18.06.2019 21:45, Hugo Mills пишет: ... > >> Is there a way to ask the block group allocator to prefer writing to >> a specific device during a balance? Something like -ddestdevid=N? >> This would just be a hint to the allocator and the usual constraints >> would always apply (and prevail over the hint when needed). > > No, there isn't. Having control over the allocator (or bypassing > it) would be pretty difficult to implement, I think. > > It would be really great if there was an ioctl that allowed you to > say things like "take the chunks of this block group and put them on > devices 2, 4 and 5 in RAID-5", because you could do a load of > optimisation with reshaping the FS in userspace with that. But I > suspect it's a long way down the list of things to do. >
It really sounds like "btrfs replace -ddrange=x..y". Replace already knows how to move chunks from one device and put it on another. Now it "just" needs to skip "replace" part and ignore chunks not covered by filter ...
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