Thin pool CoW latency

[Demi Marie Obenour]

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Like Eric, I am very concerned about CoW latency and throughput.  I
am almost certain that allocating new blocks and snapshot copy-on-write
are _the_ hot paths in Qubes OS.  In particular, I suspect that
workloads such as building an image in a throwaway VM or installing
packages onto a root volume that had just been shapshotted are dominated
by metadata operations, rather than by in-place updates.  I suspect that
frequently-snapshotted volumes will observe similar behavior in general.
- -- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab
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