Expert Oracle Exadata - Parent Directory

CHAPTER 9 RECOVERING EXADATABacking Up ExadataWhen we took delivery of our Exadata system, one of our primary concerns was, “how can we back upeverything so we can restore it to working order if something goes horribly wrong?” When our Exadataarrived in May of 2010, the latest version of the Cell software was 11.2.0.1.0. And at the time, the only wayto back up a database server was to use third-party backup software or standard Linux commands liketar. Oracle is constantly developing new features for Exadata and less than a year later, Exadata X-2database servers come configured with the native Linux Logical Volume Manager (LVM). This is a bigstep forward, because the LVM has built-in snapshot capabilities that provide an easy method of takingbackups of the operating system. Storage cells use a completely different, proprietary, method forbackup and recovery. In this section we’ll take a look at the various methods Oracle recommends forbacking up Exadata database servers and storage cells. We’ll also take a brief look at Recovery Manager(RMAN) and some of the features Exadata provides that improve the performance of database backupand recovery. After that, we’ll take a look at what it takes to recover from some of the more commontypes of system failure. It may surprise you, but the focus of this chapter is not database recovery. Thereare very few Exadata-specific considerations for database backup and recovery. A majority of theproduct-specific backup and recovery methods pertain to backup and recovery of the system volumescontaining the operating system and Exadata software, so we’ll spend a quite a bit of time discussingrecovery, from the loss of a cell disk, to the loss of a system volume on the database servers or storagecells.Backing Up the Database ServersRecently, Oracle began shipping Exadata with the Linux LVM configured for managing file systemstorage on the database servers. Logical volume managers provide an abstraction layer for physical diskpartitions similar to the way ASM does for its underlying physical storage devices. LVMs have volumegroups comparable to ASM disk groups. These volume groups are made up of one or more physical disks(or disk partitions), as ASM disk groups are made up of one or more physical disks (or disk partitions).LVM volume groups are carved up into logical volumes in which file systems can be created. In a similarway, databases utilize ASM disk groups for creating tablespaces that are used for storing tables, indexes,and other database objects. Abstracting physical storage from the file systems allows the systemadministrator to grow and shrink the logical volumes (and file systems) as needed. There are a numberof other advantages to using the LVM to manage storage for the Exadata database servers, but our focuswill be the new backup and restore capabilities the Linux LVM provides, namely LVM snapshots. Inaddition to their convenience and ease of use, LVM snapshots eliminate many of the typical challengeswe face with simple backups using the tar command or third-party backup products. For example,depending on the amount of data in the backup set, file system backups can take quite a while tocomplete. These backups are not consistent to a point in time, meaning that if you must restore a filesystem from backup, the data in your files will represent various points in time from the beginning of thebackup process to its end. Applications that continue to run during the backup cycle can hold locks onfiles, causing them to be skipped (not backed up). And once again, open applications will inevitablymake changes to data during the backup cycle. Even if you are able to back up these open files, you haveno way of knowing if they are in any usable state unless the application is shut down before the backupis taken. LVM snapshots are instantaneous because no data is actually copied. You can think of asnapshot as an index of pointers to the physical data blocks that make up the contents of your filesystem. When a file is changed or deleted, the original blocks of the file are written to the snapshotvolume. So even if it takes hours to complete a backup, it will still be consistent with the moment thesnapshot was created. Now, let’s take a look at how LVM snapshots can be used to create a consistent filesystem backup of the database server.284