On Mon, Feb 4, 2019 at 4:16 PM Gregory Farnum <[email protected]> wrote:
> On Fri, Feb 1, 2019 at 2:29 AM Mahmoud Ismail < > [email protected]> wrote: > >> Hello, >> >> I'm a bit confused about how the journaling actually works in the MDS. >> >> I was reading about these two configuration parameters (journal write >> head interval) and (mds early reply). Does the MDS flush the journal >> synchronously after each operation? and by setting mds eary reply to true >> it allows operations to return without flushing. If so, what the other >> parameter (journal write head interval) do or isn't it for MDS?. Also, can >> all operations return without flushing with the mds early reply or is it >> specific to a subset of operations?. >> > > In general, the MDS journal is flushed every five seconds (by default), > and client requests get an early reply when the operation is done in memory > but not yet committed to RADOS. Some operations will trigger an immediate > flush, and there may be some operations that can't get an early reply or > that need to wait for part of the operation to get committed (like renames > that move a file's authority to a different MDS). > IIRC the journal write head interval controls how often it flushes out the > journal's header, which limits how out-of-date its hints on restart can be. > (When the MDS restarts, it asks the journal head where the journal's > unfinished start and end points are, but of course more of the journaled > operations may have been fully completed since the head was written.) > Thanks for the explanation. Which operations trigger an immediate flush? Is the readdir one of these operations?. I noticed that the readdir operation latency is going higher under load when the OSDs are hitting the limit of the underlying hdd throughput. Can i assume that this is happening due to the journal flushing then? > >> Another question, are open operations also written to the journal? >> > > Not opens per se, but we do persist when clients have permission to > operate on files. > -Greg > > >> >> Regards, >> Mahmoud >> >> _______________________________________________ >> ceph-users mailing list >> [email protected] >> http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com >> >
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