Re: btrfs RAID5 or btrfs on md RAID5?

[Andrei Borzenkov <arvidjaar@gmail.com>]

On Mon, Oct 20, 2025 at 12:07 PM Ulli Horlacher
<framstag@rus.uni-stuttgart.de> wrote:
>
>
> Resuming this discussion...
>
> On Mon 2025-09-22 (18:57), Qu Wenruo wrote:
>
> > >>>> So you either run RAID5 for data only
> > >>>
> > >>> This is a mkfs.btrfs option?
> > >>> Shall I use "mkfs.btrfs -m dup" or "mkfs.btrfs -m raid1"?
> > >>
> > >> For RAID5, RAID1 is preferred for data.
> > >
> > > Then the real usable capacity of this volume is only the half?
> >
> > No, metadata is really a small part of the fs.
> >
> > The majority of usable space really depends on the data profile.
> >
> > If you use RAID1 metadata + RAID5 data, I believe only less than 10% of
> > real space is used on RAID1, the remaining is still RAID5.
>
> Sounds like a good compromise solution!
>
> Asuming I have 4 partitions with equal size, then the suggested command to
> create the filesystem would be:
>
> mkfs.btrfs -m raid1 -d raid5 /dev/sda4 /dev/sdb4 /dev/sdc4 /dev/sdd4
>
> Does this setup help to protect against write hole?
>

No. It simply reduces the damage caused by the write hole. Only the
content of individual files is affected, not the metadata.