Re: [RFC 00/15] btrfs: RAID5 with RAID stripe-tree (zoned + non-zoned)

On 6/23/26 12:39 PM, sun k wrote:
> I've just noticed that I'm not sure about your mindset of this raid5/6
> rst feature.
> So I want to know do you think the raid5/6 rst is extent layer raid or
> block layer raid?
> Because they are quite different and I need to align it with you
> before further discussion.

I did have a what you call block-layer parity raid without and RMW in mind.

But RST is only a mapping layer, so I think we can go "extent layer" as 
well.

Hence the early RFC, I'm still open to changes, as long as it works on 
zoned filesystems.

Qu's idea with a RAID4 style (I say style because I want to be able to 
use multiple parities in the future) with dedicated parity disks is 
something worth exploring as well IMHO.

[leaving the rest for context]

> For extent layer raid5/6, it's more like what zfs is doing now: we do
> the "raid" thing on each
> file extent, split big extent into stripes, and write them on
> different disks with parity. The issue
> of this is that the performance and efficiency on small extents is
> bad, but we can always
> do full stripe write, avoiding write hole issues.
>
> For block layer raid5/6, which is like what we currently have without
> rst, we need to deal with the
> write hole issue, possibly by
> 1. totally avoid read-modify-write for a partial stripe update. We
> always write into a new stripe
>    append only instead of modifying an existing stripe, like writing a
> zoned device even on non-zoned device.
> 2. or we allow read-modify-write for a partial stripe update, but
> write the D and P elsewhere
>    instead of in-place.
>
> Option 2 will cause another free space management issue I've mentioned before:
> let's take a 4 disk raid 5 as example.
> before the partial stripe update, logical address LA is mapped to
> physical address PA0(D0), PA1(D1), PA2(D2), PA3(P);
> after updating D2, LA is mapped to address PA0(D0), PA1(D1), PA2'(D2'), PA3'(P')
> then we need to allocate PA2' and PA3' , and free PA2 and PA3 on disk 2 and 3.
> Since currently btrfs manage freespace based on logical address, how
> to manage the allocation and manage physical free space?
>