I have a use case that I wonder if anyone might find interesting involving multiple device support and delayed raid. Let's say I have a system with two disks of equal size (to make it easy) which has sporadic, heavy, write requirements. At some points in time there will be multiple files being appended to simultaneously and at other times, there will be no activity at all. The write activity is time sensitive, however, so the filesystem must be able to provide guaranteed (only in a loose sense -- not looking for real QoS reservation semantics) bandwidths at times. Let's say slightly (but within the realm of reality) less than the bandwidth of the two disks combined. I also want both the metadata and file data mirrored between the two disks so that I can afford to lose one of the disks and not lose (most of) my data. It is not a strict requirement that all data be immediately mirrored however. So it seems that given these requirements that a filesystem should be able to keep the disks mirrored in a "loose timeframe" so as to provide redundancy (for all but the currently writing data) but also be able to provide the full bandwidth of the two disks, yes? Is this sort of idea on the btrfs roadmap at all? b.
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