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

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

On 2022/5/25 01:02, David Sterba wrote:
> On Tue, May 24, 2022 at 02:13:47PM +0800, Qu Wenruo wrote:
>> 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.
>>
>> 3) Data in the remaining reserved space
>>
>> The write time and recovery workflow is embedded into the on-disk
>> format.
>>
>> Unfortunately we will not have any optimization for the RAID56J, every
>> write will be journaled, no exception.
>>
>> Furthermore due to current write behavior of RAID56, we always submit a
>> full 64K stripe no matter what, we have every limited size for the data
>> part (at most 15 64K stripe).
>>
>> So far, I don't believe we will have a fast RAID56J at all.
>
> Well, that does not sound encouraging. One option discussed in the past
> how to fix the write hole was to always do full RMW cycle. Having a "not
> fast journal at all" would require a format change and have probably a
> comparable performance drop.
>
That's why the next step is to improve the RMW cycle, to only write the
vertical stripes first.

Which can help a little on the performance front.

Thanks,
Qu