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Re: [Xen-users] How to share data between guest domains

  • To: xen-users@xxxxxxxxxxxxxxxxxxx
  • From: "Todd D. Esposito" <Todd@xxxxxxxxxxxxxxxx>
  • Date: Sun, 19 Mar 2006 08:26:29 -0600 (CST)
  • Delivery-date: Sun, 19 Mar 2006 14:27:52 +0000
  • Importance: Normal
  • List-id: Xen user discussion <xen-users.lists.xensource.com>

On Sun, March 19, 2006 5:59, Richard Jones said:
> On Sat, Mar 18, 2006 at 09:00:28PM -0600, Todd D. Esposito wrote:
>> However, on that note, I wonder if you could mount the same file system,
>> say something like /usr, into multiple domU's READ ONLY.  That could
>> certainly save some trouble when updating applications, etc.  Anyone
>> tried
>> it?
> This would only work if the disk is read-only *and* never changes
> while the machines are up.  The reason for that is that the kernel
> keeps disk blocks cached in memory, and assumes that the disk won't
> change randomly underneath.  If they did, then it would lead to a
> situation where the kernel is randomly using a mix of stale (cached)
> and fresh blocks.
> In other words, if you wanted to upgrade parts of /usr, you'd need to
> shut down all your domUs, carry out the upgrade operation, and then
> boot them up again.

This is in line with my first instinct, but I'm one of those "try it
anyway, it might work" kinda guys, so thanks for saving me some lab time.

> Over here all our domUs have separate partitions.  You only need max
> 3G for a complete Debian with plenty of packages, and how much does 3G
> of disk cost these days?  The task then moved to automating sysadmin
> as far as possible -- see cfengine and apt-cron.

Sure, disk is cheap; my aim was really to reduce sysadmin time/overhead. 
I've glanced over cfgengine, and will likely go that way as my number of
systems (and domU/system) grows.  apt-cron won't do it for me, as I'm on
Gentoo, but there are no shortage of similar tools.  Thanks for the

- Todd

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