If I understand correctly you have one sas disk as a journal for multiple OSDs. If you do small synchronous writes it will become a IO bottleneck pretty quickly: Due to multiple journals on the same disk it will no longer be sequential writes writes to one journal but 4k writes to x journals making it fully random. I would expect a performance of 100 to 200 IOPS max. Doing an iostat -x or atop should show this bottleneck immediately. This is also the reason to go with SSDs: they have reasonable random IO performance.
Cheers, Robert van Leeuwen Sent from my iPad > On 6 dec. 2013, at 17:05, "nicolasc" <nicolas.cance...@surfsara.nl> wrote: > > Hi James, > > Thank you for this clarification. I am quite aware of that, which is why the > journals are on SAS disks in RAID0 (SSDs out of scope). > > I still have trouble believing that fast-but-not-super-fast journals is the > main reason for the poor performances observed. Maybe I am mistaken? > > Best regards, > > Nicolas Canceill > Scalable Storage Systems > SURFsara (Amsterdam, NL) > > > > On 12/03/2013 03:01 PM, James Pearce wrote: >>> I would really appreciate it if someone could: >>> - explain why the journal setup is way more important than striping >>> settings; >> >> I'm not sure if it's what you're asking, but any write must be physically >> written to the journal before the operation is acknowledged. So the overall >> cluster performance (or rather write latency) is always governed by the >> speed of those journals. Data is then gathered up into (hopefully) larger >> blocks and committed to OSDs later. >> _______________________________________________ >> ceph-users mailing list >> ceph-users@lists.ceph.com >> http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com > > _______________________________________________ > ceph-users mailing list > ceph-users@lists.ceph.com > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com _______________________________________________ ceph-users mailing list ceph-users@lists.ceph.com http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
