Add speed profiles to devices

Add speed profiles to devices

[Arthur Lutz]

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Hi,

It would be a great feature to have speed profiles linked to devices so
that btrfs could differentiate write or read operations according to
some metrics collected over time.

The use case would be : one has 3 disks as RAID1 but one of the disks is
older and slower than the two other disks. In this configuration the
third disk serves a bit as an online backup but it would be nice for a
write to be considered as done when the two fast disks have completed
the write, the third one acting asynchronously. If one of the two fast
disks fails we fall back to syncronous writing on that third device.

This would be great for mixing SSD and traditional drives too.

Issue submitted on https://bugzilla.kernel.org/show_bug.cgi?id=68671

Thanks.

Arthur
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