Can the meta and data device of thin pool to be loop back device?

[huang-jl]

I notice that the meta and data device of thin pool has to be a linear dev for now. I test locally on my machine, that directly pass a loop device to dmsetup command to create a thin pool is fine.

I also find a related issure #52 many years ago, but seems without any further updates.

I wonder, at least, if is it possible to use lookback device directly, as meta and data device of thin pool for devicemapper-rs. I doubt whether create a linear device based on loopback device maybe:

1. Add latency during create? Of course, creating the thin-pool might not be too frequent, so this might not be a big problem.
2. Performance penalty of creating linear device on loopback device? (I do not test it, but I concern it might be a problem?)

[mulkieran]

I think it should absolutely be possible to do this. Unfortunately, devicemapper-rs ThinPool does not allow it. This is a mistake in the design. If you want to use Rust to build a thinpool directly on loopbacked devices you can try to use the core functionality of devicemapper-rs, located in the core subdirectory, only. This will require you to construct your own thinpool table from primitive Rust types, but it should work.

I do not believe that the performance penalty of linear devices on top of loopbacked devices, your simpler alternative, would be very great. It can be no greater than the performance penalty of dm-crypt, which adds a 10% latency, very roughly. It does make your storage stack more complicated, but that might not bother you.

[huang-jl]

Thanks for you reply. If using the core of devicemapper-rs directly, I probably need to write my own thin pool and thin dev struct (as those fields in the struct of devicemapper-rs are private). I think it seems like "rebuild the wheel", which in unnecessary. But at least it works, so maybe I will try to rebuild my own version of thin pool. But it would be better if you can make the meta and data device to be non-linear device :)