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RAID Volume Initialize Vs Fast Initialize

Posted on 2010-08-19
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Hello Experts,

I would like to know the answers for following..

On RAID Volumes what is the difference between performing Initialize and Fast Initialize ??

Please explain me behind the scenes actions that takes place when we perform Initialize or Fast Initialize on the Raid Volumes.  
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Question by:sudhirgoogle
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stergium earned 250 total points
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Use the Slow Initialize task to initialize all physical disks included in the virtual disk. The Slow Initialize task updates the metadata on the physical disks and erases all existing data and file systems.

In comparison with the Fast Initialize task, you may want to use the Slow Initialize task if you have had trouble with a physical disk or suspect that it has bad disk blocks. The Slow Initialize task remaps bad blocks and writes zeroes to all disk blocks.

The Slow Initialize task initializes one physical disk at a time. Each physical disk displays the Clear state while being initialized. During the time that the physical disk is in the Clear state, the Cancel Clear physical disk task is available. Performing a Cancel Clear task on the physical disk causes the Slow Initialize task to be cancelled for the entire virtual disk and all member physical disks.


Use the Fast Initialize task to initialize all physical disks included in the virtual disk. The Fast Initialize task updates the metadata on the physical disks so that all disk space is available for future write operations. The initialize can be completed quickly because existing information on the physical disks is not erased, although future write operations will overwrite any information that remains on the physical disks.
      NOTE: Doing a Fast Initialize causes existing data to be inaccessible. This task should be considered data destructive.

In comparison with the Slow Initialize task, the Fast Initialize task has the following advantages:

    * The Fast Initialize task takes less time to complete.

    * The Fast Initialize task does not write zeroes to the disk blocks on the physical disks. Because the Fast Initialize task does not perform a write operation, it causes less degradation to the disk than does the Slow Initialize task.

If you have had trouble with a physical disk or suspect that it has bad disk blocks, you may want to perform a Slow Initialize task, as this task remaps bad blocks and writes zeroes to all disk blocks.

hope that helped
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by:dlethe
dlethe earned 250 total points
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Actually it is more simple then above ..

1. Both do the metadata.
2. Slow initialize only (writes ZEROS to non-parity blocks)  - So all blocks on all disks are written to
3. Both fast & slow (writes XOR parity blocks) - This necessitates a READ on all non-parity blocks for all disks

In addition, if your raid supports "background init", then this is a fast init, but whenever the O/S requires an I/O, the RAID services it, calculates/writes parity for that particular block, then goes back to what it was doing before it was interrupted.

best practices is slow init if the array has never been burned in as it insures there are no bad blocks.
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by:dlethe
dlethe earned 250 total points
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Sorry ... in #3, slow init does NOT require a READ on all blocks, because it is already writing to all blocks zeros and XOR. No need to read the disk if you just wrote to it.
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