Understanding ASM (4) strip and rebalance

(I) striping

There are two ways to strip ASM: coarse and fine-gained.

AU is the smallest space allocation unit. The default value is 1 M. Each AU is composed of eight K strip spaces by default.

In coarse striping, a disk corresponds to an AU

Therefore, this strip is suitable for consecutive I/O reads and writes, such as full table scan tables.

In fine-gained striping, eight K strip spaces in an AU are evenly dispersed on the disk.

That is to say, four disks and one AU are evenly distributed to four disks, with each disk having two 128 K disks.

Suitable for files that are sensitive to data read/write latency. Such as redo logs, control files, and spfiles

(Ii) Rebalancing

When a disk is added or deleted from A diskgroup, The RBAL process is triggered to create a rebalance plan and calculate the time overhead.

Then, send a message to the ARBn process ASM Reblance) to process the request.

The ARBn process performs locked, relocated, and unlocked operations on each extents. For details about how to execute this function, see the v $ asm_operation view.

The number of ARBn processes is determined by the ASM_POWER_LIMIT parameter.

COD (Continuting Operation Directory) is used to record rebalances

If rebalance fails, the system reads records from COD and restarts rebalance when the instance is restarted.

Note:

① Rebalance is only performed when the diskgroup is changed. It is not executed periodically.

If the server goes down during rebalance execution, the system automatically performs rebalance after restart.

② The size of each disk must be the same. If there is a small disk, because rebalance will allocate the same proportion of space to each disk, it may cause insufficient space during rebalance.

If the rebalance fails due to insufficient free space during execution, a ORA-15041 error occurs and you need to add a disk

If you need to add disks frequently, data may be moved frequently every time. To improve efficiency, it is best to add disks in batches.

If the disk size is the same, rebalance is still not performed, You need to view asm_power_limit

OCP questions:

16. Immediately after adding a new disk to or removing an existing disk from an Automatic Storage Management (ASM) instance,

You find that the performance of the database decreases initially, until the addition or removal process is completed.

Performance then gradually returns to normal levels.

Which two activities cocould you perform to maintain a consistent performance of the database while adding or removing disks? (Choose two .)

A. increase the number of checkpoint processes

B. define the POWER option while adding or removing the disks

C. increase the number of DBWR processes by setting up a higher value for DB_WRITER_PROCESSES

D. increase the number of slave database writer processes by setting up a higher value for DBWR_IO_SLAVES

E. increase the number of ASM Rebalance processes by setting upa higher value for ASM_POWER_LIMIT during the disk addition or removal

Answer: B, E

Oracle video tutorial follow: http://u.youku.com/user_video/id_UMzAzMjkxMjE2.html