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Re: [Xen-users] Backup domU

To: Simon Hobson <linux@xxxxxxxxxxxxxxxx>
Subject: Re: [Xen-users] Backup domU
From: Diego Augusto Molina <diegoaugustomolina@xxxxxxxxx>
Date: Thu, 21 Jul 2011 14:00:59 -0300
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Reading this thread I wonder about one thing: What happens with the
system time in a resumed domU? I have never tried that but I'm going
to soon (tomorrow when I get back to work).
See, my domUs and dom0 have xen.independent_clock = 0 so that I only
run ntp in my dom0 and all domUs (pv) keep very (not exactly) synced.
So If I resume a domU with this configuration which had been backed up
a couple of days ago, then the domU has the "ilusion" of time having
been changed by hand (like with date -s) because of this? This could
be disastrous to a DBMS (any!) which in the precise moment of backup
was running a silly system query (e.g., a mantainence procedure,
vacuuming, etc.). Even something as innocent as that could be
dangerous. You know how important is time to DBMSs.
And in the case the time keeps going normally from the point it
stopped, then that domU would have to have set xen.independent_clock =
1, and thus any access to the DBMS could be disastrous as well. You
couldn't connect to it using SSL. Just picture it.
I mean, it shouldn't be the end of the world but when you *have* to
use a backup ($hit happens) you want things to go back to normal as
fast as it is possible, without worrying about this kind of things.
We run multiple DBMSs so that is a concern to us. If anyone faces a
scenario like that, it may be better to try other options. If an OS
backup then choose the cold backup. If DBMS data backup make a dump.
And if availability needed then try HA tools (like Slony-I for
PostgreSQL), that would allow you to stop one server, back it up, then
the other.


2011/7/21, Simon Hobson <linux@xxxxxxxxxxxxxxxx>:
> Greg Woods wrote:
>>The trouble with backing up the guests the same way as the "real"
>>machines is that it removes one of the great advantages of
> Personally, I've never seen that as one of the advantages of
> virtualisation - but that's a separate debate ;-)
> --
> Simon Hobson
> Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
> author Gladys Hobson. Novels - poetry - short stories - ideal as
> Christmas stocking fillers. Some available as e-books.
> _______________________________________________
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Diego Augusto Molina

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