Boot problem if a ZFS log device is missing

[Maurizio Vairani]

On 30/08/2013 13.45, Ronald Klop wrote:
> On Fri, 30 Aug 2013 12:58:29 +0200, Johan Hendriks 
> <joh.hendriks at gmail.com> wrote:
>
>> Maurizio Vairani wrote:
>>> On 29/08/2013 11.01, Andriy Gapon wrote:
>>>> on 29/08/2013 11:27 Maurizio Vairani said the following:
>>>>> I am able to boot the PC without a cache device but not without a 
>>>>> log device. Why ?
>>>> The log could potentially contain uncommitted entries.  Without the 
>>>> log device
>>>> there is no knowing if it did or did not.  And if it did then the 
>>>> pool is
>>>> inconsistent state without the log device and so it can not be 
>>>> imported.
>>>>
>>>> The cache is not persistent and so there is nothing needed from it 
>>>> upon a boot.
>>>>
>>> Thank you for the clear and concise reply.
>>>
>>> Yesterday I have done some test. If I remove the stick from the USB 
>>> port, before the shutdown the PC, it  don't crash but continues to 
>>> works. Then  I am able to reboot the laptop without inserting the 
>>> stick with a pool that works in degraded mode.
>>>
>>> From the end user point of view a PC should always boot, even with a 
>>> missing ZFS log device.
>>>
>>> Regards
>>> Maurizio
>>>
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>> I do not agree with the following.
>>      From the end user point of view a PC should always boot, even 
>> with a missing ZFS log device.
>>
>> I think it should give you a option to import the pool or not import 
>> the pool!
No problem  I agree,  a step in the right direction.
>> There could be a situation when you are not sure that the ZIL is 
>> commited, in that situation it would be handy if you can suspend the 
>> boot and make sure the ZIL is there when you reboot or import after 
>> you attached the ZIL.
>> I would hate it when it corrups my data just because we always 
>> import. with or without the ZIL.
>>
>> In your test you remove the ZIL, and when you reboot then it imports 
>> correctly, as far as my knowledge goes this is ok, because when the 
>> pool is exported there is no left data in the ZIL, it was not there 
>> when we exported, so we can import even without the missing ZIL 
>> without problem.
>
> I think he was just lucky his system wasn't writing a lot to the ZIL 
> at the moment of removal. So his system was in a consistent state. 
> Otherwise you just miss data which is in the ZIL and not on disk.
I have removed the stick when there isn't R/W to the disk.
> BTW: Not everything goes through the ZIL. It is not the same as a 
> journal. Only sync writes go to the ZIL. If you don't use databases or 
> NFS or other software which wants to make sure data is on stable 
> storage, you might rarely use the ZIL.
Thanks for the explanation.
>
> Ronald.
>
>>
>> regards
>> Johan
>>
>>
>>
>>
>>
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Regards
Maurizio