On Wed, Feb 05 2020, Wols Lists wrote: > On 05/02/20 11:51, Paul Dann wrote: >> Hi there, >> >> I've got a load of data on an md RAID 1 array assembled from 2x4TB >> disks. I'm looking to expand this by adding a third 4TB disk and >> converting the array to RAID 5. Now the required procedure is >> documented on the wiki, but my question is: >> >> When I convert the RAID 1 array to RAID 5, the array will be in a >> degraded state as it rebuilds onto the new disk. However, if one of >> the original two disks were to fail during this procedure, is mdadm >> smart enough to convert the array back to degraded RAID 1, or will my >> array now be a broken RAID 5 with no path to recovery? > > There is a "revert reshape" option which will take you back to a raid 1. > This assumes, however, that it's the new disk that has failed. > > I'm pretty certain, however, that should one of the old disks fail > during the conversion you will end up with a degraded raid 5. This is correct. It is not true to say "the array will be in a degraded state as it rebuilds onto the new disk" as Paul did above. During the rebuild the array will be in 2 parts: - a non-degraded raid5 which starts small and grows - a non-degraded raid1 which starts large and shrinks. at no point in time is the array without full redundancy (unless a device fails of course). NeilBrown > > If you're worried, I would make sure you've done a SMART health check on > your two original drives, although that's no guarantee everything's okay. > > If you really are that worried, get two new disks with the intention of > ending up at raid 6. Add a 3rd drive to the mirror, fail and remove one > of the originals, add the 4th to go raid 5, then add back the first to > go raid 6. > > One thing I will say - MAKE SURE you have the latest mdadm, and if > possible make sure you're running a recent kernel. There have been > issues converting from 1 to 5. None of them serious, and all fixed by > upgrading mdadm/kernel, hence the advice to be running the latest/greatest. >> >> Many thanks, >> Paul >> > Cheers, > Wol
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