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The fact that there's two layers both doing this kind of thing (especially oblivious to one another) is a known and studied issue. Most of that study has been i
by notacoward 5y ago
The fact that there's two layers both doing this kind of thing (especially oblivious to one another) is a known and studied issue. Most of that study has been in the context of virtual-machine filesystems atop host filesystems but it's also applicable here. I'm pretty sure there was a paper at FAST about this several years ago, but can't find it right now. It's entirely likely that COW-over-COW is a bad choice just like TCP-over-TCP is.
- klodolph 5y agoAre you thinking of the classic, "Don't Stack Your Log On My Log"? https://www.usenix.org/conference/inflow14/workshop-program/presentation/yang https://www.usenix.org/conference/inflow14/workshop-program/...
- notacoward 5y agoYeah, seems about right. Thanks!
- the8472 5y agoBut the upper-level CoW can coordinate with the lower level via TRIM, that's not quite the same as TCP-in-TCP where the congestion algorithms don't talk to each other.
- notacoward 5y agoYes, TRIM exists, but it's a very limited form of coordination at best and implementations don't even do all they can with that. Every analogy is imperfect.