On 2019/11/28 下午7:24, David Sterba wrote: > 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. > Ah, makes sense. So no need for a new version. Reviewed-by: Qu Wenruo <wqu@xxxxxxxx> Thanks, Qu
