OS clock under Xen
February 23rd, 2010 | by jim |I was using s3sync to copy masses of files to Amazon S3 from a customer machine, and after the first run was complete, I just re-ran the same command to check elapsed time. It failed with 403 Forbidden errors …
Looking at the detailed error, it turned out that the machine’s clock was set about 20 minutes into the future (come back Max Headroom …), and this was breaking things. Quite reasonable.
So, I installed openntpd, and set the correct time from the date command. Well, I tried to set the date … it was being ignored. Very odd.
A bit more digging later, and I realised that this machine was a Xen guest, and had been set to follow the Xen host time — in this case, you can try to set the time until you’re blue in the face, and you will be ignored (without any errors being emitted, I note. Well, that’s under Debian 4, anyway).
The long-term answer is to get the Xen host fixed — having incorrect time is terribly inexcusable. Looking at a handy Xen FAQ, the short-term fix is to decouple the guest clock using /proc/sys/xen/independent_wallclock …
# cat /proc/sys/xen/independent_wallclock
0
# echo 1 > /proc/sys/xen/independent_wallclock
# date -u --set 03:09:45
Tue Feb 23 03:09:45 UTC 2010
# date
Tue Feb 23 03:09:47 GMT 2010
Now I can continue to sync files into S3 …