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6.6.1 Scrub Hammers CPU
And when I say hammers, I mean it runs with -1 prio and causes 6-8 kernel worker threads each vying to consume 100% of CPU. So rabid is this consumption that all other background processes, including...
Exactly! It'd be great to be able to throttle the scrub somehow. Maybe a selectable priority, or limit it to a single thread/core.
Well this is confirmed to still be happening in 6.7.1. I observe that overall I/O and wait time seem OK. Indeed the journal entries that pop up as the job starts suggest it's throttling on I/O rate. However the kernel worker processes still monopolize the CPU cores/threads. A certain amount of application CPU usage seems OK, but if you have anything like PLEX transcribes or AFS based Time Machine, which cause a fair amount of CPU activity themselves, then these other processes get starved to the point of being almost unusable.
Some of this is just Btrfs behavior, which I can compare to similar operations I do on even newer kernels on other Linux machines. But when you have a server environment where there is an expectation of responsiveness from applications, it can be problematic. On this front, the only thing I would consider a standard OS issue is Time Machine backups. These are CPU intensive and will be seriously delayed if not fail outright -- based on previous painful experiences. In this run I'm not allowing TM to even trigger until the scrub finishes. I know for a fact I'll wind up having to trash my whole backup archive if I let it try and fail.
As for PLEX, I can work around the issue by enabling direct play and disabling transcode in the client config. But hopefully we get to a point where scrubs will gracefully butt out when other activity requires CPU attention.
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