[ceph-users] Re: What is a pgmap?
Here's what I learned about PG maps from my investigation of the code. First, they don't seem to be involved in deciding what needs reconstruction when a dead OSD is revived. There is a version number stored with the PGs that is probably used for that. It looks like nothing but statistics - the ones you see in a 'ceph status' (or more specifically, 'ceph pg stat' report - and I don't think those statistics affect any automatic operation. The PG map gets updated (version incremented) mainly when an OSD sends those statistics to the monitor cluster. Each OSD sends a statistics report every 6 seconds (default - it's the osd_heartbeat_interval configuration variable) to a monitor. If those statistics differ at all from the previous report, the monitor generates a new PG map. Because the stats include I/O rates, they do tend to be different every time. But there is a limit of one update per second (default - it's the 'paxos_propose_interval' configuration variable) on updates to any of the maps in the monitor database, so on any normal size system, you'll see updates once a second. -- Bryan Henderson San Jose, California ___ ceph-users mailing list -- ceph-users@ceph.io To unsubscribe send an email to ceph-users-le...@ceph.io
[ceph-users] Re: What is a pgmap?
>I thought it was a method (the method?) to know if a PG comes back from a >crashed OSD/host, to know if it was up-to-date or old since it would have >an older timestamp. Thanks. That's a reasonable theory. Maybe I'll look in the code and see if I can confirm it. And it means on my cluster, once an hour would probably be sufficient. >I was sure it was updated exactly once per second. Because there's an infamous cluster log message every time the pgmap updates, I know for me it is about 10 times a minute, in a pattern that is neither periodic nor random. Maybe once per second is the maximum frequency and it depends upon how frequently PGs are written to. -- Bryan Henderson San Jose, California ___ ceph-users mailing list -- ceph-users@ceph.io To unsubscribe send an email to ceph-users-le...@ceph.io
[ceph-users] What is a pgmap?
I'm surprised I couldn't find this explained anywhere (I did look), but ... What is the pgmap and why does it get updated every few seconds on a tiny cluster that's mostly idle? I do know what a placement group (PG) is and that when documentation talks about placement group maps, it is talking about something else -- mapping of PGs to OSDs by CRUSH and OSD maps. -- Bryan Henderson San Jose, California ___ ceph-users mailing list -- ceph-users@ceph.io To unsubscribe send an email to ceph-users-le...@ceph.io
