Re: BTRFS subvolume RAID level

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2019/11/29 上午6:48, waxhead wrote:
> Just out of curiosity....
> 
> What are the (potential) show stoppers for implementing subvolume RAID
> levels in BTRFS? This is more and more interesting now that new RAID
> levels has been merged (RAID1c3/4) and RAID5/6 is slowly inching towards
> usable status.

My quick guesses are:
- Subvolume and RAID aware extent allocator
  Current extent allocator cares nothing about who is requesting the
  extent.
  It cares a little about the RAID profile (only for convert).
  We need some work here at least.

- Way to prevent false ENOSPC due to profile restriction
  If subvolume is using some exotic profile, while extent/csum tree is
  using regular profile, and subvolume eats too many space, making
  it impossible to fit extent in their desired profile.
  We will got ENOSPC and even goes RO.

Thanks,
Qu

> 
> I imagine that RAIDc4 for example could potentially give a grotesque
> speed increase for parallel read operations once BTRFS learns to
> distribute reads to the device with the least waitqueue / fastest devices.


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