[ceph-users] Re: increasing number of (deep) scrubs

Hi all, if you follow this thread, please see the update in "How to configure something like osd_deep_scrub_min_interval?" (https://lists.ceph.io/hyperkitty/list/ceph-users@ceph.io/thread/YUHWQCDAKP5…). I found out how to tune the scrub machine and I posted a quick update in the other thread, because the solution was not to increase the number of scrubs, but to tune parameters. Best regards, ================= Frank Schilder AIT Risø Campus Bygning 109, rum S14 ________________________________________ From: Frank Schilder Sent: Monday, January 9, 2023 9:14 AM To: Dan van der Ster Cc: ceph-users(a)ceph.io Subject: Re: [ceph-users] increasing number of (deep) scrubs Hi Dan, thanks for your answer. I don't have a problem with increasing osd_max_scrubs (=1 at the moment) as such. I would simply prefer a somewhat finer grained way of controlling scrubbing than just doubling or tripling it right away. Some more info. These 2 pools are data pools for a large FS. Unfortunately, we have a large percentage of small files, which is a pain for recovery and seemingly also for deep scrubbing. Our OSDs are about 25% used and I had to increase the warning interval already to 2 weeks. With all the warning grace parameters this means that we manage to deep scrub everything about every month. I need to plan for 75% utilisation and a 3 months period is a bit far on the risky side. Our data is to a large percentage cold data. Client reads will not do the check for us, we need to combat bit-rot pro-actively. The reasons I'm interested in parameters initiating more scrubs while also converting more scrubs into deep scrubs are, that 1) scrubs seem to complete very fast. I almost never catch a PG in state "scrubbing", I usually only see "deep scrubbing". 2) I suspect the low deep-scrub count is due to a low number of deep-scrubs scheduled and not due to conflicting per-OSD deep scrub reservations. With the OSD count we have and the distribution over 12 servers I would expect at least a peak of 50% OSDs being active in scrubbing instead of the 25% peak I'm seeing now. It ought to be possible to schedule more PGs for deep scrub than actually are. 3) Every OSD having only 1 deep scrub active seems to have no measurable impact on user IO. If I could just get more PGs scheduled with 1 deep scrub per OSD it would already help a lot. Once this is working, I can eventually increase osd_max_scrubs when the OSDs fill up. For now I would just like that (deep) scrub scheduling looks a bit harder and schedules more eligible PGs per time unit. If we can get deep scrubbing up to an average of 42PGs completing per hour with keeping osd_max_scrubs=1 to maintain current IO impact, we should be able to complete a deep scrub with 75% full OSDs in about 30 days. This is the current tail-time with 25% utilisation. I believe currently a deep scrub of a PG in these pools takes 2-3 hours. Its just a gut feeling from some repair and deep-scrub commands, I would need to check logs for more precise info. Increasing osd_max_scrubs would then be a further and not the only option to push for more deep scrubbing. My expectation would be that values of 2-3 are fine due to the increasingly higher percentage of cold data for which no interference with client IO will happen. Hope that makes sense and there is a way beyond bumping osd_max_scrubs to increase the number of scheduled and executed deep scrubs. Best regards, ================= Frank Schilder AIT Risø Campus Bygning 109, rum S14 ________________________________________ From: Dan van der Ster <dvanders(a)gmail.com> Sent: 05 January 2023 15:36 To: Frank Schilder Cc: ceph-users(a)ceph.io Subject: Re: [ceph-users] increasing number of (deep) scrubs Hi Frank, What is your current osd_max_scrubs, and why don't you want to increase it? With 8+2, 8+3 pools each scrub is occupying the scrub slot on 10 or 11 OSDs, so at a minimum it could take 3-4x the amount of time to scrub the data than if those were replicated pools. If you want the scrub to complete in time, you need to increase the amount of scrub slots accordingly. On the other hand, IMHO the 1-week deadline for deep scrubs is often much too ambitious for large clusters -- increasing the scrub intervals is one solution, or I find it simpler to increase mon_warn_pg_not_scrubbed_ratio and mon_warn_pg_not_deep_scrubbed_ratio until you find a ratio that works for your cluster. Of course, all of this can impact detection of bit-rot, which anyway can be covered by client reads if most data is accessed periodically. But if the cluster is mostly idle or objects are generally not read, then it would be preferable to increase slots osd_max_scrubs. Cheers, Dan On Tue, Jan 3, 2023 at 2:30 AM Frank Schilder <frans(a)dtu.dk> wrote: