On Mon, Feb 4, 2019 at 8:03 AM Mahmoud Ismail <mahmoudahmedism...@gmail.com> wrote:
> On Mon, Feb 4, 2019 at 4:35 PM Gregory Farnum <gfar...@redhat.com> wrote: > >> >> >> On Mon, Feb 4, 2019 at 7:32 AM Mahmoud Ismail < >> mahmoudahmedism...@gmail.com> wrote: >> >>> On Mon, Feb 4, 2019 at 4:16 PM Gregory Farnum <gfar...@redhat.com> >>> wrote: >>> >>>> On Fri, Feb 1, 2019 at 2:29 AM Mahmoud Ismail < >>>> mahmoudahmedism...@gmail.com> 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 the journal's header, which limits how out-of-date its hints on restart can 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? >>> >> >> Not directly, but a readdir might ask to know the size of each file and >> that will force the other clients in the system to flush their dirty data >> in the directory (so that the readdir can return valid results). >> -Greg >> >> > > Could it be also due to the MDS lock (operations waiting for the lock > under load)? > Well that's not going to cause high OSD usage, and the MDS lock is not held while writes are happening. But if the MDS is using 100% CPU, yes, it could be contended. > Also, i assume that the journal is using a different thread for flushing, > Right? > Yes, that's correct. > >>>>>
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