Hi all,

I am trying to better understand how the cleaner kthread (btrfs-cleaner) impacts foreground performance, specifically during snapshot deletion. My experience so far has been that it can be dramatically disruptive to foreground I/O.

Looking through the wiki at kernel.org I have not yet stumbled onto any analysis that would shed light on this specific problem. I have found numerous complaints about btrfs-cleaner online, especially relating to quotas being enabled. This has proven thus far less than helpful, as the response tends to be "use less snapshots," or "disable quotas," both of which strike me as intellectually unsatisfying answers, especially the former in a filesystem where snapshots are supposed to be "first-class citizens."

The 2007 and 2013 Rodeh papers don't do the thorough practical snapshot performance analysis I would expect to see given the assertions in the latter that "BTRFS...supports efficient snapshots..." The former is sufficiently pre-BTRFS that while it does performance analysis of btree clones, it's unclear (to me at least) if the results can be forward-propagated in some way to real-world performance expectations for BTRFS snapshot creation/deletion/modification.

Has this analysis been performed somewhere else and I'm just missing it? Also, I'll be glad to comment on my specific setup, kernel version, etc, and discuss pragmatic work-arounds, but I'd like to better understand the high-level performance implications first.

Thanks in advance to anyone who can comment on this. I am very inclined to read anything thrown at me, so if there is documentation I failed to read, please just send the link.


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