On Thu, Jul 20, 2017 at 9:19 PM, Andras Pataki <apataki@xxxxxxxxxxxxxxxxxxxxx> wrote: > We are having some difficulties with cephfs access to the same file from > multiple nodes concurrently. After debugging some large-ish applications > with noticeable performance problems using CephFS (with the fuse client), I > have a small test program to reproduce the problem. > > The core of the problem boils down to the following operation being run on > the same file on multiple nodes (in a loop in the test program): > > int fd = open(filename, mode); > read(fd, buffer, 100); > close(fd); > > Here are some results on our cluster: > > One node, mode=read-only: 7000 opens/second > One node, mode=read-write: 7000 opens/second > Two nodes, mode=read-only: 7000 opens/second/node > Two nodes, mode=read-write: around 0.5 opens/second/node (!!!) > Two nodes, one read-only, one read-write: around 0.5 opens/second/node (!!!) > Two nodes, mode=read-write, but remove the 'read(fd, buffer,100)' line from > the code: 500 opens/second/node > > > So there seems to be some problems with opening the same file read/write and > reading from the file on multiple nodes. That operation seems to be 3 > orders of magnitude slower than other parallel access patterns to the same > file. The 1 second time to open files almost seems like some timeout is > happening somewhere. I have some suspicion that this has to do with > capability management between the fuse client and the MDS, but I don't know > enough about that protocol to make an educated assessment. You're pretty much spot on. Things happening at 0.5 per second is characteristic of a particular class of bug where we are not flushing the journal soon enough, and instead waiting for the next periodic (every five second) flush. Hence there is an average 2.5 second dely, hence operations happening at approximately half an operation per second. > [And an aside - how does this become a problem? I.e. why open a file > read/write and read from it? Well, it turns out gfortran compiled code does > this by default if the user doesn't explicitly says otherwise]. > > All the nodes in this test are very lightly loaded, so there does not seems > to be any noticeable performance bottleneck (network, CPU, etc.). The code > to reproduce the problem is attached. Simply compile it, create a test file > with a few bytes of data in it, and run the test code on two separate nodes > on the same file. > > We are running ceph 10.2.9 both on the server, and we use the 10.2.9 fuse > client on the client nodes. > > Any input/help would be greatly appreciated. If you have a test/staging environment, it would be great if you could re-test this on the 12.1.1 release candidate. There have been MDS fixes for similar slowdowns that were shown up in multi-mds testing, so it's possible that the issue you're seeing here was fixed along the way. John > > Andras > > > _______________________________________________ > ceph-users mailing list > ceph-users@xxxxxxxxxxxxxx > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com > _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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