Re: [PATCH DRAFT] btrfs: RAID56J journal on-disk format draft

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

On 2022/6/1 17:07, Wang Yugui wrote:
> Hi,
>
>> On 2022/6/1 10:25, Wang Yugui wrote:
>>> Hi,
>>>
>>>> On 2022/6/1 10:06, Wang Yugui wrote:
>>>>> Hi,
>>>>>
>>>>>> This is the draft version of the on-disk format for RAID56J journal.
>>>>>>
>>>>>> The overall idea is, we have the following elements:
>>>>>>
>>>>>> 1) A fixed header
>>>>>>       Recording things like if the journal is clean or dirty, and how many
>>>>>>       entries it has.
>>>>>>
>>>>>> 2) One or at most 127 entries
>>>>>>       Each entry will point to a range of data in the per-device reserved
>>>>>>       range.
>>>>>
>>>>> Can we put this journal in a device just like 'mke2fs -O journal_dev'
>>>>> or 'mkfs.xfs -l logdev'?
>>>>>
>>>>> A fast & small journal device may help the performance.
>>>>
>>>> Then that lacks the ability to lose one device.
>>>>
>>>> The journal device must be there no matter what.
>>>>
>>>> Furthermore, this will still need a on-disk format change for a special type of device.
>>>
>>> If we save journal on every RAID56 HDD, it will always be very slow,
>>> because journal data is in a different place than normal data, so HDD
>>> seek is always happen?
>>>
>>> If we save journal on a device just like 'mke2fs -O journal_dev' or 'mkfs.xfs
>>> -l logdev', then this device just works like NVDIMM?  We may not need
>>> RAID56/RAID1 for journal data.
>>
>> That device is the single point of failure. You lost that device, write
>> hole come again.
>
> The HW RAID card have 'single point of failure'  too, such as the NVDIMM
> inside HW RAID card.
>
> but  power-lost frequency > hdd failure frequency  > NVDIMM/ssd failure
> frequency

It's a completely different level.

For btrfs RAID, we have no special treat for any disk.
And our RAID is focusing on ensuring device tolerance.

In your RAID card case, indeed the failure rate of the card is much lower.
In journal device case, how do you ensure it's still true that the
journal device missing possibility is way lower than all the other devices?

So this doesn't make sense, unless you introduce the journal to
something definitely not a regular disk.

I don't believe this benefit most users.
Just consider how many regular people use dedicated journal device for
XFS/EXT4 upon md/dm RAID56.

>
> so It still help a lot.
>
>> RAID56 can tolerant one or two device failures for sure.
>> Thus one point failure is against RAID56.
>>
>>
>> If one is not bothered with writehole, then they doesn't need any
>> journal at all.
>
> I though 'degraded read-only' will help more case than 'degraded
> read-write' with writehole.

I don't get what you're talking about here.

Thanks,
Qu

>
> Best Regards
> Wang Yugui (wangyugui@e16-tech.com)
> 2022/06/01
>
>