On Tue, 18 Jun 2019 at 19:45, Hugo Mills <h...@carfax.org.uk> wrote: > > 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. >
This combined with a flag that prevents btrfs from allocating on specific devices automatically would finally enable my dream filesystem where all writes always go to SSDs and are gradually migrated to slower storage after the fact (and eventually to RAID-6 if the data is really cold) as well as hot data being migrated to faster storage. All this could be done via a userspace daemon with controllable policy with this ioctl() and flag.
