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. Thanks, Qu
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