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I'm considering the services of a Virtual Hosting Provider. Are there a set of vendors that support exporting and importing system images, such that a system running on provider A can be migrated to provider B?

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The migration of VM's between providers is perfectly possible, technically. The problem your going to have is trying to find a host that will do it. Obviously hosts want to do their up most to keep hold of customers, and physical inconveniences to moving providers is one of these ways. I would imagine any host that does support this will want a reasonable fee for doing so.

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    Well, if I narrow my selection to companies that accept images, that'd be a helpful market force I think.
    – jldugger
    May 24, 2009 at 23:31
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    A VM provider that would not give you an image of your system if asked is pretty untrustworthy. It's identical to not giving you an image of your hard drives if they host real hardware. Obviously this can be costly and I only mean this in exchange for reasonable payment. A VM provider has no reason to not run any image they're asked to, for a fair fee. This fee has no reason to be wildly different from the fee for an OS they've chosen. No significant extra cost for setup or maintenance for them. I'd be wary of any VM provider that didn't provide both if asked.
    – carlito
    Jun 29, 2009 at 4:04
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I've migrated Xen instances from one provider to another, by hand (i.e., with no coordination between the providers). The approach below tries to minimise downtime, though there will still be some.

Basically, I do a full rsync from the provider 1 to provider 2. Obviously, with daemons still running, this won't be a consistent image. Then, I re-run rsync, to bridge the gap a little more. At this stage, the two systems shouldn't be too different.

Then, shut down all the daemons on provider 1, so that no files are going to get changed. This goes double for database servers and email servers. Do a final rsync; this shouldn't take long, given how similar provider 1 and provider 2's files should now be.

Then, start up daemons on provider 2. Shut down provider 1. Done.

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  • You can't just rsync / can you? What about /dev or other special files?
    – Chloe
    Mar 31, 2014 at 15:06
  • @Chloe I usually use rsync with the -a option. With that option, the device nodes will be copied (and not their contents). Having said that, I don't want to sync /proc or /sys or the like, so what I actually do is bind-mount all the mounts I care about into a special mount point, and rsync that.
    – C. K. Young
    Mar 31, 2014 at 15:09
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    @Chloe e.g., mkdir /root/backup-feeder; mount --bind / /root/backup-feeder; mount --bind /boot/backup-feeder/boot, and so on for all your other mount points of interest, then only back /root/backup-feeder up. Don't forget to unmount everything afterwards.
    – C. K. Young
    Mar 31, 2014 at 15:12
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    That third command is mount --bind /boot /root/backup-feeder/boot, by the way. But I've only caught the typo now, and it's too late to edit the message. :-)
    – C. K. Young
    Apr 1, 2014 at 1:19
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First of all, both providers must use the same virtualization technology. If that is not the case, forget it (or get lucky).

Second, you must be able to get the raw image of your VM which should roughly be the weight of all the data in your VM if it's a sparse image or the size of your partition if it's a full image. This image is usually stored on a SAN or NAS at the provider's datacentre. I have never seen this being possible...

An alternative could be backup software. I know that there exists software that are capable of backing up an entire OS but I don't have any names in mind.

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Its probably not worth the hassle. chances are that the hosts will be using different systems etc and having a clean install of the OS will ensure generally smoother running of the system in the long term. it wont take too long to migrate the actual data you need to the new server and you will probably be more happy with it in the end.

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  • This is the right answer. Manageability of systems is greatly increased by doing a fresh install and knowing what you changed to set it up. This includes reinstalling "perfectly good" machines to make sure they are clean. Any excuse to reinstall the machine is one to make sure that your deployment procedures are sound, that you understand what you're running, and that you can replace or fix it if needed. If you're managing a single-digit number of machines and moving stuff to a new machine is an issue at -all-, you're in big trouble!
    – carlito
    Jun 29, 2009 at 3:57
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Migrating a linux guest can be quite easy (for guru's) even from within. The biggest issue with migrating windows would probably be the licensing, even if you, not the VM host own the licence the activation "fun" would probably make it not worth the hassle.

Again, if you have proper documentation and backups re-building a server shouldn't be a huge pain.

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