Re: about raid5 recovery when created

[hank peng <pengxihan@gmail.com>]

2009/12/8 Robin Hill <robin@robinhill.me.uk>:
> On Tue Dec 08, 2009 at 09:01:23PM +0800, hank peng wrote:
>
>> Hi, all:
>> As we know, when a raid5 array is created, recovery will be going on
>> which involves some read, one xor and one write. Since there is no
>> real data in the disk at the time, besides, if I am willing to wait
>> for recovery to complete and then use this raid5, how about adding
>> support for a fast recovery method? Right now, what is in my mind is
>> zero all disks which belong to this raid5. I think it will increase
>> raid5 recovery speed when created and decrease CPU usage, since all
>> zero is also XORed.
>> What do raid developers think?
>>
> It'll decrease CPU usage but increase I/O - you're now needing to write
> to all disks.  Most systems will be I/O limited rather than CPU limited,
> so the current approach works better.  If you want to zero the disks
> then do this before creating the array - you can then use --assume-clean
> to skip the resync process.
>
I think --assume-clean is used mostly when doing performance test and
can't be used when creating a raid5 array using new disk, because
later read and write operation make assumption that all stripe is
XORed. Correct me if I am wrong.

> Cheers,
>    Robin
> --
>     ___
>    ( ' }     |       Robin Hill        <robin@robinhill.me.uk> |
>   / / )      | Little Jim says ....                            |
>  // !!       |      "He fallen in de water !!"                 |
>



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