Re: [PATCH 3/3] btrfs: volumes: Allocate degraded chunks if rw devices can't fullfil a chunk

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2019/11/28 下午8:30, Qu WenRuo wrote:
> 
> 
> On 2019/11/28 下午7:24, David Sterba wrote:
>> On Thu, Nov 28, 2019 at 07:36:41AM +0800, Qu Wenruo wrote:
>>> On 2019/11/28 上午3:23, David Sterba wrote:
>>>> On Tue, Nov 19, 2019 at 06:41:49PM +0800, Qu Wenruo wrote:
>>>>> On 2019/11/19 下午6:05, Anand Jain wrote:
>>>>>> On 11/7/19 2:27 PM, Qu Wenruo wrote:
>>>>>>> [PROBLEM]
>>>>>>> Btrfs degraded mount will fallback to SINGLE profile if there are not
>>>>>>> enough devices:
>>>>>>
>>>>>>  Its better to keep it like this for now until there is a fix for the
>>>>>>  write hole. Otherwise hitting the write hole bug in case of degraded
>>>>>>  raid1 will be more prevalent.
>>>>>
>>>>> Write hole should be a problem for RAID5/6, not the degraded chunk
>>>>> feature itself.
>>>>>
>>>>> Furthermore, this design will try to avoid allocating chunks using
>>>>> missing devices.
>>>>> So even for 3 devices RAID5, new chunks will be allocated by using
>>>>> existing devices (2 devices RAID5), so no new write hole is introduced.
>>>>
>>>> That this would allow a 2 device raid5 (from expected 3) is similar to
>>>> the reduced chunks, but now hidden because we don't have a detailed
>>>> report for stripes on devices. And rebalance would be needed to make
>>>> sure that's the filesystem is again 3 devices (and 1 device lost
>>>> tolerant).
>>>>
>>>> This is different to the 1 device missing for raid1, where scrub can
>>>> fix that (expected), but the balance is IMHO not.
>>>>
>>>> I'd suggest to allow allocation from missing devices only from the
>>>> profiles with redundancy. For now.
>>>
>>> But RAID5 itself supports 2 devices, right?
>>> And even 2 devices RAID5 can still tolerant 1 missing device.
>>
>>> The tolerance hasn't changed in that case, just unbalanced disk usage then.
>>
>> Ah right, the constraints are still fine. That the usage is unbalanced
>> is something I'd still consider a problem because it's silently changing
>> the layout from the one that was set by user.
>>
>> As there are two conflicting ways to continue from the missing device state:
>>
>> - try to use remaining devices to allow writes but change the layout
>> - don't allow writes, let user/admin sort it out
>>
>> I'd rather have more time to understand the implications and try to
>> experiment with that.
>>
> Ah, makes sense.
> 
> So no need for a new version.
> 
> Reviewed-by: Qu Wenruo <wqu@suse.com>
> 
> Thanks,
> Qu
> 
Facepalm, that's for another thread....

Reviewing patch from myself, WTF....


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