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

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

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.

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