[ceph-users] Do not use SSDs with (small) SLC cache

Hi all, digging around debugging, why our (small: 10 Hosts/~60 OSDs) cluster is so slow even while recovering I found out one of our key issues are some SSDs with SLC cache (in our case Samsung SSD 870 EVO) - which we just recycled from other use cases in the hope to speed up our mainly hdd based cluster. We know it's a little bit random which objects get accelerated when not used as cache. However the opposite was the case. These type's ssds are only fast when operating in their SLC cache, which is only several Gigabytes in a multi-TB ssd [1]. When doing a big write or a backfill onto these SSDs we got really low IO-rates (around 10 MB/s even with 4M-objects). But it got even worse. Disclaimer: This is my view as a user, maybe a more technically involved person is able to correct me. Cause seems to be the mclock-scheduler which measures the iops an osd is able to do. As in the blog measured [2], this is usually a good thing as there is done some profiling and queing is done different. But in our case the osd_mclock_max_capacity_iops_ssd for most of the corresponding osds was very low. But not for everyone. I assume that it depends when mclock-scheduler measured the iops capacity. That led to a broken scheduling where backfills were at low speed and the ssd itself had nearly no disk usage because it was operating in it's cache again and could work faster. That issue could be solved by switching back to wpq scheduler for the affected SSDs. This scheduler seems to just queue up ios without throttling because of maximum iops reached. Now we see a still bad IO situation because of the slow SSDs but at least they are operating at their maximum (having typical settings like osd_recovery_max_active and osd_recovery_sleep* tuned). We are going to replace the SSDs to hopefully more consistent performing ones (even if their peak performance would be not as good). I hope this may help somebody in the future when being stuck in low performance recoverys. Refs: [1] https://www.tomshardware.com/reviews/samsung-870-evo-sata-ssd-review-the-be… [2] https://ceph.io/en/news/blog/2022/mclock-vs-wpq-testing-with-background-ops… Happy Storing! Michael Wodniok -- Michael Wodniok M.Sc. WorNet AG Bürgermeister-Graf-Ring 28 82538 Geretsried Simply42 und SecuMail sind Marken der WorNet AG. http://www.wor.net/ Handelsregister Amtsgericht München (HRB 129882) Vorstand: Christian Eich Aufsichtsratsvorsitzender: Dirk Steinkopf