I think it still make sense to try with 'noatime'. Here is the reason. 'relatime' requires a write for the first read after a write, but the 'atime' requires a write for every read. But with noatime each read is free of a write.
Thanks & Regards Somnath -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Christian Balzer Sent: Wednesday, April 16, 2014 5:43 PM To: Dan van der Ster Cc: [email protected] Subject: Re: [ceph-users] RBD write access patterns and atime On Wed, 16 Apr 2014 17:08:09 +0200 Dan van der Ster wrote: > Dear ceph-users, > > I've recently started looking through our FileStore logs to better > understand the VM/RBD IO patterns, and noticed something interesting. > Here is a snapshot of the write lengths for one OSD server (with 24 > OSDs) -- I've listed the top 10 write lengths ordered by number of > writes in one day: > > Writes per length: > 4096: 2011442 > 8192: 438259 > 4194304: 207293 > 12288: 175848 > 16384: 148274 > 20480: 69050 > 24576: 58961 > 32768: 54771 > 28672: 43627 > 65536: 34208 > 49152: 31547 > 40960: 28075 > > There were ~4000000 writes to that server on that day, so you see that > ~50% of the writes were 4096 bytes, and then the distribution drops > off sharply before a peak again at 4MB (the object size, i.e. the max > write size). (For those interested, read lengths are below in the > P.S.) > > I'm trying to understand that distribution, and the best explanation > I've come up with is that these are ext4/xfs metadata updates, > probably atime updates. Based on that theory, I'm going to test > noatime on a few VMs and see if I notice a change in the distribution. > That strikes me as odd, as since kernel 2.6.30 the default option for mounts is relatime, which should have an effect quite close to that of a strict noatime. Regards, Christian -- Christian Balzer Network/Systems Engineer [email protected] Global OnLine Japan/Fusion Communications http://www.gol.com/ _______________________________________________ ceph-users mailing list [email protected] http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com ________________________________ PLEASE NOTE: The information contained in this electronic mail message is intended only for the use of the designated recipient(s) named above. If the reader of this message is not the intended recipient, you are hereby notified that you have received this message in error and that any review, dissemination, distribution, or copying of this message is strictly prohibited. If you have received this communication in error, please notify the sender by telephone or e-mail (as shown above) immediately and destroy any and all copies of this message in your possession (whether hard copies or electronically stored copies). _______________________________________________ ceph-users mailing list [email protected] http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
