They are SAS disks, 900GB 15K (spinning disks, not SSDs) configured in RAID10. They are not exactly slow, but you obviously wouldn’t get the IOPS you’d get from an SSD.
As for the database: the postgres_data directory is 4.5GB.
I doubt it’s redis in this case though - I stupidly enough forgot to grab a full screenshot from iotop but I can’t remember seeing redis in there. Did see postgres UPDATE at the top of the list with IO activity of 98% and up.
BTW: I’ll check tomorrow as I have no doubt it will happen again. And I’ll grab a screenshot from iotop.