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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.
That's unfortunate to hear. I was hoping that it would be somewhat better. I think it's going on day four for my scrub at about 90% complete now.
Are you using compression on any of your volumes/shares?I am running compression on one of my shares. I think I'll copy that data off to a new uncompressed share. I'll run another scrub to see what happens.
Hmm. I've read more recently that IO throttling changed in more recent releases. I stated a scrub off while transcoding a show using PLEX. In the past, this would have immediately resulted in complaints that the server couldn't keep up with the transcode. In two hours I've had one such complaint. But overall it's been doing alright.
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