eBPF is really a superpower, it lets you do things which are incomprehensible if you don’t know about it.
I first confirmed that nvme drives were healthy according to SMART, I then worked up the stack and used BCC tools to look at block io latency. Block io latency was quite low for the NVME drives (microseconds) but was hundreds of milliseconds for the loopback block devices.
This lead me to believe that something was wrong with the loopback devices and not the underlying NVMEs. I used cachestat/cachetop and found that the page cache miss rate was very high and that we were thrashing the page cache constantly paging in and out data. From there I inspected the loopback devices using losetup and found that direct io was disabled and the sector size of the loopback device did not match the sector size of the backing filesystem.
I modified the loopback devices to use the same sector size as the block size of the underlying file system and enabled direct io. Instantly, the majority of the page cache was freed, iowait went way down, and io throughout went way up.
Without BCC tools I would have never been able to figure this out.
Double caching loopback devices is quite the footgun.
Another interesting thing we hit is that our version of losetup would happily fail to enable direct io but still give you a loopback device, this has since been fixed: https://github.com/util-linux/util-linux/commit/d53346ed082d...
https://github.com/containers/composefs https://github.com/project-machine/puzzlefs
And how did you know that tweaking the sector size to equal the underlying filesystem's block size would prevent double caching? Where can one get this sort of knowledge?
I knew that enabling direct io would most likely disable double caching because that is literally the point of enabling direct io on a loopback device. Initially I just tried enabling direct io on the loopback devices, but that failed with a cryptic “invalid argument” error. After some more research I found that direct IO needs the sector size to match the filesystems block size in some cases to work.