Re: about raid5 recovery when created

[hank peng <pengxihan@gmail.com>]

2009/12/9 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')?
> --
Yes, that's what I am thinking.
I think it is better to implement this function in kernel's md layer.
I wonder what Neil Brown think of this?

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