Re: [RFC v3 00/11] btrfs: raid-stripe-tree draft patches

[Josef Bacik <josef@toxicpanda.com>]

On Mon, Oct 17, 2022 at 04:55:18AM -0700, Johannes Thumshirn wrote:
> Here's a yet another draft of my btrfs zoned RAID patches. It's based on
> Christoph's bio splitting series for btrfs.
> 
> Updates of the raid-stripe-tree are done at delayed-ref time to safe on
> bandwidth while for reading we do the stripe-tree lookup on bio mapping time,
> i.e. when the logical to physical translation happens for regular btrfs RAID
> as well.
> 
> The stripe tree is keyed by an extent's disk_bytenr and disk_num_bytes and
> it's contents are the respective physical device id and position.
> 

So generally I'm good with this design and everything, I just have a few asks

1. I want a design doc for btrfs-dev-docs that lays out the design and how it's
   meant to be used.  The ondisk stuff, as well as the post update after the
   delayed ref runs.
2. Additionally, I would love to see it written down where exactly you want to
   use this in the future.  I know you've talked about using it for other raid
   levels, but I have a hard time paying attention to my own stuff so I'd like
   to see what the long term vision is for this, again this would probably be
   well suited for the btrfs-dev-docs update.
3. I super don't love the fact that we have mirrored extents in both places,
   especially with zoned stripping it down to 128k, this tree is going to be
   huge.  There's no way around this, but this makes the global roots thing even
   more important for scalability with zoned+RST.  I don't really think you need
   to add that bit here now, I'll make it global in my patches for extent tree
   v2.  Mostly I'm just lamenting that you're going to be ready before me and
   now you'll have to wait for the benefits of the global roots work.

Thanks,

Josef