[Bug 1900438] Re: Bcache bypasse writeback on caching device with fragmentation

** Changed in: linux (Ubuntu)
Status: Incomplete => Confirmed

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https://bugs.launchpad.net/bugs/1900438

Title:
Bcache bypasse writeback on caching device with fragmentation

Status in linux package in Ubuntu:
Confirmed

Bug description:
Hello,

An upstream bug has been opened on the matter for quite some time now [0].
I can reproduce easily on our production compute node instance, which are trusty host with xenial hwe kernels (4.15.0-101-generic).
However due to heavy backport and such, doing real tracing is a bit hard there.

I was able to reproduce the behavior on a hwe-bionic kernel as well.
Since most of our critical deployments use bcache, I think this is a kinda nasty bug to have.

Reproducing the issue is relatively easy with the script provided in the bug [1].
The script used to capture the stats is this one [2].

[0]: https://bugzilla.kernel.org/show_bug.cgi?id=206767
[1]: https://pastebin.ubuntu.com/p/YnnvvSRhXK/
[2]: https://pastebin.ubuntu.com/p/XfVpzg32sN/

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