On 2019/11/28 下午8:30, Qu WenRuo wrote: > > > 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 > Facepalm, that's for another thread.... Reviewing patch from myself, WTF....
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