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

[Wang Yugui <wangyugui@e16-tech.com>]

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

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.

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