Hi Jason, thanks for your feedback. I did now some tests over the weekend to verify the memory overhead. I was using qemu 2.8 (taken from the Ubuntu Cloud Archive) with librbd 10.2.7 on Ubuntu 16.04 hosts. I suspected the ceph rbd cache to be the cause of the overhead so I just generated a lot of IO with the help of fio in the VMs (with a datasize of 80GB) . All VMs had 3GB of memory. I had to run fio multiple times, before reaching high RSS values. I also noticed that when using larger blocksizes during writes (like 4M) the memory overhead in the KVM process increased faster. I ran several fio tests (one after another) and the results are: KVM with writeback RBD cache: max. 85% memory overhead (2.5 GB overhead) KVM with writethrough RBD cache: max. 50% memory overhead KVM without RBD caching: less than 10% overhead all the time KVM with local storage (logical volume used): 8% overhead all the time I did not reach those >200% memory overhead results that we see on our live cluster, but those virtual machines have a way longer uptime as well. I also tried to reduce the RSS memory value with cache dropping on the physical host and in the VM. Both did not lead to any change. A reboot of the VM also does not change anything (reboot in the VM, not a new KVM process). The only way to reduce the RSS memory value is a live migration so far. Might this be a bug? The memory overhead sounds a bit too much for me. Best Regards Sebastian On Thursday, April 27, 2017 10:08:36 AM you wrote: > I know we noticed high memory usage due to librados in the Ceph > multipathd checker [1] -- the order of hundreds of megabytes. That > client was probably nearly as trivial as an application can get and I > just assumed it was due to large monitor maps being sent to the client > for whatever reason. Since we changed course on our RBD iSCSI > implementation, unfortunately the investigation into this high memory > usage fell by the wayside. > > [1] > http://git.opensvc.com/gitweb.cgi?p=multipath-tools/.git;a=blob;f=libmultip > ath/checkers/rbd.c;h=9ea0572f2b5bd41b80bf2601137b74f92bdc7278;hb=HEAD > On Thu, Apr 27, 2017 at 5:26 AM, nick <nick@xxxxxxx> wrote: > > Hi Christian, > > thanks for your answer. > > The highest value I can see for a local storage VM in our infrastructure > > is a memory overhead of 39%. This is big, but the majority (>90%) of our > > local storage VMs are using less than 10% memory overhead. > > For ceph storage based VMs this looks quite different. The highest value I > > can see currently is 244% memory overhead. So that specific allocated 3GB > > memory VM is using now 10.3 GB RSS memory on the physical host. This is a > > really huge value. In general I can see that the majority of the ceph > > based VMs has more than 60% memory overhead. > > > > Maybe this is also a bug related to qemu+librbd. It would be just nice to > > know if other people are seeing those high values as well. > > > > Cheers > > Sebastian > > > > On Thursday, April 27, 2017 06:10:48 PM you wrote: > >> Hello, > >> > >> Definitely seeing about 20% overhead with Hammer as well, so not version > >> specific from where I'm standing. > >> > >> While non-RBD storage VMs by and large tend to be closer the specified > >> size, I've seen them exceed things by few % at times, too. > >> For example a 4317968KB RSS one that ought to be 4GB. > >> > >> Regards, > >> > >> Christian > >> > >> On Thu, 27 Apr 2017 09:56:48 +0200 nick wrote: > >> > Hi, > >> > we are running a jewel ceph cluster which serves RBD volumes for our > >> > KVM > >> > virtual machines. Recently we noticed that our KVM machines use a lot > >> > more > >> > memory on the physical host system than what they should use. We > >> > collect > >> > the data with a python script which basically executes 'virsh > >> > dommemstat > >> > <virtual machine name>'. We also verified the results of the script > >> > with > >> > the memory stats of 'cat /proc/<kvm PID>/status' for each virtual > >> > machine > >> > and the results are the same. > >> > > >> > Here is an excerpt for one pysical host where all virtual machines are > >> > running since yesterday (virtual machine names removed): > >> > > >> > """ > >> > overhead actual percent_overhead rss > >> > ---------- -------- ---------------- -------- > >> > 423.8 MiB 2.0 GiB 20 2.4 GiB > >> > 460.1 MiB 4.0 GiB 11 4.4 GiB > >> > 471.5 MiB 1.0 GiB 46 1.5 GiB > >> > 472.6 MiB 4.0 GiB 11 4.5 GiB > >> > 681.9 MiB 8.0 GiB 8 8.7 GiB > >> > 156.1 MiB 1.0 GiB 15 1.2 GiB > >> > 278.6 MiB 1.0 GiB 27 1.3 GiB > >> > 290.4 MiB 1.0 GiB 28 1.3 GiB > >> > 291.5 MiB 1.0 GiB 28 1.3 GiB > >> > 0.0 MiB 16.0 GiB 0 13.7 GiB > >> > 294.7 MiB 1.0 GiB 28 1.3 GiB > >> > 135.6 MiB 1.0 GiB 13 1.1 GiB > >> > 0.0 MiB 2.0 GiB 0 1.4 GiB > >> > 1.5 GiB 4.0 GiB 37 5.5 GiB > >> > """ > >> > > >> > We are using the rbd client cache for our virtual machines, but it is > >> > set > >> > to only 128MB per machine. There is also only one rbd volume per > >> > virtual > >> > machine. We have seen more than 200% memory overhead per KVM machine on > >> > other physical machines. After a live migration of the virtual machine > >> > to > >> > another host the overhead is back to 0 and increasing slowly back to > >> > high > >> > values. > >> > > >> > Here are our ceph.conf settings for the clients: > >> > """ > >> > [client] > >> > rbd cache writethrough until flush = False > >> > rbd cache max dirty = 100663296 > >> > rbd cache size = 134217728 > >> > rbd cache target dirty = 50331648 > >> > """ > >> > > >> > We noticed this behavior since we are using the jewel librbd libraries. > >> > We > >> > did not encounter this behavior when using the ceph infernalis librbd > >> > version. We also do not see this issue when using local storage, > >> > instead > >> > of ceph. > >> > > >> > Some version information of the physical host which runs the KVM > >> > machines: > >> > """ > >> > OS: Ubuntu 16.04 > >> > kernel: 4.4.0-75-generic > >> > librbd: 10.2.7-1xenial > >> > """ > >> > > >> > We did try to flush and invalidate the client cache via the ceph admin > >> > socket, but this did not change any memory usage values. > >> > > >> > Does anyone encounter similar issues or does have an explanation for > >> > the > >> > high memory overhead? > >> > > >> > Best Regards > >> > Sebastian > > > > -- > > Sebastian Nickel > > Nine Internet Solutions AG, Albisriederstr. 243a, CH-8047 Zuerich > > Tel +41 44 637 40 00 | Support +41 44 637 40 40 | www.nine.ch > > _______________________________________________ > > ceph-users mailing list > > ceph-users@xxxxxxxxxxxxxx > > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com -- Sebastian Nickel Nine Internet Solutions AG, Albisriederstr. 243a, CH-8047 Zuerich Tel +41 44 637 40 00 | Support +41 44 637 40 40 | www.nine.ch
Attachment:
signature.asc
Description: This is a digitally signed message part.
_______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
