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 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.

Thanks,
Qu


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