This is a description of the clock synchronization issue we are facing
in Hadoop:
Components of Hadoop use mtime as a versioning mechanism. Here is an
example where Client B tests the expected 'version' of a file created
by Client A:
Client A: create file, write data into file.
Client A: expected_mtime <-- lstat(file)
Client A: broadcast expected_mtime to client B
...
Client B: mtime <-- lstat(file)
Client B: test expected_mtime == mtime
Since mtime may be set in Ceph by both client and MDS, inconsistent
mtime view is possible when clocks are not adequately synchronized.
Here is a test that reproduces the problem. In the following output,
issdm-18 has the MDS, and issdm-22 is a non-Ceph node with its time
set to an hour earlier than the MDS node.
nwatkins@issdm-22:~$ ssh issdm-18 date && ./test
Tue Nov 20 11:40:28 PST 2012 // MDS TIME
local time: Tue Nov 20 10:42:47 2012 // Client TIME
fstat time: Tue Nov 20 11:40:28 2012 // mtime seen after file
creation (MDS time)
lstat time: Tue Nov 20 10:42:47 2012 // mtime seen after file write
(client time)
Here is the code used to produce that output.
#include <errno.h>
#include <sys/fcntl.h>
#include <sys/time.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <dirent.h>
#include <sys/xattr.h>
#include <stdio.h>
#include <string.h>
#include <assert.h>
#include <cephfs/libcephfs.h>
#include <time.h>
int main(int argc, char **argv)
{
struct stat st;
struct ceph_mount_info *cmount;
struct timeval tv;
/* setup */
ceph_create(&cmount, "admin");
ceph_conf_read_file(cmount, "/users/nwatkins/Projects/ceph.conf");
ceph_mount(cmount, "/");
/* print local time for reference */
gettimeofday(&tv, NULL);
printf("local time: %s", ctime(&tv.tv_sec));
/* create a file */
char buf[256];
sprintf(buf, "/somefile.%d", getpid());
int fd = ceph_open(cmount, buf, O_WRONLY|O_CREAT, 0);
assert(fd > 0);
/* get mtime for this new file */
memset(&st, 0, sizeof(st));
int ret = ceph_fstat(cmount, fd, &st);
assert(ret == 0);
printf("fstat time: %s", ctime(&st.st_mtime));
/* write some data into the file */
ret = ceph_write(cmount, fd, buf, sizeof(buf), -1);
assert(ret == sizeof(buf));
ceph_close(cmount, fd);
memset(&st, 0, sizeof(st));
ret = ceph_lstat(cmount, buf, &st);
assert(ret == 0);
printf("lstat time: %s", ctime(&st.st_mtime));
ceph_shutdown(cmount);
return 0;
}
Note that this output is currently using the short patch from
http://marc.info/?l=ceph-devel&m=133178637520337&w=2 which forces
getattr to always go to the MDS.
diff --git a/src/client/Client.cc b/src/client/Client.cc
index 4a9ae3c..2bb24b7 100644
--- a/src/client/Client.cc
+++ b/src/client/Client.cc
@@ -3858,7 +3858,7 @@ int Client::readlink(const char *relpath, char
*buf, loff_t \
size)
int Client::_getattr(Inode *in, int mask, int uid, int gid)
{
- bool yes = in->caps_issued_mask(mask);
+ bool yes = false; //in->caps_issued_mask(mask);
ldout(cct, 10) << "_getattr mask " << ccap_string(mask) << "
issued=" << yes << \
dendl; if (yes)
--
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