[Xen-devel] Xen domU Timekeeping

[Qrux <qrux.qed@xxxxxxxxx>]

Howdy, all.

Is there definitive documentation about accurate timekeeping on Linux PV domUs 
(Xen-4.1.2, Linux-3.1-pvops)?

Specifically:

        * Is there a way to keep good time (i.e., bare-metal accuracy) on domU?

        * What's happened to /proc/sys/xen/independent_wallclock?

        * What should be done with /sbin/hwclock (if copied from a dom0)?

        * Does NTP on domU "work"?  Does adjtimex do anything?

        * Are there "bad side-effects" to "bad time" on domUs (see below)...?

I'd be happy for an: "RTFM @ http://...,"; response, if the docs were definitive.

===============================================================================

In addition, I'm having hard-to-track issues using ext4 on a domU, which I 
suspect may be related to the timekeeping issue(s).  When I boot this domU the 
first time, everything comes up nicely.  The kernel has ext2, ext3, and ext4 
drivers built in.  I see this on the console:

        [ 0.283049] EXT3-fs (xvda1): error: couldn't 
mount...unsupported...features
        [ 0.288476] EXT2-fs (xvda1): error: couldn't 
mount...unsupported...features

which is expected--and makes perfect sense--because I expect the next line to 
be this:

        [ 0.318273] EXT4-fs (xvda1): mounted filesystem with ordered data 
mode...

And, that is what I observe.  At least, on the first boot...

But, upon reboot of the domU, I get stuck at the ext3/ext2 errors.  Guessing, I 
destroyed the nonfunctional domU, and reformatted its drive as ext3.  This 
worked.  I was able to reboot that domU without a problem.  Google didn't find 
too much information except this:

        http://lists.openwall.net/linux-ext4/2009/10/12/12

But this is from 2009.  And, I'm not sure how relevant it is, directly, but it 
did make me wonder...Does not having "good time" on domUs affect the ability of 
the kernel to mount filesystems?  Could that be breaking ext4 on a domU with 
NTP?  And, despite the article's age, is using ext4 with "barriers=0" still 
valid advice...?

Then...Accidentally, when I had the domU disk device mounted on dom0 
(debugging, and forgot to umount before xl create), the domU came up 
fine--albeit slightly pissed off because it thought that the filesystem had 
errors.  But, it "repaired" the "errors" just fine (I assume they were related 
to the double-rw-mount), and booted.

I'm assuming all of this is documented somewhere; I just need a pointer to 
where to find this info.

===============================================================================

Thanks,
        Q


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