Re: [PATCH 3/3] btrfs: volumes: Allocate degraded chunks if rw devices can't fullfil a chunk

David Sterba
2019-11-28T11:24:54Z

On Thu, Nov 28, 2019 at 07:36:41AM +0800, Qu Wenruo wrote:
> On 2019/11/28 上午3:23, David Sterba wrote:
> > On Tue, Nov 19, 2019 at 06:41:49PM +0800, Qu Wenruo wrote:
> >> On 2019/11/19 下午6:05, Anand Jain wrote:
> >>> On 11/7/19 2:27 PM, Qu Wenruo wrote:
> >>>> [PROBLEM]
> >>>> Btrfs degraded mount will fallback to SINGLE profile if there are not
> >>>> enough devices:
> >>>
> >>>  Its better to keep it like this for now until there is a fix for the
> >>>  write hole. Otherwise hitting the write hole bug in case of degraded
> >>>  raid1 will be more prevalent.
> >>
> >> Write hole should be a problem for RAID5/6, not the degraded chunk
> >> feature itself.
> >>
> >> Furthermore, this design will try to avoid allocating chunks using
> >> missing devices.
> >> So even for 3 devices RAID5, new chunks will be allocated by using
> >> existing devices (2 devices RAID5), so no new write hole is introduced.
> >
> > That this would allow a 2 device raid5 (from expected 3) is similar to
> > the reduced chunks, but now hidden because we don't have a detailed
> > report for stripes on devices. And rebalance would be needed to make
> > sure that's the filesystem is again 3 devices (and 1 device lost
> > tolerant).
> >
> > This is different to the 1 device missing for raid1, where scrub can
> > fix that (expected), but the balance is IMHO not.
> >
> > I'd suggest to allow allocation from missing devices only from the
> > profiles with redundancy. For now.
>
> But RAID5 itself supports 2 devices, right?
> And even 2 devices RAID5 can still tolerant 1 missing device.

> The tolerance hasn't changed in that case, just unbalanced disk usage then.

Ah right, the constraints are still fine. That the usage is unbalanced
is something I'd still consider a problem because it's silently changing
the layout from the one that was set by user.

As there are two conflicting ways to continue from the missing device state:

- try to use remaining devices to allow writes but change the layout
- don't allow writes, let user/admin sort it out

I'd rather have more time to understand the implications and try to
experiment with that.