My Plex server is running Debian 10 and I'm using mdadm for RAID 6. I recently added 2 more drives to the array and during the reshape, when I was watching a movie via my Plex client, streaming would frequently pause. Some times Plex would just stop playing back and give me an error. Under normal circumstances, my RAID array can sustain 500MB/s read/write. When I add a drive or 2 and mdadm is doing a reshape, that speed drops like a rock, which is understandable.

But, is there a command(s) I can issue to mdadm while it's adding drives to say..."hey, if something needs to read data from the array....slow down on the reshape"?

See if changing speed_limit_max has an effect.

"Raid 6 is the only raid level where mdadm may complain that it cannot carry out a conversion directly between levels 4, 5 and 6. This is down to the way the parity blocks are scattered amongst the devices. The reshape is almost certainly possible, but it may require doing it in a couple of stages rather than directly."

https://raid.wiki.kernel.org/index.php/A_guide_to_mdadm

Basically saying it takes a lot of movements and that those require overheard to perform.

Next time I do a re-shape, I'll give that try, thanks!

Thanks for the link! If berndbausch's tip doesn't help, guess I'll just let my Plex streamers know that the server might be slow for a couple of days the next time I expand the array.

I was wondering if you were subject to a yo-yo effect. The reshape is trundling along then Plex reads a bunch of content into memory. mdadm sees that and throttles itself - then while Plex is serving that content, mdadm sees no (disk) I/O so ramps back up. Then Plex wants more content ...

I'd simply run mdadm at the min (set max equal to the current min) while people are glued to their devices, then when they all go to bed set min and max to current max. Couple of simple timers or cron. Only needed when you need to reshape.