Ric Wheeler wrote:
I think that the btrfs plan is still to push more complicated RAID schemes off to MD (RAID6, etc) so this is an issue even with a JBOD. It will be interesting to map out the possible ways to use built in mirroring, etc vs the external RAID and actually measure the utilized capacity and performance (online & during rebuilds).
That's leaving a lot of performance and features on the table, IMO. We definitely want to have metadata and small files using mirroring (perhaps even three copies for some metadata). Use RAID[56] for large files. Perhaps even start files at RAID1, and have the scrubber convert them to RAID[56] when it notices they are only ever read. Keep temporary or unimportant files at RAID0. Play games with asymmetric setups (small fast disks + large slow disks). etc etc etc.
Delegating things to MD throws out a lot of metadata so these things become impossible.
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