Hi Marc, I would try something the near your example, yes. You could add "size=<yourdisksize>" for testing until once written the whole disk (by io, not by all available cells) independent of time. If the result is either high deviation or the results are far from the specification it's very likely there is a SLC cache in use. Best regards, Michael -----Ursprüngliche Nachricht----- Von: Marc <Marc@xxxxxxxxxxxxxxxxx> Gesendet: Dienstag, 21. Februar 2023 11:27 An: Michael Wodniok <wodniok@xxxxxxx>; ceph-users@xxxxxxx; Phil Regnauld <pr@xxxxx> Betreff: RE: [ceph-users] Re: Do not use SSDs with (small) SLC cache What fio test would indicate this behaviour up front? I guess something like this, but with a duration larger than this disk cache? [randwrite-4k-seq] stonewall bs=4k rw=randwrite fsync=1 > thank you for your hint - any input is appreciated. Please note that > Ceph does highly random IO (especially when having small object > sizes), AnandTech also states: > > "Some of our other tests have shown a few signs that the 870 EVO's > write performance can drop when the SLC cache runs out, but this > straightforward sequential write pass over the entire drive doesn't > reveal any such behavior. The 870 EVO's sequential write performance > is extremely consistent, even on the second write pass." [3] > > So this kind of cache handling is very interesting under the hood, > because it seems the Samsung SSD 870 Evo is able to handle sequential > io at nearly SATA line speed. However the random behaviour seems to be > inconsistent - maybe I'm able to run a big fio on the ssd once I have > one of these on my desk, but I can't promise I'll have time to. > > More interesting: We also have some Kingston SEDC450. Kingston even > promises constant write speed [4] - even when specifying lower > performance. At least those drives did not fell into my eye (but I did > not examine the situation specially). > > [4] > https://www.kingston.com/en/ssd/dc450-data-center-solid-state-drive > > Best regards, > Michael > > -----Ursprüngliche Nachricht----- > Von: mailing-lists <mailing-lists@xxxxxxxxx> > Gesendet: Dienstag, 21. Februar 2023 10:21 > An: ceph-users@xxxxxxx > Betreff: [ceph-users] Re: Do not use SSDs with (small) SLC cache > > Dear Michael, > > I don't have an explanation for your problem unfortunately, but I just > wondered that you experience a drop in performance, that this SSD > shouldn't have. Your SSDs drives (Samsung 870 EVO) should not get > slower on large writes. You can verify this on the post you've > attached [1] or here [3]. > > I am curious if replacing them with other disks will improve it. > > > [3] > https://www.anandtech.com/show/16480/the-samsung-870-evo-ssd-1tb-4tb- > review/4 > > > Best > > Ken
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