about your paintolerance where it comes to system outages. Normal redundancy providesmore storage and less protection from disk/cell failures. Unless you can affordfor your databases to be down for an extended period of time, you should leantoward high redundancy for the DATA disk group. If you can afford an outagewhile transient disk/cell failures are resolved, or in a worst-case scenario, waitfor a full database recovery, then perhaps high redundancy for the RECO diskgroup is a better fit. If space is very tight and you can tolerate these types ofoutages, then you may consider setting redundancy for all disk groups tonormal.The redundancy level for your SYSTEM_DG (or DBFS_DG) disk group, which storesthe OCR and Voting files, will be automatically set to the maximum levelsupported by your Exadata rack configuration. High redundancy is notsupported on quarter rack configurations, so normal redundancy is the bestyou can do for them. For half rack and full rack configurations, redundancy willalways be set to high for the SYSTEM_DG disk group. Kevin Says: Protection level is a very sensitive and seemingly politically charged topic. The manner in whichASM chooses partner disks leaves any normal redundancy disk group at much greater risk of data loss from adouble-disk failure than would be the case with traditional RAID-style protection. While there may be hundreds ofdisks that could fail simultaneously in a double disk failure, it’s just a matter of (bad) luck which ones fail. If the“wrong two disks” in a normal redundancy situation happen to fail, the administrator will have to deal with dataloss. For this reason, the authors are correct to point out which high-redundancy scheme is recommended basedon the backup strategy protecting the system.246