Expert Oracle Exadata - Parent Directory

CHAPTER 9 RECOVERING EXADATASYS:+ASM2> CREATE DISKGROUP SCRATCH_DG NORMAL REDUNDANCYFAILGROUP CELL01 DISK 'o/192.168.12.3/SCRATCH_DG_CD_05_cell01'FAILGROUP CELL02 DISK 'o/192.168.12.4/SCRATCH_DG_CD_05_cell02'FAILGROUP CELL03 DISK 'o/192.168.12.5/SCRATCH_DG_CD_05_cell03'attribute 'compatible.rdbms'='11.2.0','compatible.asm' ='11.2.0','au_size'='4M','cell.smart_scan_capable'='true';Notice that this disk group is created using three grid disks. Following Exadata best practices, we’veused one grid disk from each storage cell. It’s interesting to note that even if we hadn’t specified threefailure groups with one disk in each, ASM would have done so automatically. We then created a small,single-instance database called SCRATCH using this disk group. The disk group is configured with normalredundancy (two mirror copies for each block of data), which means our database should be able tosuffer the loss of one cell disk without losing access to data or causing a crash. Since each grid diskresides on a separate storage cell, we could even suffer the loss of an entire storage cell without losingdata. We’ll discuss what happens when a storage cell fails later in the chapter.In a moment we’ll take a look at what happens when a grid disk is removed from the storage cell (asimulated disk failure). But before we do, there are a few things we need to check:• Verify that no rebalance or other volume management operations are running.• Ensure that all grid disks for the SCRATCH_DG disk group are online.• Verify that taking a disk offline will not impact database operations.• Check the disk repair timer to ensure the disk is not automatically dropped beforewe can bring it back online again.There are a couple of ways to verify that volume management activity is not going on. First let’scheck the current state of the disk groups using asmcmd. The ls –l command shows the disk groups, thetype of redundancy, and whether or not a rebalance operation is currently under way. By the way, youcould also get this information using the lsdg command, which also includes other interestinginformation like space utilization, online/offline status, and more. The Rebal column in the followinglisting indicates that no rebalance operations are executing at the moment.> asmcmd -pASMCMD [+] > ls -lState Type Rebal NameMOUNTED NORMAL N DATA_DG/MOUNTED NORMAL N RECO_DG/MOUNTED NORMAL N SCRATCH_DG/MOUNTED NORMAL N STAGE_DG/MOUNTED NORMAL N SYSTEM_DG/Notice that not all volume management operations are shown in the asmcmd commands. If a griddisk has been offline for a period of time, there may be a considerable amount of backlogged data thatmust be copied to it in order to bring it up to date. Depending on the volume of data, it may take severalminutes to finish resynchronizing a disk. Although this operation is directly related to maintainingbalance across all disks, it is not technically a “rebalance” operation. As such, it will not appear in thelisting. For example, even though the ls –l command in the previous listing showed a status of N forrebalance operations, you can clearly see that a disk is currently being brought online by running thenext query:303