Thanks Hans. Sorry for the top-post, but I'm boiling things down here
so I don't have a clear line-item to respond to. The take-aways I see
here to my original queries are:
1. Nobody has done a thorough analysis of the impact of snapshot
manipulation WITHOUT qgroups enabled on foreground I/O performance
2. Nobody has done a thorough analysis of the impact of snapshot
manipulation WITH qgroups enabled on foreground I/O performance
3. I need to look at the code to understand the interplay between
qgroups, snapshots, and foreground I/O performance as there isn't
existing architecture documentation to point me to that covers this
4. I should be cautioned that CoW in BTRFS can exhibit pathological (if
expected) capacity consumption for very random-write-oriented datasets
with or without snapshots, and nocow (or in my case transparently
absorbing and coalescing writes at a higher tier) is my friend.
5. I should be cautioned that CoW is broken across snapshots when
defragmentation is run.
I will update a test system to the most recent kernel and will perform
tests to answer #1 and #2. I will plan to share it when I'm done. If I
have time to write-up my findings for #3 I will similarly share that.
Thanks to all for your input on this issue.
ellis
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