Hi Janne. > To be fair, this number could just be something vaguely related to > "spin drives have 100-200 iops" ... It could be, but is it? Or is it just another rumor? I simply don't see how the PG count could possibly impact Io load on a disk. How about this guess: It could be dragged along from a time when HDDs were <=1T and it simply means to have PGs not larger than 10G. Sounds reasonable, but is it? I think we should really stop second-guessing here. This discussion was not meant to be a long thread where we all just guess but never know. I would appreciate if someone who actually knows something about why this recommendation is really there would ship in here. As far as I can tell, it could be anything or nothing. I actually tend toward its nothing, it was just never updated along with new developments and nowadays nobody knows any more. Best regards, ================= Frank Schilder AIT Risø Campus Bygning 109, rum S14 ________________________________________ From: Janne Johansson <icepic.dz@xxxxxxxxx> Sent: Thursday, October 10, 2024 8:51 AM To: Frank Schilder Cc: Anthony D'Atri; ceph-users@xxxxxxx Subject: Re: Re: What is the problem with many PGs per OSD Den ons 9 okt. 2024 kl 20:48 skrev Frank Schilder <frans@xxxxxx>: > The PG count per OSD is a striking exception. Its just a number (well a range with 100 recommended and 200 as a max: https://docs.ceph.com/en/latest/rados/operations/pgcalc/#keyDL). It just is. And this doesn't make any sense unless there is something really evil lurking in the dark. > For comparison, a guidance that does make sense is something like 100PGs per TB. That I would vaguely understand: to keep the average PG size constant at a max of about 10G. To be fair, this number could just be something vaguely related to "spin drives have 100-200 iops", and while cent/rhel linux kernels 10 years ago did have some issues in getting io done in parallel as much as possible towards a single device, doing multiple OSDs on flash devices would have been both a way to get around this limitation in the IO middle layer, and a way to "tell" ceph it can send more IO to the device since it has multiple OSDs on it. -- May the most significant bit of your life be positive. _______________________________________________ ceph-users mailing list -- ceph-users@xxxxxxx To unsubscribe send an email to ceph-users-leave@xxxxxxx
 |