Re: about raid5 recovery when created

[Michael Evans <mjevans1983@gmail.com>]

On Tue, Dec 8, 2009 at 6:03 AM, hank peng <pengxihan@gmail.com> wrote:
> 2009/12/8 Robin Hill <robin@robinhill.me.uk>:
>> On Tue Dec 08, 2009 at 09:49:48PM +0800, hank peng wrote:
>>
>>> 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.
>>>
>> You're correct - that's why I said to zero all the disks first so the
>> XOR data is all correct.
>>
> I think this function is better to be implemented in kernel raid
> layer, not in user space(for example using dd command).
> In this way, we can get good performance and lower cpu usage, also, we
> can make this function be part of raid code so that it can be managed
> by mdadm
>> Cheers,
>>    Robin
>> --
>>     ___
>>    ( ' }     |       Robin Hill        <robin@robinhill.me.uk> |
>>   / / )      | Little Jim says ....                            |
>>  // !!       |      "He fallen in de water !!"                 |
>>
>
>
>
> --
> The simplest is not all best but the best is surely the simplest!
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How about documenting this better?  'zeroing all underlying devices
then creating with --assume-clean' will be clean because the parity
algorithm is even (or similar to 'even parity')?
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