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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:
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. [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. Andras |
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <sys/time.h>
#define INTERVAL 2
double now()
{
struct timeval tv;
gettimeofday(&tv, NULL);
return tv.tv_sec + tv.tv_usec / 1e6;
}
int main(int argc, char *argv[])
{
if (argc != 3) {
fprintf(stderr, "Usage: %s <filename> r|rw\n", argv[0]);
exit(1);
}
const char *filename = argv[1];
int mode = 0;
if (strcmp(argv[2], "r") == 0) {
mode = O_RDONLY;
} else if (strcmp(argv[2], "rw") == 0) {
mode = O_RDWR;
} else {
fprintf(stderr, "Second argument must be 'r' or 'rw'\n");
exit(1);
}
while (1) {
char buffer[100];
double t0 = now();
double dt;
int count = 0;
while (1) {
dt = now() - t0;
if (dt > INTERVAL) {
break;
}
int fd = open(filename, mode);
if (fd < 0) {
printf("Could not open file '%s' for read/write", filename);
exit(1);
}
read(fd, buffer, 100);
close(fd);
count++;
}
printf("File open rate: %8.2f\n", count / dt);
}
return 0;
}
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