On 02/11/2018 01:03 PM, Hans van Kranenburg wrote:
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

Well, the excellent write-up of Qu this morning shows some explanation
from the design point of view.

Sorry, I may have missed this email. Or perhaps you are referring to a wiki or blog post of some kind I'm not following actively? Either way, if you can forward me the link, I'd greatly appreciate it.

nocow only keeps the cows on a distance as long as you don't start
snapshotting (or cp --reflink) those files... If you take a snapshot,
then you force btrfs to keep the data around that is referenced by the
snapshot. So, that means that every next write will be cowed once again,
moo, so small writes will be redirected to a new location, causing
fragmentation again. The second and third write can go in the same (new)
location of the first new write, but as soon as you snapshot again, this
happens again.

Ah, very interesting.  Thank you for clarifying!


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